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Spirometry: use of percent of predicted is inappropriate

Lower limit of normal for FEV1 and FVC as percent predictedFig. 1 - The lower limit of normal (LLN) for FEV1 and FVC expressed as a percentage of the GLI-2012 predicted values in the 3-95 year age range.

When interpretating spirometric data, it is an ingrained habit in respiratory medicine to express measured values as percent of predicted. This tradition probably arose from a recommendation by Bates and Christie [1]: “a useful general rule is that a deviation of 20% from the predicted normal value probably is significant”. This leads to considering 80% of predicted as the “lower limit of normal” (LLN). This rule of thumb was uncritically adopted. The rule is only valid if the scatter around the predicted value is proportional to that value; hence, large if the predicted value is large, and proportionally smaller if the predicted value is small. As shown in figure 1 there is no proportionality, so that the use of percent of predicted will inevitably lead to erroneous interpretation of test results, as has been explained in scores of publications [2-9]. In fact, Sobol wrote [5]: “Nowhere else in medicine is such a naïve view taken of the limit of normal”. As the GLI group had tens of thousands of records available, this provided an opportunity to estimate the lower limit of normal more accurately (see later). Expressing the loer limit of normal as %predicted leads to the picture in figure 6: over a large age range the lower limit of normal is well below the 80% predicted line.

Percentage of healthy males and females with FEV1 and FVC < 80% predictedFig. 2 - Percentage of healthy males and females in whom the measured FEV1 or FVC is <80% predicted.

We can subsequently assess in what percentage of a healthy, non-smoking population (25,827 males, 31,568 females) the measured FEV1 and FVC are below the 80% predicted mark (fig. 2). It will be clear that the large proportion of erroneous assessments of test results, in particular in those aged over 50 years, should lead to abandoning the use of %predicted.

References

  1. Bates DV, Christie RV. (1964). Respiratory Function in Disease, p. 91. Saunders, Philadelphia and London.
  2. Quanjer PH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault J-C. Lung volume and forced ventilatory flows. Report Working Party Standardization of Lung Function Tests, European Community for Steel and Coal. Official Statement of the European Respiratory Society. Eur Respir J 1993; 6: Suppl. 16, 5–40. Erratum Eur Respir J 1995; 8: 1629.
  3. 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. Manuscript
  4. Sobol BJ. Assessment of ventilatory abnormality in the asymptomatic subject: an exercise in futility. Thorax 1966; 2: 445-449. Manuscript
  5. Sobol BJ, Sobol PG. Editorial. Percent of predicted as the limit of normal in pulmonary function testing: a statistically valid approach. Thorax 1979; 34: 1-3. Manuscript
  6. Miller MR, Pincock AC. Predicted values: how should we use them? Thorax 1988; 43: 265-267. Manuscript
  7. ATS Statement. Lung function testing: selection of reference values and interpretative strategies. Am Rev Resp Dis 1991; 144: 1202-1218. Manuscript
  8. 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. PubMed
  9. 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. PubMed

Acknowledgment: Illustrations modified and reproduced with permission of the European Respiratory Society. Eur Respir J December 2012 40:1324-1343; published ahead of print June 27, 2012, doi:10.1183/09031936.00080312.

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