Indian J Pediatr DOI 10.1007/s12098-015-1781-8
CORRESPONDENCE
Comparison of Two New Generation Pulse Oximeters with Arterial Oxygen Saturation in Critically Ill Children: Correspondence Viswas Chhapola 1 & Sandeep Kumar Kanwal 1
Received: 21 December 2014 / Accepted: 16 April 2015 # Dr. K C Chaudhuri Foundation 2015
To the Editor: We read with great interest the article by Jose et al. [1] in December 2014 issue, comparing oxygen saturation (SpO2) between arterial blood gas (ABG) and two new generation pulse oximeters. We critically analysed the article and found few issues worth highlighting. In the objectives the authors state that they checked for the correlation, however it is incorrect because correlation is a poor surrogate for agreement [2]. It was not clear how possibility of bias due to air bubbles was taken care of. Authors preferably should have used at least 1 ml sample for ABG instead of low volume (0.3 ml) because pO2 in low volume samples is sensitive even to small air bubbles [3]. Authors rightly chose Bland-Altman (BA) analysis for agreement assessment. BA analysis recommends reporting of results on 6 criteria [2]. A previous study has reported deficient reporting of BA method in laboratory research literature [4]. The current study too had missing information on at least 4 criteria of BA analysis. 1. For repeatability assessment the authors could have either analysed ABG samples in duplicate or mentioned coefficient of variation of ABG analyser (as per manufacturer claims) so as to provide readers an idea regarding inherent imprecision of ABG analyser. 2. Authors gave estimates of bias, the limits of agreement (LOA) but did not explain the significance i.e., comparison of LOA with permissible a priori clinical error limits.
3. Visual inspection of both BA plots revealed higher scatter (poor agreement) in hypoxic range and narrow scatter (good agreement) at higher SpO 2 range i.e., the heteroscedastic distribution of scatter. Most other laboratory analytes show increasing scatter at higher values of analyte being studied [2]. The converse happens with SpO2 where scatter is more at lower values and is probably explained by sigmoid shape of oxygen dissociation curve. To deal with heteroscedasticity, authors could have constructed either a funnel shaped BA plot or broken the data into three subsets of low, normal and high SpO2 for more meaningful inferences. 4. Also the confidence interval for LOA were not mentioned. As far as sample size is concerned, it was not a limitation as sample size in agreement analysis is related to total number of paired samples and not with number of patients from whom the samples were collected [5]. These issues need consideration because inadequate reporting of BA analysis adversely affects the generalizability of results. Conflict of Interest None. Source of Funding None.
References 1.
* Viswas Chhapola [email protected] 2. 1
Division of PICU, Department of Pediatrics, Kalawati Saran Children’s Hospital and Lady Hardinge Medical College, New Delhi 110001, India
3.
Jose B, Lodha R, Kabra SK. Comparison of two new generation pulse oximeters with arterial oxygen saturation in critically ill children. Indian J Pediatr. 2014;81:1297–301. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8:135–60. Biswas CK, Ramos JM, Agroyannis B, Kerr DNS. Blood gas analysis: effect of air bubbles in syringe and delay in estimation. Br Med J. 1982;284:923–7.
Indian J Pediatr 4.
Chhapola V, Kanwal SK, Brar R. Reporting standards for BlandAltman agreement analysis in laboratory research: a cross-sectional survey of current practice. Ann Clin Biochem. 2014. doi:10.1177/ 0004563214553438.
5.
Stockl D, Cabaleiro DR, Van Uytfanghe K, Thienpont LM. Interpreting method comparison studies by use of the Bland–Altman plot: reflecting the importance of sample size by incorporating confidence limits and predefined error limits in the graphic. Clin Chem. 2004;50:2216–8.
CORRESPONDENCE
Comparison of Two New Generation Pulse Oximeters with Arterial Oxygen Saturation in Critically Ill Children: Correspondence Viswas Chhapola 1 & Sandeep Kumar Kanwal 1
Received: 21 December 2014 / Accepted: 16 April 2015 # Dr. K C Chaudhuri Foundation 2015
To the Editor: We read with great interest the article by Jose et al. [1] in December 2014 issue, comparing oxygen saturation (SpO2) between arterial blood gas (ABG) and two new generation pulse oximeters. We critically analysed the article and found few issues worth highlighting. In the objectives the authors state that they checked for the correlation, however it is incorrect because correlation is a poor surrogate for agreement [2]. It was not clear how possibility of bias due to air bubbles was taken care of. Authors preferably should have used at least 1 ml sample for ABG instead of low volume (0.3 ml) because pO2 in low volume samples is sensitive even to small air bubbles [3]. Authors rightly chose Bland-Altman (BA) analysis for agreement assessment. BA analysis recommends reporting of results on 6 criteria [2]. A previous study has reported deficient reporting of BA method in laboratory research literature [4]. The current study too had missing information on at least 4 criteria of BA analysis. 1. For repeatability assessment the authors could have either analysed ABG samples in duplicate or mentioned coefficient of variation of ABG analyser (as per manufacturer claims) so as to provide readers an idea regarding inherent imprecision of ABG analyser. 2. Authors gave estimates of bias, the limits of agreement (LOA) but did not explain the significance i.e., comparison of LOA with permissible a priori clinical error limits.
3. Visual inspection of both BA plots revealed higher scatter (poor agreement) in hypoxic range and narrow scatter (good agreement) at higher SpO 2 range i.e., the heteroscedastic distribution of scatter. Most other laboratory analytes show increasing scatter at higher values of analyte being studied [2]. The converse happens with SpO2 where scatter is more at lower values and is probably explained by sigmoid shape of oxygen dissociation curve. To deal with heteroscedasticity, authors could have constructed either a funnel shaped BA plot or broken the data into three subsets of low, normal and high SpO2 for more meaningful inferences. 4. Also the confidence interval for LOA were not mentioned. As far as sample size is concerned, it was not a limitation as sample size in agreement analysis is related to total number of paired samples and not with number of patients from whom the samples were collected [5]. These issues need consideration because inadequate reporting of BA analysis adversely affects the generalizability of results. Conflict of Interest None. Source of Funding None.
References 1.
* Viswas Chhapola [email protected] 2. 1
Division of PICU, Department of Pediatrics, Kalawati Saran Children’s Hospital and Lady Hardinge Medical College, New Delhi 110001, India
3.
Jose B, Lodha R, Kabra SK. Comparison of two new generation pulse oximeters with arterial oxygen saturation in critically ill children. Indian J Pediatr. 2014;81:1297–301. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8:135–60. Biswas CK, Ramos JM, Agroyannis B, Kerr DNS. Blood gas analysis: effect of air bubbles in syringe and delay in estimation. Br Med J. 1982;284:923–7.
Indian J Pediatr 4.
Chhapola V, Kanwal SK, Brar R. Reporting standards for BlandAltman agreement analysis in laboratory research: a cross-sectional survey of current practice. Ann Clin Biochem. 2014. doi:10.1177/ 0004563214553438.
5.
Stockl D, Cabaleiro DR, Van Uytfanghe K, Thienpont LM. Interpreting method comparison studies by use of the Bland–Altman plot: reflecting the importance of sample size by incorporating confidence limits and predefined error limits in the graphic. Clin Chem. 2004;50:2216–8.
