Predicting mortality and hospital admission in patients with COPD: significance of NT pro-BNP, clinical and echocardiographic assessment.

AIMS: To quantify the ability of N-terminal pro-brain natriuretic peptide (NT pro-BNP) to predict mortality and hospitalization in patients with chronic obstructive airways disease (COPD). METHODS: Prospective single-centre observational study of 140 consecutive patients aged at least 18 years with COPD between 27 March 2004 and 28 February 2008 (median follow-up 3.9 years). RESULTS: Sixty-five (46%) men, 26 (19%) O2 therapy, 115 (82%) smokers, 38 (27%) patients receiving diuretics, 15 (11%) left-ventricular ejection fraction less than 45%. Median [interquartile range (IQR)] NT pro-BNP concentration 16.2 (25.4) pmol/l. NT pro-BNP was higher in those with a dilated left atrium (P<0.001), aortic stenosis (P=0.02), left-ventricular systolic dysfunction (P=0.027), right ventricular impairment (P=0.011), atrial fibrillation (P<0.001), patients receiving diuretics (P=0.010) and angiotensin-converting enzyme (ACE) inhibitors (P=0.006). One-year mortality and hospitalization rates were 2.9 and 25.4%. The median (IQR) time to hospitalization and length of first hospital stay: 383.5 (605) and 4.0 (7.0) days. NT pro-BNP was an excellent discriminator of right-ventricular impairment (C statistic=0.90) and predicted survival (highest quartile versus lowest quartile relative risk=3.02, P=0.001), but not hospital admission. After adjustment this association was not significant. CONCLUSION: NT pro-BNP predicts survival, but not hospital admission in patients with COPD. The ability of NT pro-BNP to independently predict death or hospitalization is superseded by the presence of a dilated left atrium, aortic stenosis and left-ventricular systolic dysfunction.

Disproportionate sampling for population subgroups in telephone surveys.

Population studies often seek to examine phenomena in important population subgroups or to compare results among these and other subgroups. When subgroups of interest comprise a relatively small percentage of the population and acceptable subgroup member lists are not available to serve as sampling frames, it may be prohibitively expensive even by telephone to screen through a sample of the entire population. This paper considers some statistical effects of estimation from a class of two-stratum telephone sample designs where part of the frame with a higher subgroup concentration is disproportionately sampled compared to the rest of the frame. Using proportionate sampling as a reference, the relative impact of this disproportionate design is determined for nominal and effective sample sizes, where the latter are tied to the effect of variation in sample weights that occurs in disproportionately allocated samples. Findings are illustrated using two recent telephone surveys. Whereas nominal subgroup sample sizes may be improved by disproportionate sampling, we conclude that both the survey designer and analyst should use this type of design cautiously in telephone surveys.