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To the Editor:
I read with interest the article by Mowafy and Abd Ellatif and would like to bring a few points for due consideration [1].
- 1.
Sample size should be based on the primary aim. Instead of using a predefined sensitivity/specificity or area under the curve (AUC) of receiver operating characteristic (ROC) curve, the authors chose to find an undisclosed difference of mean of cerebral artery velocity [1, 2]. Moreover, the cited study does not report the values used by authors [1]. Thus, it is unclear whether their study was adequately powered to examine their primary aim.
- 2.
The standard deviation (SD) of mean velocity and pulsatility index is very narrow compared to the difference between the means of the groups [1]. Previous study suggests some overlap of the dispersion of those two parameters between the groups [1, 3]. This peculiar dispersion may have led to a `significant` p value, very narrow AUC of ROC and a near ideal sensitivity and specificity.
- 3.
The reported 95% Confidence Interval (CI) of AUC of ROC of PI is 1–1 [1]. It is extremely unlikely that the observed sample parameter will depict the population parameter with such outstanding confidence.
- 4.
Sensitivity and specificity values observed in a sample are estimates for the corresponding population parameters. Thus, the authors are expected to report their CI.
References
Mowafy SMS, Abd Ellatif SE. Transcranial Doppler role in prediction of post-dural puncture headache in parturients undergoing elective cesarean section: prospective observational study. J Anesth. 2019;33:426–34.
Hajian-Tilaki K. Sample size estimation in diagnostic test studies of biomedical informatics. J Biomed Inform. 2014;48:193–204.
Serra‐Serra V, Kyle PM, Chandran R, Redman CW. Maternal middle cerebral artery velocimetry in normal pregnancy and postpartum. BJOG. 1997;104:904–9.
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Saikia, P. Predictive performance of transcranial Doppler for PDPH. J Anesth 34, 158 (2020). https://doi.org/10.1007/s00540-019-02677-7
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DOI: https://doi.org/10.1007/s00540-019-02677-7