Assessment of severity and prognosis in COPD: moving beyond percent of predicted

Abstract

@ERSpublicationsNewer methods of grading severity of airflow limitation perform better than the percent predicted of the FEV1 and deserve consideration in both prognostic models and individual patient assessment http://ow.ly/oQB030kvVEWCite this article as: Culver BH. Assessment of severity and prognosis in COPD: moving beyond percent of predicted. Eur Respir J 2018; 52: 1801005 [https://doi.org/10.1183/13993003.01005-2018.There is broad consensus that airflow limitation, the primary physiological abnormality of chronic obstructive pulmonary disease (COPD), is best defined by a significant reduction in the ratio of forced expiratory volume in 1 s (FEV1) to forced vital capacity (FVC) or to the slow vital capacity. There is also abundant evidence that this reduction is most accurately identified as an individual value less than the lower limit of the normal range (LLN) specific to that individual, as determined from an appropriate healthy, non-smoking, reference population [1]. Because the FEV1/FVC ratio declines normally with age, using a non-individualised cut-off, such as 0.70, has been shown to cause an unacceptable level of misclassification with age and sex bias; this leads to over-diagnosis of 30% or more of older men and under-diagnosis of younger women [2][3][4][5]. There is much less consensus, however, on methods to indicate disease severity or to assess the likelihood of future outcomes. The most commonly used index is the percent of the predicted value of FEV1, with various cut-off points proposed for categories of severity, and this was endorsed in the 2005 American Thoracic Society (ATS)/European Respiratory Society (ERS) pulmonary function documents [1]. The use of standardised residuals (z-scores) to establish the normal range was recommended by the ERS in 1993 [6] and, more recently, they have also been evaluated as an index of severity [7]. Their use in pulmonary function reporting, particularly as part of a visual scale, has been endorsed in a current ATS technical statement [8]. Both percent of predicted and z-score depend upon the predicted value of FEV1, but predicted values have inherent uncertainty and may not accurately reflect some individuals. Other indices, based upon the absolute value of FEV1, and thus not dependent upon a reference value, have been proposed but are not yet in wide use. Recently in the European Respiratory Journal, HUANG et al. [9] reported an evaluation of seven methods to categorise reductions in FEV1, comparing their correlations to the outcomes of acute exacerbations and mortality. While this study in a Taiwanese population with confirmed COPD was relatively small (n=296) and predominantly male (94%), the results draw attention to some of the less commonly used indices and may cause us to rethink our dependence on percent of predicted.For each of the methods studied, the population was divided into four stages of severity and the reliability of the index was assessed by the correlation of these strata to increasingly worse outcomes. This analysis tests both the ...

Relative and absolute lung function change in a general population aged 60–102 years

“…One interpretation of this is that the longitudinal overall reductions in FEV 1 Q by age can be broadly applied to both sexes [32]. This may be useful since FEV 1 Q has been suggested as a staging tool for COPD, and forecasting future levels may be of clinical use [38].…”

Cut to the Chase

“…1 Rather than using somewhat arbitrary fixed cut-points to define normal, a more statistically based approach that uses the distribution of normal values about the mean predicted value would seem more appropriate. 3,4 With this approach, an abnormal value is defined as below the fifth percentile of the predicted value (lower limit of normal [LLN]), recognizing that 5% of the normal population will be considered "abnormal" and interpreting values near the LLN will continue to be problematic. Importantly, the LLN changes with age (higher in younger subjects, lower in older subjects), whereas a single cut-point does not.…”

“…Numerous studies have reported better correlation with disease based on this approach, and the consensus among most professional pulmonary societies is that an LLN cutoff for defining normal vs abnormal results is the appropriate approach to take. [3][4][5] Quantifying the severity of an impairment takes this discussion to another level. Arbitrary cut-points have been recommended by multiple groups using percent predicted values to define the impairments as "mild," "moderate," and "severe" (or more extensively as "moderately severe" or "very severe").…”

“…Importantly, degrees of abnormality can also be expressed by the deviation from the mean using z scores (multiples of the SD from the mean of the normal distribution). [3][4][5]9 One approach has been proposed by Quanjer et al, 4 in which mild is a z score of -2 or below, moderate is a z score of -2.5 to -2, moderately severe is a z score of -3 to -2.5, severe is a z score of -4 to -3, and very severe is a z score below -4. The z scores correlate with percent predicted values, but increasing discordance occurs as subjects age.…”