Insufficient evidence for chest CT scan in TB outbreak investigation.

Correspondence result that FRC TLC and RV TLC ratios are disproportionately elevated. We have speculated about the infl uence of erroneously elevated volume ratios on the interpretation of published COPD outcome studies. For example, Fessler et al 2 developed a model in which plethysmographically measured RV TLC ratios predicted improvement in FEV 1 following lung volume reduction surgery (LVRS). Because any plethysmographic error would be proportionately greater for RV, the RV TLC ratio would be artifactually elevated. Among LVRS candidates in our sample, average plethysmographic RV TLC was 64.9%, whereas average helium dilution RV TLC was only 58.4%. When incorporated into the model of Fessler and colleagues, this difference in RV TLC yields a twofold difference in predicted post-LVRS improvement in FEV 1 (30% for plethysmographic vs 15% for He estimates). Also, the data of Casanova et al 3 indicated an approximate 5% annual increase in mortality for each 1% decrement of IC TLC ratio in COPD. Among our subjects, plethysmographic IC TLC averaged 20.7%, whereas helium IC TLC averaged 27.6%. This difference in the estimate of IC TLC ratio yields an approximate 30% difference in predicted annual mortality. We believe these examples demonstrate potential consequences of plethysmographic error for predicting clinical outcomes and indicate the need to take a new look at an accepted measure. With respect to the observed lack of signifi cant association between TLC and degree of airfl ow obstruction in the study of Dykstra et al, 4 we point out two things. First, the authors used a conservative criterion of P , .01 for inclusion of TLC predictors in the statistical model: The actual P value for the association of TLC and FEV 1 in their multivariate model was 0.04. Second, we observed a threshold effect in the relationship between FEV 1 and plethysmographic overestimation of TLC such that the magnitude of apparent error became much greater at FEV 1 values , 30% of predicted. Analyses that treat the relationship between FEV 1 and plethysmographic TLC as a linear continuum may be prone to underestimating its magnitude. Finally, the authors state that “TLC in COPD patients varies as a function of phenotype.” Unfortunately, no citation is provided, and we are unaware of any good evidence for the relationship of TLC to COPD phenotype (emphysema vs chronic bronchitis) that is not potentially biased by the reported limitations in plethysmographic volume measurement.