Per cent 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), unless in children and adolescents. 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. The indiscriminate use of 80% of predicted as the ‘lower limit of normal’ cannot be justified at all. |
When is ‘% predicted
value’ 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 left upper diagram illustrates the relationship
between residual scatter in FEV1 and standing height in asymptomatic lifelong non-smoking males.
The figure also shows the regression line and the 95% confidence
interval for predicted values in individuals. There is no
sign whatsoever that the scatter is small in short persons
(with a low FEV1),
and high in tall subjects (high FEV1);
the same lack of trend is observed if the residual scatter
in FEV1 is plotted
as a function of age. This corroborates what we saw earlier in females, in whom the scatter was similarly independent of age (see ref. 2).
See also:
Bias due to percent predicted in adults
Scatter independent of level
| Ref. 1 - Do not use per cent predicted in adults | |
| 1 | Sobol BJ. Assessment of ventilatory abnormality in the asymptomatic subject: an exercise in futility. Thorax 1966; 21: 445-449. |
| 2 | Oldham PD. Percent of predicted as the limit of normal in pulmonary function testing: a statistically valid approach. Thorax 1979; 34: 569. |
| 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. |
| 4 | Miller MR, Pincock AC. Predicted values: how should we use them? Thorax 1988; 43: 265-267. |
| 5 | Quanjer PhH. Predicted values: how should we use them (letter). Thorax 1988; 43: 663-664. |
| 6 | ATS Statement. Lung function testing: selection of reference values and interpretative strategies. Am Rev Respir Dis 1991; 144: 1202-1218. |
| 7 | 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. |
| Ref. 2 - Constant scatter about predicted in adults | |
| 1 | Drouet D, Kauffmann F, Brille D, Lellouch J. Valeurs spirographiques de référence. Modèles mathématiques et utilisation pratique. Bull Europ Physiopath Resp 1980; 16: 745-767. |
| 2 | Glindmeyer HW, Lefante JJ, McColloster C, Jones RN, Weill H. Blue-collar normative spirometric values for Causasian and African-American men and women aged 18 to 65. Am J Respir Crit Care Med 1995; 151: 412-423. |