TRANSFUSION MEDICINE Original Article
Thawing Fresh Frozen Plasma in a Microwave Oven A Comparison with Thawing in a 37 °C Waterbath W. HALLOWELL CHURCHILL, M.D.,1 BIRGITTA SCHMIDT, M.D.,2 JANE LINDSEY, Sc.B.,3 MARK GREENBERG, B.S.,1 SUSAN BOUDROW, M.T. (ASCP)1 AND CARLO BRUGNARA, M.D.1
Thawing fresh frozen plasma (FFP) encased in a plastic overjacket in a conventional 37 °C circulating waterbath (WB) takes about 20 to 30 minutes, an interval that is often too long when fresh plasma is urgently needed to treat coagulation disorders. In addition, immersion in a WB increases the likelihood of bacterial contamination. Increasing the temperature of the WB to 45 or 56 °C shortens thaw time with satisfactory retention of coagulation components but does not eliminate the hazard of bacterial contamination.1"2 An alternate approach that reduces thaw time to about 5 to 10 minutes and eliminates need for immersion in water is the use of microwave energy. Initial studies used standard microwave ovens (MWO).3"5 Thaw time was reduced to about 5 minutes without significant destruction
thawed plasma showed that MWO-thawed FFP temperature was 20.4 + 2.5° C, whereas WB-thawed FFP was 15.4 + 3.3 °C (n = 24; P < 0.005). Except for thrombin time (MWO = 20.1 seconds; WB = 19.8 seconds; n = 24; P = 0.023), no significant differences were observed in the 23 other coagulation parameters and plasma proteins studied. Faster thawing and freedom from risk of contamination may make MWO the method of choice for emergency thawing of FFP. (Key words: Plasma; Microwave; Waterbath; Transfusion) Am J Clin Pathol 1992;97: 227-232
of coagulation components. In one of these studies, FFP thawed by both techniques was infused into patients and appeared to cause no side effects and have equivalent beneficial effects.4 Significant improvement was achieved by Rock and associates,6 who modified the MWO so that continuous mixing would occur during the thaw process and an internal sensor would prevent plasma from reaching temperatures higher than 21 °C. In these studies, the thawing time was consistently less than 6 minutes and no significant differences in coagulation factors or serum proteins were demonstrated on analysis of the paired samples.6 Similar results were obtained with another type of MWO in Germany. 7 Because this approach appeared to be the best method available to achieve rapid thawing of FFP; we used a model of the MWO specifically produced by Westmorland Laboratories to thaw FFP. Because previous studies had not adequately addressed the issue of temperature of thawed From the 'Blood Bank and Clinical Laboratories, and department FFPand by MWO, we used rapid calorimetry to obtain a fast of Pathology, Brigham Women's Hospital, Boston, Massachusetts, and reliable measurement of the temperature after thawthe ^Department of Biostatistics, Harvard School of Public Health. ing. Furthermore, we evaluated the percentage recovery Supported by grants from Rush Enterprises, Inc., Hudson, New of several noncoagulation proteins after thawing by MWO Hampshire. or WB. The presence of hot spots in thawing by MWO Received March 12, 1991; received revised manuscript and accepted for publication July 24, 1991. has always been a great concern in the application of this Address reprint requests to Dr. Churchill: Department of Pathology, technique to FFP thawing. To address this issue, we obBrigham and Women's Hospital, 75 Francis Street, Boston Massachusetts tained early samples from FFP thawed by MWO and 02115. 227
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We show in this report that fresh frozen plasma (FFP) can be thawed faster using a specifically designed microwave oven (MWO) (WesLabs Plasma Defroster, Westmorland Laboratories, Inc., New Brunswick, Canada) than using 37 °C water bath (WB) and that the thawed product was equivalent to FFP thawed by WB. Paired plasma bags (200 mL/bag) from plasma pools were frozen, stored at —35 °C, and thawed in parallel, one bag in MWO, the other in WB. Mean thaw time (mean + SD) by MWO was 6.99 + 1.3 minutes; by WB the time was 17.6 + 1.7 minutes (n = 24; P < 0.005). Rapid calorimetry of
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compared them with the same FFP samples after completion of thawing and with early samples of FFP thawed with WB. Our experiments indicate that there are no biologically significant differences in coagulation proteins or other serum proteins in FFP thawed with MWO. These findings suggest that microwave energy under precisely controlled conditions is an effective rapid method to thaw FFP. MATERIALS AND METHODS Blood Collection and Storage
Thawing Procedures Circulating waterbath. One of the paired samples was placed in a 37 °C WB in a plastic overwrap according to the established blood bank procedure. The bags were checked periodically for mixing and removed when ice crystals could no longer be palpated. Microwave oven. The WesLabs Plasma Defroster (Westmorland Laboratories) is a production model of the instrument used by Rock and associates6 to thaw FFP by microwave energy. The MWO was equipped with four bag holders. However, no more than two bags of plasma were thawed at a time in the MWO. After initiating the thaw process, the bags were rotated intermittently by the MWO. The temperature of each bag was monitored by a sensor and the MWO was programmed to shut off automatically when any bag reached a temperature immediately adjacent to sensor probe of 21 °C, at which temperature the FFP was fully thawed. Immediately after thawing, rapid calorimetry of each bag was performed using a Rayteck Model R2-LT infrared pistol with a digital display. This measurement yielded an average temperature for the entire bag.
Samples from plasma thawed by MWO and by WB were collected and the thrombin time, the prothrombin time, the partial thromboplastin time, fibrinogen, fibrinogen-split products, factors VIII, IX, XI, von Willebrand factor, anti-thrombin III, and protein C were determined. We also determined total proteins, and by nephelometry we measured albumin, alpha-1 antitrypsin, haptoglobin, transferrin, orosomucoid, C4, C3, IgG, IgA, IgM, properdin, factor B, and B-lipoprotein. Methods used in each of these assays were those used in the Clinical Laboratory of the Brigham and Women's Hospital and the Center for Blood Research.8"12 Statistical
Analysis
Microwave oven- and WB-thawed samples were compared by the Mest for paired samples. In experiments comparing post-thaw to prefreeze values and in experiments comparing the levels of parameters at two time points during the thawing process, repeated measure analysis of variance was used.13 Relative error is defined as the ratio of the absolute value of the larger of the upper and lower 95% confidence limits for the mean difference to the mean value for all samples. R
( I C u | , |C1|) X 100 Mean MWO + Mean WB
Cu equals the upper limit of 95% confidence interval for the expected difference between values from MWOand WB-thawed samples. CI equals the lower limit of 95% confidence interval for the expected difference between values from MWO- and WB-thawed samples. RESULTS In the initial series of experiments, we measured prothrombin time, partial thromboplastin time, thrombin time, and the levels of coagulation proteins in pairs of plasma thawed by either MWO or WB (Table 1). In addition, the total protein and the levels of 12 other plasma proteins in these paired samples also were compared (Table 2). The end point for MWO thawing was determined by the MWO thermal sensor. This sensor shuts the oven off when the temperature reaches 21 °C. Absence of ice crystals was used as the endpoint for plasma thawed in the 37 °C WB. The mean thaw time for 24 samples thawed by MWO was 6.99 minutes (± 1.3 minutes), whereas thawing in a WB took 17.5 minutes (± 1.7 minutes). The average tem-
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Blood was collected in standard Fenwal blood bags (Fenwal Laboratories, Inc., Deerfield, IL) that contained citrate-phosphate-dextrose-adenine 1. Plasma was separated from red blood cells within 6 hours of collection. The plasma from two donors were pooled and 200-mL portions were transferred into two separate 300-mL bags. All units were frozen at —80 °C. Bags were frozen flat with ports extended. This configuration was required to fit bags into the bag holder in the MWO. The paired aliquots of FFP were stored at —35 °C and were thawed within 1 month of collection. No more than two samples were thawed simultaneously. Bags were weighed just before thawing. Average weight of bags thawed by MWO was 209.4 ± 2.6 (M + SE) (n = 24); for bags thawed by WB, average weight was 207.8 ± 2.6 (M + SE) (n = 24).
Laboratory Tests
CHURCHILL ET
AL.
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Oven Thawing Fresh Frozen PlasmaMicrowave by TABLE 1. COMPARISON OF COAGULATION SCREENING TEST AND LEVELS OF COAGULATION PROTEINS IN SAMPLES THAWED BY MWO OR WB WB
MWO
TT (sec) PT (sec) aPTT (sec) Fibr g/L ATIIIFu % ATIII ag % Prot CFu % Prot C ag % Factor VIII % Factor IX % Factor XI % VWF%
n
Mean ± SE
N
Mean ± SE
24 24 24 24 23 21 22 22 24 24 24 23
20.1 11.6 26.3 3.2 104.3 103.1 91.0 112.5 137.1 110.2 90 113.3
24 24 24 24 23 21 22 22 24 24 24 23
19.8 11.6 26.4 3.2 104.4 103.5 89.1 113.0 131.6 113.3 89.0 116.4
±.40 ±.07 ± .58 ±0.14 ±2.1 ±2.8 ±6.73 ±4.3 ±7.0 ±5.2 ± 4.67 ±6.4
Probability Value
±0.39 ±0.06 ± 0.49 ±0.13 ±2.1 ± 3.0 ±8.03 ±4.1 ±6.8 ±4.9 ± 4.23 ±6.5
0.023 0.118 0.567 0.934 0.916 0.785 0.701 0.845 0.090 0.667 0.440 0.065
antigen; Prot CFu = protein C functional units; Prot C ag = protein C antigen; VWF = Von Willebrand Factor.
perature of FFP thawed by MWO was 20.4 ± 2.5 °C, which was 5 °C higher than the mean temperature of FFP thawed in the WB (15.4 ± 3.3). The results of comparison of coagulation function in thawed FFP is shown in Table 1. In this and subsequent tables, the mean and standard error of the mean of all samples thawed by MWO or by WB is shown as a measure of variability between plasma pools. In 11 of 12 parameters studied, no statistically significant differences between the paired samples were found. For the thrombin time, the mean of the differences of the paired samples was 0.35 seconds (P < 0.02). Fibrin-split products were not increased (data not shown) and fibrinogen levels were identical, so this small difference of 0.35 seconds is unexplained. However, a difference of this magnitude has no clinical significance and could be due to chance because of multiple tests being run on the same sample. The levels
of the noncoagulation proteins studied are shown in Table 2. No differences were found in any of the 12 proteins studied. We estimated the confidence interval for the mean difference of paired samples as a percentage of the mean of all samples (relative error). This value represents the largest relative error that is consistent with the observed data at the 95% confidence level. Protein CFu has the largest relative error with 14%. This result is not surprising because the measurement of protein C, both by functional assay and by antigen, has wide scatter and consequently the largest confidence interval for the differences of the paired samples. For all of the other coagulation proteins and screening tests of coagulation, the relative error was less than 8.8%. For the noncoagulation proteins, the relative error is smaller than for the coagulation proteins, with the maximum relative error being 5.6%.
TABLE 2. COMPARISON OF LEVELS OF PROTEINS IN SAMPLES THAWED BY MWO OR WB MWO-WB MWO
Total protein g/L Albumin g/L a 1 antitrypsin % nl Haptoglobin g/L Transferrin g/L Orosomucoid g/L C4 % nl C3g/L IgG g/L IgAg/L IgM g/L Properdin factor B g/L B-lipoprotein g/L
WB
n
Mean ± SE
N
Mean ± SE
23 23 23 23 23 23 23 23 23 23 23 23 23
61 33 9.32 1.05 2.41 0.94 72.1 1.2 7.7 1.8 0.86 0.25 0.77
23 23 23 23 23 23 23 23 23 23 23 23 23
61 33 92.7 1.03 2.43 0.94 71.4 1.2 7.7 1.8 0.88 0.25 0.77
±0.7 ± 0.5 ± 2.3 ±0.07 ±0.08 ± 0.05 ±2.2 ±0.03 ± 0.03 ±0.1 ±0.1 ±0.01 ± 0.04
See Table 1 for definitions of abbreviations.
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±0.7 ± 0.5 ± 2.70 ±0.07 ± 0.08 ± 0.05 ±2.4 ±0.03 ± 0.04 ±0.1 ±0.1 ±0.01 ± 0.04
Probability Value 0.233 0.759 0.141 0.193 0.932 0.386 0.199 0.910 0.594 0.414 0.266 0.385
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N = Number of samples; MWO = microwave oven; WB = 37° C waterbath. TT = thrombin time; PT = prothrombin time; aPTT = activated partial thromboplastin time; Fibr = fibrinogen; ATIIIFu = antithrombin III functional units; ATIII ag = antithrombin III
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Percentage Recovery in Thawed Specimens To measure the percentage recovery after thawing, we compared parameters measured in a sample of each pool taken before freezing to the levels measured after thawing by either MWO or WB. In Table 3, results are presented for the noncoagulation proteins. Only 2 of 12 of these proteins were significantly different from the value measured before freezing. Albumin and plasma levels of C3 were significantly higher in the samples thawed by MWO or WB than in the samples taken before freezing. Comparison of Levels Early and Late in the Thawing Process
DISCUSSION FFP can be stored in the frozen state with retention of clotting protein activity for long periods of time. Because thawed FFP contains all of the coagulation proteins, it is often urgently needed to replace clotting factors in bleeding patients.'4 Conventional methods ofthawing in a 37 °C WB have the disadvantage of being slow and of increasing the risk of bacterial contamination. Microwave energy has the potential for fast, contamination-free thawing, but its use requires carefully controlled conditions that avoid overheating and formation of hot spots within small areas of the thawing FFP. We have used a production model of the WesLabs Microwave Plasma Defroster (model 601) to compare thawing of FFP by microwave energy to thawing by a 37 °C WB. The MWO thawed the FFP in one third of the time required by the WB and yielded thawed plasma that was 5 °C warmer than the FFP thawed by WB. This temperature difference could be of clinical importance. In a massively transfused patient, temperature of the transfused blood products has important effects on core body tem-
TABLE 3. PERCENTAGE RECOVERY OF SERUM PROTEINS AFTER THAWING BY MWO OR WB Percentage Recovery
Total protein g/L Albumin g/L a\ antitrypsin % nL Haptoglobin g/L Transferrin g/L Orosomucoid g/L C4 % nL C3g/L IgG g/L IgAg/L IgM g/L Properdin factor B g/L B-lipoprotein g/L
Mean Prior to Freeze
MWO
WB
Probability* Value
61.2 30.4 97.00 0.92 2.32 0.84 74.60 1.16 8.88 1.77 0.90 0.26 0.66
99.1 115 103 93.5 112 102 94.1 105 98.9 101 90 82.6 120
91.7' 116 101 91.1 112 102 92.8 106 96.6 102 88.7 86.4 121
0.890 0.0001 0.839 0.312 0.387 0.864 0.100 0.032 0.678 0.965 0.552 0.157 0.069
See Table 1 for definitions of abbreviations. ' Probability refers to the differences between pre- and post-thaw values.
A.J.C.P. * February 1992
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Early in the process of thawing FFP, small areas are thawed, but most of the FFP is still frozen. Because such areas could represent hot spots of increased energy absorption, we performed experiments in which samples were obtained early in the thawing process and compared with samples drawn from the same FFP after thawing had been completed. The initial sample was obtained 3 minutes after thawing by MWO was begun and 5.3 minutes after thawing by 37 °C WB was initiated. The results of these experiments are summarized in Table 4. In contrast to thefindingsafter complete thawing by either MWO or WB, highly significant differences in changes from early to late sampling between MWO and WB thawed samples were found for some of the coagulation proteins. The levels offibrinogen,ATIIIag, ATIIIFu, Prot Cag, Factors VIII, IX, and XI were all significantly higher in the specimens collected early in the MWO thawing process than they were in the specimens collected after thawing had been completed. In contrast, the levels of these coagulation proteins were the same in the specimens sampled early in the process of thawing by 37 °C WB as they were in
the completely thawed specimens. These results showed a statistically significant interaction between the thawing method and the time of sampling for these coagulation proteins (Table 4). Three tests of coagulation function also were measured early in the thawing process (prothrombin time, partial thromboplastin time, and thrombin time). No differences between MWO and WB thawing were found. Because these particular tests are normal with as little as 30 to 40% of clotting proteins, it is not surprising that these screening tests were not affected by higher levels of coagulation proteins in specimens collected early in the MWO thawing.
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TABLE 4. COMPARISON OF COAGULATION TESTS AND LEVELS OF COAGULATION PROTEINS SAMPLED AT TWO TIME POINTS DURING THAWING BY MWO AND WB«t Late
Earlyf
TT (sec) PT (sec) PTT (sec) Fibg/L ATIIIFu % ATIII ag % Prot CFu % Prot C ag % F VIII % FIX% FXI% VWF%
n
MWO Mean ± SE
n
WB Mean ± SE
n
MWO Mean ± SE
n
WB Mean + SE
5 5 5 5 5 4 3 4 5 5 5 5
21.2 ± 0 . 7 11.8 ± 0 . 3 28.3 ± 1.5 4.3 ± 0.3 128.4 ± 4 . 2 132 ± 11 74.3 ± 4.5 151 ± 2 3 140 ± 4 134 ± 16 124 ± 11 116±4
5 5 5 5 5 5 3 5 5 5 5 5
19.5 ± 0 . 3 11.9 + 0.2 27.7 ± 1.3 3.2 ± 0.3 101 ± 6 87.6 ± 5 . 1 84.0 ± 1.5 106 ± 5 105 ± 10 101 ± 13 79.0 ± 9.4 101 ± 9
5 5 5 5 5 4 3 5 5 5 5 5
20.3 ± 0.6 11.7 ± 0 . 2 26.6 ± 0.9 3.2 ± 0.3 104 ± 5 96.5 ± 6.8 76 ± 0 101 ± 5 104 ± 5 105 ± 11 79.4 ± 8.0 114± 10
5 5 5 5 4 4 3 4 5 5 5 5
19.6 ±0.2 11.6+0.1 26.3 ±0.6 3.3 ± 0 . 4 102 ± 1 95.0 ± 4.5 89.3 ± 15.4 111 ± 3 105 ± 5 102 ± 11 87.2 ± 8.2 109 ± 13
Interaction P Value
0.0001 0.006 0.0009 0.0227 0.0022 0.017 0.0001
X Early for MWO = 3 minutes after the start of thawing; for WB it was 5.33 minutes after the start of thawing. See Table 1 for definitions of abbreviations.
perature. Infusing warmer blood products helps to minimize decreases in body temperature. 15 Our study on the pre-/post-thaw comparison of noncoagulation proteins showed no important differences between thawing by MWO or WB (Table 3). However, recovery of both albumin and C'3 was significantly enhanced after thawing. We suspect that this is an artifact of the method of measurement. Nephelometry measures light scattering of antigen-antibody complexes as a measure of protein concentration. Any change in aggregation after thawing by either method could yield an apparent enhancement of protein concentration. Measurement by an independent method will be required to settle this issue. The possible presence of hot spots has been a source of concern for the use of MWO in thawing FFP. Our approach to this issue has been to sample early in the thawing process and compare these samples with others taken at the end of thawing. The rationale was that these areas could be potential hot spots. We found no loss of coagulation proteins in these areas. Some of the coagulation proteins were significantly increased early in the MWO thaw period even though they were indistinguishable from WB-thawed specimens when the thawing was complete. These proteins were fibrinogen, ATIII, protein Cag, and factors VIII, IX, and XI. Further experiments are required to clarify whether these results indicate that these proteins are relatively better preserved in early MWO-thawed specimens than they are in early WBthawed specimens. Possible explanations include variation in sampling or in homogeneity in concentration of certain, but not all, proteins during the freezing process. This latter explanation seems unlikely because the increase in factors was observed only in samples collected early in the MWO
thawing process. Salt concentrations and ionic strength also could be involved because ionic strength is important for assays dependent on fibrin polymerization. Further kinetic experiments are necessary to resolve these issues. Our results confirm observations made by Rock and associates6 using an earlier prototype of this MWO 6 and are very similar to observations recently reported by Sohngen and co-workers.7 Results from measurement of nine coagulation proteins and three screening tests of coagulation function in paired samples thawed by MWO or WB were indistinguishable, except for a 0.35-second prolongation of the thrombin time in MWO-thawed specimens. A prolongation of this magnitude is not biologically significant and could be due to chance because of multiple tests of significance on the same sample. The absence of fibrin-split products in these samples and the similar levels of fibrinogen provides further evidence that there was no measurable degradation of fibrinogen. Some of the coagulation proteins were measured by both functional and antigenic assays (ATIII and Protein C) to determine if a loss of function with retention of antigenic activity could be demonstrated, but no differences between MWO- and WB-thawed samples were found. Measurement of 12 different serum proteins by nephelometry yielded similar results. These findings confirm previous reports.6'7 One could suggest the use of MWO for thawing other plasma products, particularly cryoprecipitate. However, the presence of a very small amount of fluid in a large bag makes it impossible for the temperature sensor to be in contact with enough liquid, with consequent probable overheating of the product. Furthermore, the limiting factor in the time required to issue cryoprecipitate is not the thawing time but rather the time required to pool 10
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* For tests lacking significant overall interactions, MWO-WB difference for TT was significant (P = 00.37) and early-late difference for PTT was significant (0.0099). t Only significant probability values are given.
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It is difficult to estimate savings generated from the prompt availability of FFP. It is clear that ordering behavior in the operating room will be affected dramatically by the availability of a product in 10 minutes rather than in 30. An effect of MWO thawing on reducing FFP waste also can be anticipated. The effect of MWO thawing on FFP use will be the object of further studies.
Acknowledgments. This work was performed in the Blood Bank, Clinical Laboratories, Brigham and Women's Hospital, Boston, Massachusetts. The authors thank Professor James Ware, Department of Biostatistics, Harvard School of Public Health, for expert statistical consultation.
REFERENCES 1. Westphal RG, Tindle B, Howard PL, Golden EA, Page GA. Rapid thawing of fresh frozen plasma. Am J Clin Pathol 1982; 78:220222. 2. Plotz RD, Ciotola RT. Thawing of fresh-frozen plasma at 45° versus 37 °C. Am J Clin Pathol 1988;89:381-384. 3. Sherman LA, Dorner IM. A new rapid method for thawing fresh frozen plasma. Transfusion 1974; 14:595-597. 4. Thompson KS, O'Kell RT. Comparison of fresh-frozen plasmas thawed in a microwave oven and in a 37°C water bath. Am J Clin Pathol 1981;75:851-853. 5. Mead JH, Boucock BP, Russell RE, Robinson CS, Harris G. Water environment microwave thawing. Am J Clin Pathol 1986;85: 510-513. 6. Rock G, Tackaberry ES, Dunn JG, Kashyap S. Rapid controlled thawing of fresh-frozen plasma in a modified microwave oven. Transfusion 1984;24:60-65. 7. Sohngen D, Kretschmer V, Franke K, Pelzer H, Walker WH. Thawing of fresh-frozen plasma with a new microwave oven. Transfusion 1988;28:576-580. 8. Technicon Instruments Corporation. Preliminary: Automated immunoprecipitin method for determining specific proteins (AIP/ PEG method). Technicon Method No. SE4-00375D5, 1985; 111. 9. Rizza> CR, Walker W. One stage prothrombin time techniques, thrombosis and bleeding disorders. In: Bang NU. Medical Laboratory Automation. New York: Academic Press, 1971, pp 92100. 10. Bowie EJW, Thompson JH, Didischeim P, Owen CA. Mayo Clinic Laboratory Manual of Hemostasis. Philadelphia: WB Saunders, Philadelphia, 1971, pp 65-69. 11. Simone JV, Vanderheiden J, Abildgaard CF. A semi-automatic onestage factor VIII assay with a commercially prepared standard. J Lab and Clin Pathol 1967;69:706-712. 12. Allain JP, Cooper HA, Wagner RH, Brinkhous KM. Platelets fixed with paraformaldehyde: A new reagent for assay of vWF and PIT aggregating factor. J Lab Clin Med 1975;85:318-328. 13. Winer BJ. Statistical Principles in Experimental Design. New York: McGraw-Hill, 1971. 14. Office of Medical Applications of Research. National Institutes of Health. Fresh Frozen Plasma: Indications and Risk. JAMA 1985;253:551-553. 15. Dybkjaer E, Elkjaer. P. The use of heated blood in massive blood replacement. Acta Anaesth Scand 1964;8:271-276. 16. Manual for Laboratory Workload Recording Method. College of American Pathologists. Northfield, IL, 1991.
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different bags into 1 single bag. The thawing of cryoprecipitate with WB takes substantially less time than for FFP because of the small amount of fluid condensed into the cryoprecipitate bags. Therefore, microwave thawing is not likely to provide a significant improvement in the time required for issuing cryoprecipitate. It is difficult to quantify, in monetary terms, the benefits of using MWO thawing rather than the traditional WB thawing. In fact, the technologist's time is probably not different for the two procedures. The College of American Pathologists code for FFP thawing with the WB method is 5 minutes (no. 86805).16 A similar time also is spent by the technologist for the thawing of FFP by MWO. The main advantage of the MWO procedure is the shorter turnaround time. This advantage probably will be greater when multiple FFP units are needed. In fact, the 18-minute thaw time for FFP by WB was estimated in ideal conditions, which clearly are different from working conditions. The presence of multiple FFP thawing in the same WB significantly increases the turnaround time for FFP thawing, which can easily be 45 minutes. These problems do not occur in the thawing of FFP by MWO.
Thawing Fresh Frozen Plasma in a Microwave Oven A Comparison with Thawing in a 37 °C Waterbath W. HALLOWELL CHURCHILL, M.D.,1 BIRGITTA SCHMIDT, M.D.,2 JANE LINDSEY, Sc.B.,3 MARK GREENBERG, B.S.,1 SUSAN BOUDROW, M.T. (ASCP)1 AND CARLO BRUGNARA, M.D.1
Thawing fresh frozen plasma (FFP) encased in a plastic overjacket in a conventional 37 °C circulating waterbath (WB) takes about 20 to 30 minutes, an interval that is often too long when fresh plasma is urgently needed to treat coagulation disorders. In addition, immersion in a WB increases the likelihood of bacterial contamination. Increasing the temperature of the WB to 45 or 56 °C shortens thaw time with satisfactory retention of coagulation components but does not eliminate the hazard of bacterial contamination.1"2 An alternate approach that reduces thaw time to about 5 to 10 minutes and eliminates need for immersion in water is the use of microwave energy. Initial studies used standard microwave ovens (MWO).3"5 Thaw time was reduced to about 5 minutes without significant destruction
thawed plasma showed that MWO-thawed FFP temperature was 20.4 + 2.5° C, whereas WB-thawed FFP was 15.4 + 3.3 °C (n = 24; P < 0.005). Except for thrombin time (MWO = 20.1 seconds; WB = 19.8 seconds; n = 24; P = 0.023), no significant differences were observed in the 23 other coagulation parameters and plasma proteins studied. Faster thawing and freedom from risk of contamination may make MWO the method of choice for emergency thawing of FFP. (Key words: Plasma; Microwave; Waterbath; Transfusion) Am J Clin Pathol 1992;97: 227-232
of coagulation components. In one of these studies, FFP thawed by both techniques was infused into patients and appeared to cause no side effects and have equivalent beneficial effects.4 Significant improvement was achieved by Rock and associates,6 who modified the MWO so that continuous mixing would occur during the thaw process and an internal sensor would prevent plasma from reaching temperatures higher than 21 °C. In these studies, the thawing time was consistently less than 6 minutes and no significant differences in coagulation factors or serum proteins were demonstrated on analysis of the paired samples.6 Similar results were obtained with another type of MWO in Germany. 7 Because this approach appeared to be the best method available to achieve rapid thawing of FFP; we used a model of the MWO specifically produced by Westmorland Laboratories to thaw FFP. Because previous studies had not adequately addressed the issue of temperature of thawed From the 'Blood Bank and Clinical Laboratories, and department FFPand by MWO, we used rapid calorimetry to obtain a fast of Pathology, Brigham Women's Hospital, Boston, Massachusetts, and reliable measurement of the temperature after thawthe ^Department of Biostatistics, Harvard School of Public Health. ing. Furthermore, we evaluated the percentage recovery Supported by grants from Rush Enterprises, Inc., Hudson, New of several noncoagulation proteins after thawing by MWO Hampshire. or WB. The presence of hot spots in thawing by MWO Received March 12, 1991; received revised manuscript and accepted for publication July 24, 1991. has always been a great concern in the application of this Address reprint requests to Dr. Churchill: Department of Pathology, technique to FFP thawing. To address this issue, we obBrigham and Women's Hospital, 75 Francis Street, Boston Massachusetts tained early samples from FFP thawed by MWO and 02115. 227
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We show in this report that fresh frozen plasma (FFP) can be thawed faster using a specifically designed microwave oven (MWO) (WesLabs Plasma Defroster, Westmorland Laboratories, Inc., New Brunswick, Canada) than using 37 °C water bath (WB) and that the thawed product was equivalent to FFP thawed by WB. Paired plasma bags (200 mL/bag) from plasma pools were frozen, stored at —35 °C, and thawed in parallel, one bag in MWO, the other in WB. Mean thaw time (mean + SD) by MWO was 6.99 + 1.3 minutes; by WB the time was 17.6 + 1.7 minutes (n = 24; P < 0.005). Rapid calorimetry of
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compared them with the same FFP samples after completion of thawing and with early samples of FFP thawed with WB. Our experiments indicate that there are no biologically significant differences in coagulation proteins or other serum proteins in FFP thawed with MWO. These findings suggest that microwave energy under precisely controlled conditions is an effective rapid method to thaw FFP. MATERIALS AND METHODS Blood Collection and Storage
Thawing Procedures Circulating waterbath. One of the paired samples was placed in a 37 °C WB in a plastic overwrap according to the established blood bank procedure. The bags were checked periodically for mixing and removed when ice crystals could no longer be palpated. Microwave oven. The WesLabs Plasma Defroster (Westmorland Laboratories) is a production model of the instrument used by Rock and associates6 to thaw FFP by microwave energy. The MWO was equipped with four bag holders. However, no more than two bags of plasma were thawed at a time in the MWO. After initiating the thaw process, the bags were rotated intermittently by the MWO. The temperature of each bag was monitored by a sensor and the MWO was programmed to shut off automatically when any bag reached a temperature immediately adjacent to sensor probe of 21 °C, at which temperature the FFP was fully thawed. Immediately after thawing, rapid calorimetry of each bag was performed using a Rayteck Model R2-LT infrared pistol with a digital display. This measurement yielded an average temperature for the entire bag.
Samples from plasma thawed by MWO and by WB were collected and the thrombin time, the prothrombin time, the partial thromboplastin time, fibrinogen, fibrinogen-split products, factors VIII, IX, XI, von Willebrand factor, anti-thrombin III, and protein C were determined. We also determined total proteins, and by nephelometry we measured albumin, alpha-1 antitrypsin, haptoglobin, transferrin, orosomucoid, C4, C3, IgG, IgA, IgM, properdin, factor B, and B-lipoprotein. Methods used in each of these assays were those used in the Clinical Laboratory of the Brigham and Women's Hospital and the Center for Blood Research.8"12 Statistical
Analysis
Microwave oven- and WB-thawed samples were compared by the Mest for paired samples. In experiments comparing post-thaw to prefreeze values and in experiments comparing the levels of parameters at two time points during the thawing process, repeated measure analysis of variance was used.13 Relative error is defined as the ratio of the absolute value of the larger of the upper and lower 95% confidence limits for the mean difference to the mean value for all samples. R
( I C u | , |C1|) X 100 Mean MWO + Mean WB
Cu equals the upper limit of 95% confidence interval for the expected difference between values from MWOand WB-thawed samples. CI equals the lower limit of 95% confidence interval for the expected difference between values from MWO- and WB-thawed samples. RESULTS In the initial series of experiments, we measured prothrombin time, partial thromboplastin time, thrombin time, and the levels of coagulation proteins in pairs of plasma thawed by either MWO or WB (Table 1). In addition, the total protein and the levels of 12 other plasma proteins in these paired samples also were compared (Table 2). The end point for MWO thawing was determined by the MWO thermal sensor. This sensor shuts the oven off when the temperature reaches 21 °C. Absence of ice crystals was used as the endpoint for plasma thawed in the 37 °C WB. The mean thaw time for 24 samples thawed by MWO was 6.99 minutes (± 1.3 minutes), whereas thawing in a WB took 17.5 minutes (± 1.7 minutes). The average tem-
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Blood was collected in standard Fenwal blood bags (Fenwal Laboratories, Inc., Deerfield, IL) that contained citrate-phosphate-dextrose-adenine 1. Plasma was separated from red blood cells within 6 hours of collection. The plasma from two donors were pooled and 200-mL portions were transferred into two separate 300-mL bags. All units were frozen at —80 °C. Bags were frozen flat with ports extended. This configuration was required to fit bags into the bag holder in the MWO. The paired aliquots of FFP were stored at —35 °C and were thawed within 1 month of collection. No more than two samples were thawed simultaneously. Bags were weighed just before thawing. Average weight of bags thawed by MWO was 209.4 ± 2.6 (M + SE) (n = 24); for bags thawed by WB, average weight was 207.8 ± 2.6 (M + SE) (n = 24).
Laboratory Tests
CHURCHILL ET
AL.
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Oven Thawing Fresh Frozen PlasmaMicrowave by TABLE 1. COMPARISON OF COAGULATION SCREENING TEST AND LEVELS OF COAGULATION PROTEINS IN SAMPLES THAWED BY MWO OR WB WB
MWO
TT (sec) PT (sec) aPTT (sec) Fibr g/L ATIIIFu % ATIII ag % Prot CFu % Prot C ag % Factor VIII % Factor IX % Factor XI % VWF%
n
Mean ± SE
N
Mean ± SE
24 24 24 24 23 21 22 22 24 24 24 23
20.1 11.6 26.3 3.2 104.3 103.1 91.0 112.5 137.1 110.2 90 113.3
24 24 24 24 23 21 22 22 24 24 24 23
19.8 11.6 26.4 3.2 104.4 103.5 89.1 113.0 131.6 113.3 89.0 116.4
±.40 ±.07 ± .58 ±0.14 ±2.1 ±2.8 ±6.73 ±4.3 ±7.0 ±5.2 ± 4.67 ±6.4
Probability Value
±0.39 ±0.06 ± 0.49 ±0.13 ±2.1 ± 3.0 ±8.03 ±4.1 ±6.8 ±4.9 ± 4.23 ±6.5
0.023 0.118 0.567 0.934 0.916 0.785 0.701 0.845 0.090 0.667 0.440 0.065
antigen; Prot CFu = protein C functional units; Prot C ag = protein C antigen; VWF = Von Willebrand Factor.
perature of FFP thawed by MWO was 20.4 ± 2.5 °C, which was 5 °C higher than the mean temperature of FFP thawed in the WB (15.4 ± 3.3). The results of comparison of coagulation function in thawed FFP is shown in Table 1. In this and subsequent tables, the mean and standard error of the mean of all samples thawed by MWO or by WB is shown as a measure of variability between plasma pools. In 11 of 12 parameters studied, no statistically significant differences between the paired samples were found. For the thrombin time, the mean of the differences of the paired samples was 0.35 seconds (P < 0.02). Fibrin-split products were not increased (data not shown) and fibrinogen levels were identical, so this small difference of 0.35 seconds is unexplained. However, a difference of this magnitude has no clinical significance and could be due to chance because of multiple tests being run on the same sample. The levels
of the noncoagulation proteins studied are shown in Table 2. No differences were found in any of the 12 proteins studied. We estimated the confidence interval for the mean difference of paired samples as a percentage of the mean of all samples (relative error). This value represents the largest relative error that is consistent with the observed data at the 95% confidence level. Protein CFu has the largest relative error with 14%. This result is not surprising because the measurement of protein C, both by functional assay and by antigen, has wide scatter and consequently the largest confidence interval for the differences of the paired samples. For all of the other coagulation proteins and screening tests of coagulation, the relative error was less than 8.8%. For the noncoagulation proteins, the relative error is smaller than for the coagulation proteins, with the maximum relative error being 5.6%.
TABLE 2. COMPARISON OF LEVELS OF PROTEINS IN SAMPLES THAWED BY MWO OR WB MWO-WB MWO
Total protein g/L Albumin g/L a 1 antitrypsin % nl Haptoglobin g/L Transferrin g/L Orosomucoid g/L C4 % nl C3g/L IgG g/L IgAg/L IgM g/L Properdin factor B g/L B-lipoprotein g/L
WB
n
Mean ± SE
N
Mean ± SE
23 23 23 23 23 23 23 23 23 23 23 23 23
61 33 9.32 1.05 2.41 0.94 72.1 1.2 7.7 1.8 0.86 0.25 0.77
23 23 23 23 23 23 23 23 23 23 23 23 23
61 33 92.7 1.03 2.43 0.94 71.4 1.2 7.7 1.8 0.88 0.25 0.77
±0.7 ± 0.5 ± 2.3 ±0.07 ±0.08 ± 0.05 ±2.2 ±0.03 ± 0.03 ±0.1 ±0.1 ±0.01 ± 0.04
See Table 1 for definitions of abbreviations.
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±0.7 ± 0.5 ± 2.70 ±0.07 ± 0.08 ± 0.05 ±2.4 ±0.03 ± 0.04 ±0.1 ±0.1 ±0.01 ± 0.04
Probability Value 0.233 0.759 0.141 0.193 0.932 0.386 0.199 0.910 0.594 0.414 0.266 0.385
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N = Number of samples; MWO = microwave oven; WB = 37° C waterbath. TT = thrombin time; PT = prothrombin time; aPTT = activated partial thromboplastin time; Fibr = fibrinogen; ATIIIFu = antithrombin III functional units; ATIII ag = antithrombin III
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Percentage Recovery in Thawed Specimens To measure the percentage recovery after thawing, we compared parameters measured in a sample of each pool taken before freezing to the levels measured after thawing by either MWO or WB. In Table 3, results are presented for the noncoagulation proteins. Only 2 of 12 of these proteins were significantly different from the value measured before freezing. Albumin and plasma levels of C3 were significantly higher in the samples thawed by MWO or WB than in the samples taken before freezing. Comparison of Levels Early and Late in the Thawing Process
DISCUSSION FFP can be stored in the frozen state with retention of clotting protein activity for long periods of time. Because thawed FFP contains all of the coagulation proteins, it is often urgently needed to replace clotting factors in bleeding patients.'4 Conventional methods ofthawing in a 37 °C WB have the disadvantage of being slow and of increasing the risk of bacterial contamination. Microwave energy has the potential for fast, contamination-free thawing, but its use requires carefully controlled conditions that avoid overheating and formation of hot spots within small areas of the thawing FFP. We have used a production model of the WesLabs Microwave Plasma Defroster (model 601) to compare thawing of FFP by microwave energy to thawing by a 37 °C WB. The MWO thawed the FFP in one third of the time required by the WB and yielded thawed plasma that was 5 °C warmer than the FFP thawed by WB. This temperature difference could be of clinical importance. In a massively transfused patient, temperature of the transfused blood products has important effects on core body tem-
TABLE 3. PERCENTAGE RECOVERY OF SERUM PROTEINS AFTER THAWING BY MWO OR WB Percentage Recovery
Total protein g/L Albumin g/L a\ antitrypsin % nL Haptoglobin g/L Transferrin g/L Orosomucoid g/L C4 % nL C3g/L IgG g/L IgAg/L IgM g/L Properdin factor B g/L B-lipoprotein g/L
Mean Prior to Freeze
MWO
WB
Probability* Value
61.2 30.4 97.00 0.92 2.32 0.84 74.60 1.16 8.88 1.77 0.90 0.26 0.66
99.1 115 103 93.5 112 102 94.1 105 98.9 101 90 82.6 120
91.7' 116 101 91.1 112 102 92.8 106 96.6 102 88.7 86.4 121
0.890 0.0001 0.839 0.312 0.387 0.864 0.100 0.032 0.678 0.965 0.552 0.157 0.069
See Table 1 for definitions of abbreviations. ' Probability refers to the differences between pre- and post-thaw values.
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Early in the process of thawing FFP, small areas are thawed, but most of the FFP is still frozen. Because such areas could represent hot spots of increased energy absorption, we performed experiments in which samples were obtained early in the thawing process and compared with samples drawn from the same FFP after thawing had been completed. The initial sample was obtained 3 minutes after thawing by MWO was begun and 5.3 minutes after thawing by 37 °C WB was initiated. The results of these experiments are summarized in Table 4. In contrast to thefindingsafter complete thawing by either MWO or WB, highly significant differences in changes from early to late sampling between MWO and WB thawed samples were found for some of the coagulation proteins. The levels offibrinogen,ATIIIag, ATIIIFu, Prot Cag, Factors VIII, IX, and XI were all significantly higher in the specimens collected early in the MWO thawing process than they were in the specimens collected after thawing had been completed. In contrast, the levels of these coagulation proteins were the same in the specimens sampled early in the process of thawing by 37 °C WB as they were in
the completely thawed specimens. These results showed a statistically significant interaction between the thawing method and the time of sampling for these coagulation proteins (Table 4). Three tests of coagulation function also were measured early in the thawing process (prothrombin time, partial thromboplastin time, and thrombin time). No differences between MWO and WB thawing were found. Because these particular tests are normal with as little as 30 to 40% of clotting proteins, it is not surprising that these screening tests were not affected by higher levels of coagulation proteins in specimens collected early in the MWO thawing.
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TABLE 4. COMPARISON OF COAGULATION TESTS AND LEVELS OF COAGULATION PROTEINS SAMPLED AT TWO TIME POINTS DURING THAWING BY MWO AND WB«t Late
Earlyf
TT (sec) PT (sec) PTT (sec) Fibg/L ATIIIFu % ATIII ag % Prot CFu % Prot C ag % F VIII % FIX% FXI% VWF%
n
MWO Mean ± SE
n
WB Mean ± SE
n
MWO Mean ± SE
n
WB Mean + SE
5 5 5 5 5 4 3 4 5 5 5 5
21.2 ± 0 . 7 11.8 ± 0 . 3 28.3 ± 1.5 4.3 ± 0.3 128.4 ± 4 . 2 132 ± 11 74.3 ± 4.5 151 ± 2 3 140 ± 4 134 ± 16 124 ± 11 116±4
5 5 5 5 5 5 3 5 5 5 5 5
19.5 ± 0 . 3 11.9 + 0.2 27.7 ± 1.3 3.2 ± 0.3 101 ± 6 87.6 ± 5 . 1 84.0 ± 1.5 106 ± 5 105 ± 10 101 ± 13 79.0 ± 9.4 101 ± 9
5 5 5 5 5 4 3 5 5 5 5 5
20.3 ± 0.6 11.7 ± 0 . 2 26.6 ± 0.9 3.2 ± 0.3 104 ± 5 96.5 ± 6.8 76 ± 0 101 ± 5 104 ± 5 105 ± 11 79.4 ± 8.0 114± 10
5 5 5 5 4 4 3 4 5 5 5 5
19.6 ±0.2 11.6+0.1 26.3 ±0.6 3.3 ± 0 . 4 102 ± 1 95.0 ± 4.5 89.3 ± 15.4 111 ± 3 105 ± 5 102 ± 11 87.2 ± 8.2 109 ± 13
Interaction P Value
0.0001 0.006 0.0009 0.0227 0.0022 0.017 0.0001
X Early for MWO = 3 minutes after the start of thawing; for WB it was 5.33 minutes after the start of thawing. See Table 1 for definitions of abbreviations.
perature. Infusing warmer blood products helps to minimize decreases in body temperature. 15 Our study on the pre-/post-thaw comparison of noncoagulation proteins showed no important differences between thawing by MWO or WB (Table 3). However, recovery of both albumin and C'3 was significantly enhanced after thawing. We suspect that this is an artifact of the method of measurement. Nephelometry measures light scattering of antigen-antibody complexes as a measure of protein concentration. Any change in aggregation after thawing by either method could yield an apparent enhancement of protein concentration. Measurement by an independent method will be required to settle this issue. The possible presence of hot spots has been a source of concern for the use of MWO in thawing FFP. Our approach to this issue has been to sample early in the thawing process and compare these samples with others taken at the end of thawing. The rationale was that these areas could be potential hot spots. We found no loss of coagulation proteins in these areas. Some of the coagulation proteins were significantly increased early in the MWO thaw period even though they were indistinguishable from WB-thawed specimens when the thawing was complete. These proteins were fibrinogen, ATIII, protein Cag, and factors VIII, IX, and XI. Further experiments are required to clarify whether these results indicate that these proteins are relatively better preserved in early MWO-thawed specimens than they are in early WBthawed specimens. Possible explanations include variation in sampling or in homogeneity in concentration of certain, but not all, proteins during the freezing process. This latter explanation seems unlikely because the increase in factors was observed only in samples collected early in the MWO
thawing process. Salt concentrations and ionic strength also could be involved because ionic strength is important for assays dependent on fibrin polymerization. Further kinetic experiments are necessary to resolve these issues. Our results confirm observations made by Rock and associates6 using an earlier prototype of this MWO 6 and are very similar to observations recently reported by Sohngen and co-workers.7 Results from measurement of nine coagulation proteins and three screening tests of coagulation function in paired samples thawed by MWO or WB were indistinguishable, except for a 0.35-second prolongation of the thrombin time in MWO-thawed specimens. A prolongation of this magnitude is not biologically significant and could be due to chance because of multiple tests of significance on the same sample. The absence of fibrin-split products in these samples and the similar levels of fibrinogen provides further evidence that there was no measurable degradation of fibrinogen. Some of the coagulation proteins were measured by both functional and antigenic assays (ATIII and Protein C) to determine if a loss of function with retention of antigenic activity could be demonstrated, but no differences between MWO- and WB-thawed samples were found. Measurement of 12 different serum proteins by nephelometry yielded similar results. These findings confirm previous reports.6'7 One could suggest the use of MWO for thawing other plasma products, particularly cryoprecipitate. However, the presence of a very small amount of fluid in a large bag makes it impossible for the temperature sensor to be in contact with enough liquid, with consequent probable overheating of the product. Furthermore, the limiting factor in the time required to issue cryoprecipitate is not the thawing time but rather the time required to pool 10
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* For tests lacking significant overall interactions, MWO-WB difference for TT was significant (P = 00.37) and early-late difference for PTT was significant (0.0099). t Only significant probability values are given.
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It is difficult to estimate savings generated from the prompt availability of FFP. It is clear that ordering behavior in the operating room will be affected dramatically by the availability of a product in 10 minutes rather than in 30. An effect of MWO thawing on reducing FFP waste also can be anticipated. The effect of MWO thawing on FFP use will be the object of further studies.
Acknowledgments. This work was performed in the Blood Bank, Clinical Laboratories, Brigham and Women's Hospital, Boston, Massachusetts. The authors thank Professor James Ware, Department of Biostatistics, Harvard School of Public Health, for expert statistical consultation.
REFERENCES 1. Westphal RG, Tindle B, Howard PL, Golden EA, Page GA. Rapid thawing of fresh frozen plasma. Am J Clin Pathol 1982; 78:220222. 2. Plotz RD, Ciotola RT. Thawing of fresh-frozen plasma at 45° versus 37 °C. Am J Clin Pathol 1988;89:381-384. 3. Sherman LA, Dorner IM. A new rapid method for thawing fresh frozen plasma. Transfusion 1974; 14:595-597. 4. Thompson KS, O'Kell RT. Comparison of fresh-frozen plasmas thawed in a microwave oven and in a 37°C water bath. Am J Clin Pathol 1981;75:851-853. 5. Mead JH, Boucock BP, Russell RE, Robinson CS, Harris G. Water environment microwave thawing. Am J Clin Pathol 1986;85: 510-513. 6. Rock G, Tackaberry ES, Dunn JG, Kashyap S. Rapid controlled thawing of fresh-frozen plasma in a modified microwave oven. Transfusion 1984;24:60-65. 7. Sohngen D, Kretschmer V, Franke K, Pelzer H, Walker WH. Thawing of fresh-frozen plasma with a new microwave oven. Transfusion 1988;28:576-580. 8. Technicon Instruments Corporation. Preliminary: Automated immunoprecipitin method for determining specific proteins (AIP/ PEG method). Technicon Method No. SE4-00375D5, 1985; 111. 9. Rizza> CR, Walker W. One stage prothrombin time techniques, thrombosis and bleeding disorders. In: Bang NU. Medical Laboratory Automation. New York: Academic Press, 1971, pp 92100. 10. Bowie EJW, Thompson JH, Didischeim P, Owen CA. Mayo Clinic Laboratory Manual of Hemostasis. Philadelphia: WB Saunders, Philadelphia, 1971, pp 65-69. 11. Simone JV, Vanderheiden J, Abildgaard CF. A semi-automatic onestage factor VIII assay with a commercially prepared standard. J Lab and Clin Pathol 1967;69:706-712. 12. Allain JP, Cooper HA, Wagner RH, Brinkhous KM. Platelets fixed with paraformaldehyde: A new reagent for assay of vWF and PIT aggregating factor. J Lab Clin Med 1975;85:318-328. 13. Winer BJ. Statistical Principles in Experimental Design. New York: McGraw-Hill, 1971. 14. Office of Medical Applications of Research. National Institutes of Health. Fresh Frozen Plasma: Indications and Risk. JAMA 1985;253:551-553. 15. Dybkjaer E, Elkjaer. P. The use of heated blood in massive blood replacement. Acta Anaesth Scand 1964;8:271-276. 16. Manual for Laboratory Workload Recording Method. College of American Pathologists. Northfield, IL, 1991.
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different bags into 1 single bag. The thawing of cryoprecipitate with WB takes substantially less time than for FFP because of the small amount of fluid condensed into the cryoprecipitate bags. Therefore, microwave thawing is not likely to provide a significant improvement in the time required for issuing cryoprecipitate. It is difficult to quantify, in monetary terms, the benefits of using MWO thawing rather than the traditional WB thawing. In fact, the technologist's time is probably not different for the two procedures. The College of American Pathologists code for FFP thawing with the WB method is 5 minutes (no. 86805).16 A similar time also is spent by the technologist for the thawing of FFP by MWO. The main advantage of the MWO procedure is the shorter turnaround time. This advantage probably will be greater when multiple FFP units are needed. In fact, the 18-minute thaw time for FFP by WB was estimated in ideal conditions, which clearly are different from working conditions. The presence of multiple FFP thawing in the same WB significantly increases the turnaround time for FFP thawing, which can easily be 45 minutes. These problems do not occur in the thawing of FFP by MWO.
