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Dear Editor, I read with great interest the original study by Al Lawati et al. published in the November 2019 issue of SQUMJ.1 Among a cohort of Omani obstructive sleep apnoea syndrome patients, the authors nicely compared spirometry parameters between obese and non-obese patients when patients changed position from sitting to supine.1 They found a significant difference in the forced expiratory volume in 1 second/forced vital capacity (FVC) in obese and non-obese patients when changing position (P = 0.03). Moreover, there was a significant gender difference in FVC percentages (P <0.05) whereby male patients had a decreased FVC compared to females. Considering the remaining spirometry elements, there was no significant difference with patients’ change of position.1 I assume that the study’s results must be taken cautiously as, in addition to the limitations addressed by the authors, there is an additional relevant methodological limitation. The evaluation of lung function in a given population is usually done by examination of the absolute values of various spirometry components, comparing them with the predicted values and ultimately, checking the shape of the resultant curves. This requires the employment of populationspecific reference values.2 Normal pulmonary function is controlled by genetic, physiological, environmental, psychological, nutritional, socioeconomic and racial determinants.2 Consequently, many population-specific reference values of spirometry have been formulated to be used in clinical settings and research institutions.3,4 Interestingly, Omani population-specific prediction equations for reference values of spirometry have already been established.5 However, the authors did not clearly state which reference values they used in their study methodology to analyse the readings of various spirometry parameters. Consequently, this may bring into question the results of the study.

Re: The Effect of Change in Posture on Spirometry in Patients with Obstructive Sleep Apnoea Syndrome