Comments on “Increase in the circulating levels of malondialdehyde in patients with obstructive sleep apnea: a systematic review and meta-analysis”

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I read with interest a systematic review and meta-analysis of Fadaei et al. [1] entitled “Increase in the circulating levels of malondialdehyde in patients with obstructive sleep apnea: a systematic review and meta-analysis”. The authors tried to pool available data to show the association between obstructive sleep apnea (OSA) and malondialdehyde (MDA) levels by meta-analysis of 14 published reports. MDA levels in the control groups varied in the range of 0.004 to 14.1 µmol/L (µM), and those of OSA groups were 0.0052 to 20.1 µM. Extreme heterogenecity was observed which could have originated from a number of possible sources: type of employed analytical method for determination of MDA (colorimetric, UV, spectroflourimetric and/or chromatographic methods), age of the sample donors in both control and OSA groups, their gender, and storage time of the samples. [2]. The large values of relative standard deviations in both control (varying between 17.4 to 312.8%) and OSA (varying between 0.5 to 62.2%) groups reveal that there are serious confounding factors affecting MDA levels in both groups. In addition, there are some points dealing with collecting MDA data reported in Table 2 of the review paper [1]; as examples, (1) MDA levels for control group (CG) and patient group (PG) were reported as 1.6 ± 0.4 and 2.6 ± 0.7 mM, whereas the reported data in the original paper (Ref #20 of [1]) were as 1.6 (1.5–1.8) and 2.6 (1.9–3.7) μM. Beside misquoted concentration unit of mM instead of μM, there is no straight and accurate procedure to calculate the standard deviations (as reported by Fadaei et al. [1]) from the range of MDA values. This is also the case for MDA data of Ref #26. A common way to estimate the standard deviation in these cases is dividing the range by 6, but this should be noted in the meta-analysis report. (2) The normal MDA values reported for the analytical method was 4.6–9.4 μM [3]; Asker et al. (Ref #28 of [1]) reported MDA levels for control group as 0.2 ± 0.1 Hmol/L! Interestingly, Hmol/L was a typographical error in the original paper of Asker et al., and it has been exactly repeated in Table 2 of the meta-analysis [1]. (3) Units of MDA were reported as nmol/L, mM or mmol/L, and μM and ng/mL. Using different concentration units poses difficulties in direct comparison of the data among various studies. We do not know if the values with different units were used in pooling the data or not? To facilitate the comparison, we convert all these data to μM and list in Table 1. (4) There were also typographical errors with MDA units of Refs. #20, #23, and #26. (5) There were more subgroups in the Yardim-Akaydin et al. study [18]; however, Fadaei et al. [1] discussed them in only three subgroups. …