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confusion by using the phrase "breath alcohol corresponding to a blood alcohol concentration of ..." or by writing "blood alcohol" instead of "breath alcohol". Department of Anaesthesiology, Huddinge University Hospital, S-141 86 Huddinge, Sweden

ROBERT HAHN

latex test for diagnosis of invasive aspergillosis

Aspergillus antigen

SiR,—Dr Warnock and his colleagues have drawn attention (Oct 19, p 1023) to defects in the performance of the ’Pastorex Aspergillus’ latex agglultination test introduced for detection of

antigenaemia when invasive aspergillosis is suspected. Our findings are similar to theirs. In one series latex tests were positive in 32 out of 277 sera from 74 neutropenic patients on chemotherapy for haematological malignancy. On retesting with a different batch, only 18 samples remained positive. All were from patients on prophylactic amphotericin B. Invasive aspergillosis was considered possible on clinical grounds in only 1 of the 13 patients involved. The loss of reactivity on retesting noted by Warnock et al has proved to be both common and troublesome, rendering interpretation impossible. Another difficulty is dual reactivity of sera to tests marketed for aspergillosis and candidosis. Sera giving positive tests for aspergillosis may also be positive for candidosis. In our series, coexisting infection could not always be ruled out, but the frequency of dual reactions strongly suggested a cross-reaction. Indeed, the manufacturers state that sera containing high levels (microgram range per ml) of aspergillus galactomannan will also with the candida latex reagent. we have observed positive reactions in the ’Pastorex Candida’ test with sera from a patient with myeloma who had no evidence of systemic candidosis. False-positive reactions with sera containing high levels of paraprotein might readily occur in the latex-based system, and inclusion of a control latex in the new test kit should perhaps have been considered. Rapid and reliable tests for invasive infections caused by aspergillus and candida are needed, and the marketing of kits designed to do this was very welcome. Warnock’s group and ours conclude that performance of the tests has so far been disappointing. Aspergillus galactomannan is an important target molecule for antigenaemia tests, and latex procedures, because of their rapidity and simplicity, have considerable attractions to the routine screening of patients at risk. It is therefore greatly to be hoped that the manufacturers will develop these tests further so that they fulfill their earlier promise.

react

Moreover,

Mycological Reference Laboratory, Central Public Health Laboratory, London NW9 5HT, UK

FRANCES KNIGHT D. W. R. MACKENZIE

*** This letter has been shown to the manufacturers, whose reply to it and to an earlier one follows.-ED. L. SIR,-Dr Warnock and colleagues and Mrs Knight and Professor Mackenzie report defects in the performance of ’Pastorex’ (Diagnostics Pasteur) latex agglutination tests for the detection of circulating antigens in invasive aspergillosis and candidosis. Both groups of investigators observed that positive samples had become negative on later retesting, and Knight and Mackenzie found sera reacting in both tests. When a positive sample is negative on retesting it is either a true positive turned false negative or false positive turned true negative. In the first paper on the aspergillus test Dupont et aP reported that samples from patients with invasive aspergillosis may turn negative

during storage in a freezer, and when we re-tested 15 positive plasma samples from guineapigs with experimental invasive aspergillosis after 13 months of conservation at - 30°C, 6 had unchanged titres,1 increased by 1 dilution, but 8 dropped by 1, 2, or 3 dilution steps. Given the usually low amounts of circulating galactomannan in patients, this instability during storage may account for true-positive samples turning negative. It is difficult to estimate how far this might explain the observations of your correspondents since the clinical information provided does not tell us how many patients definitely had an invasive aspergillus infection.

Both letters do contain evidence that many of the positive reactions were false positives. Pastorex latex agglutination tests have very low detection limits and a drawback of this high sensitivity is increased susceptibility to interference. Good results can only be obtained under good test conditions. Warnock et al did not follow the manufacturer’s instructions. Volumes of samples and latex reagent were halved to 20 pl and 5 ul, respectively. During development of this test special attention was paid to volumes, and they cannot be changed without altering the characteristics of the test itself. Halving the volumes makes it more difficult to homogenise samples and latex suspension and to read the result, and it increases the risk of false-positive reactions as a result of desiccation. By reducing test volumes Warnock et al could do more tests with every kit. They either recycled the disposable agglutination cards and the mixing sticks or used material from other sources; both practices risk introducing errors (eg, mixing with wooden toothpicks may cause false positives). We know that to save money many UK users of these kits halve test volumes. We discourage this very strongly. False-positive agglutination may also occur when latex reagents are not equilibrated to room temperature. We also recommend the use of microcentrifuge tubes that remain well closed during the boiling step of serum treatment; we have observed false positives when water from the water bath enters opened tubes. Finally, the serum should be of good quality. Bacterial contamination causes false-positive reactions.! Serum must be separated from the clot within 24 h of sampling. Transport should be avoided if possible; this can interfere with the reliability of the tests done in reference

laboratories. The dual reactivity described by Knight and Mackenzie has three possible explanations, two of which they mention-namely, double infection (possible but rare) and cross-reactivity. The candida test kit does react with aspergillus galactomannan but only in the Ilgjml range, levels rarely seen in patients. The third explanation is false agglutination, and whenever dual reactivity is frequently observed all the above-mentioned potential causes of false reactions should be

explored. Pastorex candida and aspergillus agglutination tests do require precautions than most latex tests, this being an unfortunate consequence of the very low detection limits that have to be achieved. We now realise that potential interferences deserve more emphasis in the package insert. Some are already mentioned and those that are not will soon be included. We shall also recommend that positive results be re-tested immediately for confirmation. Despite the problems raised by Warnock et al and by Knight and Mackenzie, we believe that both tests are satisfactory if the manufacturer’s protocol is followed and if the pitfalls are avoided. A multicentre study underway in France proves that under those conditions false-positive results are negligible or even non-existent.

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Sanofi Diagnostics Pasteur, B-3600 Genk, Belgium

D. STYNEN L. MEULEMANS

Sanofi

Diagnostics Pasteur, Marnes-la-Coquette, France

M. L. GARRIGUES

1. Dupont B, Improvisi L, Provost F. Detection de galactomannane dans les aspergilloses invasives humaines et animales avec un test au latex. Bull Soc Fr Mycol Méd 1990; 19: 35-42.

Rise in intracranial pressure with intravenous adenosine SIR,-Adenosine, a purine nucleoside has recently received a product licence (Sanofi) for treatment of paroxysmal supraventricular tachycardia (PSVT). Recognised side-effects include facial flushing, nausea, chest tightness, dizziness, and lightheadedness, this last perhaps as a consequence of either arterial hypotension or cerebral vasodilatation, as reported by Sollevi.1 We report a rise in intracranial pressure after intravenous adenosine. A 58-year-old man was admitted with head injuries after a fall. A subdural haematoma was evacuated, and intracranial pressure monitoring was started perioperatively. Initial intracranial pressure (ICP) measurements ranged from 15-20 cm cerebrospinal fluid (CSF) for 4 days, at which time SVT developed, with associated

Aspergillus antigen latex test for diagnosis of invasive aspergillosis.

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