THE VALUE OF FORCED EXPIRATORY VOLUME IN IS IN SCREENING SUBJECTS WITH STABLE COFD FOR PAO2 LESS-THAN-7-CENTER-DOT-3 KPA QUALIFYING FOR LONG-TERM OXYGEN-THERAPY

Guidelines on the management of chronic obstructive pulmonary disease (COPD) issued by the European Respiratory Society (ERS), British Thora cic Society (BTS), American Thoracic Society (ATS), and Department of Health for England and Wales (DoH) suggest differing values of forced expiratory volume in 1 s (FEV1) below which arterial blood gas analysi s should be performed to determine the presence of severe hypoxaemia a nd possible long-term oxygen therapy (LTOT) requirement. This study ai med to determine the value of FEV1 at these different levels in screen ing for LTOT requirement defined as PaO2 <7.3 kPa in subjects with sta ble COPD. Comparative measures were taken against other lung function tests of volume and diffusing capacity. A retrospective analysis of pa ired lung function and arterial oxygen measurements in 491 subjects wa s made. The positive and negative predictive values, sensitivity and s pecificity of FEV1 <70% predicted (ERS), FEV1 <50% predicted (ATS), FE V1 <40% predicted (BTS) and FEV1 <1.5 l (DoH) were determined for fulf illing LTOT criteria (PaO2 <7.3 kPa). The correlation between lung fun ction variables and PaO2 was established. Logistic regression analysis was used to classify subjects with PaO2 <7.3 kPa and PaO2 greater tha n or equal to 7.3 kPa. Using FEV1 to screen for LTOT requirement produ ced a high negative predictive value at all four suggested limits (FEV 1 <70% 100%, FEV1 <50% 96%. FEV1 <40% 95%, FEV1 <1.5 l 97%). However, the positive predictive values were low (FEV1 <70% 13%, FEV1 <50% 16%, FEV1 <40% 19%, FEV1 <1.5 l 15%) as were sensitivities. No single lung function variable was a strong determinant of PaO2. FEV1 % pred (r=0. 40), FVC % pred (r=0.34) and TLCO % pred (r=0.27) had the strongest re lationships. Logistic regression also placed FEV1 % pred and TLCO % pr ed as the best predictors of PaO2 <7.3 kPa. We conclude no lung functi on variable correlates well with PaO2 in subjects with stable COPD. Th e best predictor of PaO2 <7.3 kPa was FEV1 % pred. Whilst a low FEV1 i s a poor predictor of LTOT requirement in an individual, PaO2 <7.3 kPa is only found in subjects with a low FEV1. A high FEV1 may be used to exclude subjects from further investigation for LTOT and prevent unne cessary arterial sampling.