Per cent of predicted is misleading
The percentage of publications that do not use ‘per cent predicted’ for spirometric indices, is negligible. This is because this usage is copied unthinkingly; in spite of its widespread adoption per cent predicted is not scientifically founded (see ref. 1). In fact, ‘Nowhere else in medicine is such a naive view taken of the limit of normal’ (Sobol). Similarly it is inappropriate to express a change in spirometric indices, for example due to an intervention such as bronchodilatation, as a percentage of the initial value because this introduces bias due to regression to the mean. The indiscriminate use of 80% of predicted as the ‘lower limit of normal’ (LLN) cannot be justified.
When would ‘% of predicted value’ be correct? Simply, when the scatter is proportional to the predicted value. In other words, if in a reference group a low predicted value is associated with a small scatter, and a high predicted value associated with a proportionally larger scatter. The figure illustrates the relationship between scatter in FEV1 and age in asymptomatic lifelong non-smoking males. The figure also shows the regression line (solid line) and the lower limit of normal (5th centile) for predicted values in these individuals. There is no sign whatsoever that the scatter is small in persons with a low FEV1, and proportionately higher in subjects with a high FEV1.
Ref. 1 - Do not use per cent predicted
- Sobol BJ. Assessment of ventilatory abnormality in the asymptomatic subject: an exercise in futility. Thorax 1966; 21: 445-449.
- Sobol BJ, Sobol PG. Percent of predicted as the limit of normal in pulmonary function testing: a statistically valid approach. Thorax 1979; 34: 1-3.
- Miller A. Prediction equations and ‘normal values’. In: Miller A, ed. Pulmonary function tests in clinical and occupational lung disease. New York, Grune & Stratton, 1986; 197-213.
- Miller MR, Pincock AC. Predicted values: how should we use them? Thorax 1988; 43: 265-267.
- Quanjer PhH. Predicted values: how should we use them (letter). Thorax 1988; 43: 663-664.
- ATS Statement. Lung function testing: selection of reference values and interpretative strategies. Am Rev Respir Dis 1991; 144: 1202-1218.
- Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629.
- Stanojevic S, Wade A, Stocks J, et al. Reference ranges for spirometry across all ages. A new approach. Am J Respir Crit Care Med 2008; 177: 253–260.
- Miller MR, Quanjer PH, Swanney MP, Ruppel G, Enright PL. Interpreting lung function data using 80% predicted and fixed thresholds misclassifies more than 20% of patients. Chest 2011; 139: 52-59.
- Quanjer PH, Stanojevic S, Cole TJ et al. and the ERS Global Lung Function Initiative. Multi-ethnic reference values for spirometry for the 3-95 years age range: the Global Lung Function 2012 equations. Eur Respir J 2012; 40: 1324–1343.