a b trace. The value of FVC will be significantly reduced, but the ratio of FEV1/FVC should still be below 70%. It is not easy to differentiate this from more severe obstruction with loss of FVC without more detailed lung function tests.

The choice of severity values in COPD guidelines

Over the last few years, national and international clinical guidelines have been produced for the management of COPD. Each set of guidelines has chosen values of FEV1 percent predicted to categorize mild, moderate and severe levels of COPD.

Values of FEV1 greater than 80% are within two standard residuals of the predicted mean and are thus considered to be within the normal range. The American thoracic Society (ATS), in assessing levels of respiratory disability [16], choose 60% and 40% FEV1 as indicators for mild, moderate and severe disability. The same levels were used for the British Thoracic Society (BTS) guidelines [17].

The ATS COPD guidelines [18] decided on three levels of severity based on 80%, 50% and 35% predicted. The European Respiratory Society guidelines [19] opted for 90%, 70% and 50% predicted. There has been no scientific evidence quoted for justifying these values (Table 4.2).

How does FEV1 correlate with various clinical parameters?

FEV1 is the best predictor of progress and mortality in COPD. It also corre-

Table 4.2 Chronic obstructive pulmonary disease guidelines — severity levels of forced expiratory volume in 1 s (FEVj) % of predicted.

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