Using arm span to derive height: Impact of three estimates of height on interpretation of spirometry

BACKGROUND: When standing height required to calculate forced vital capacity (FVC) cannot be measured, it can be derived from arm span using different methods. OBJECTIVES: To compare three different estimates of height derived from arm span and investigate their impact on interpretation of spirometric data. METHODS: In a cross-sectional study, 517 subjects aged 7 to 76 years, with various respiratory diseases underwent spirometry. Three estimates of height were obtained from arm span: (a) by direct substitution (Ht(AS)); (b) estimated height (Ht(est)), obtained from the mean arm span:standing height ratio; and (c) predicted height (Ht(pred)), obtained from arm span by linear regression analysis. Predicted values of forced vital capacity (FVC) obtained from these estimates were compared with those obtained from actual standing height (Ht(act)), followed by Bland Altman analysis of agreement in the patterns of ventilatory impairment. RESULTS: The arm span was 5%-6% greater than the height. The difference increased with increasing height. Ht(AS) and the FVC predicted from it were significantly greater than the other measures of height and the related predicted FVCs respectively. Compared to Ht(act), Ht(AS) gave a misclassification rate of 23.7% in taller subjects (Ht(act) > 150 cm) and 14.2% in shorter subjects in the patterns of ventilatory impairment. Misclassification rates were 6%-8% with Ht(est) and Ht(pred). Agreement analysis showed that FVCs predicted from Ht(pred) had the best agreement with the FVC predicted from Ht(act). CONCLUSIONS: Among several methods of estimating height from the arm span, prediction by regression is most appropriate as it gives least errors in interpretation of spirometric data