SEARCH

SEARCH BY CITATION

To the Editor: —I applaud Straus et al.1 for evaluating a combination of clinical findings—history of chronic obstructive pulmonary disease (COPD), wheezes, and forced expiratory time (FET)—in evaluating test measures for COPD and for assessing time intervals for performing bedside maneuvers. However, I take issue with several points in the article. First, the authors downplay the significance of a forced expiratory time >9 seconds because the post-test probability increases only from 10% to 40%. A positive FET does have merit, insofar as spirometric testing, the gold standard, will be ordered. Second, the authors infer that the absence of the combination excludes COPD. A pretest probability of 40% decreases only to 17% with a likelihood ratio (LR) of 0.3. I doubt any clinician would dismiss COPD with a post-test probability of 17%. Third, the authors arrived at the LR +59 for the combination by multiplying LRs for each. They have included “crude” LRs without corresponding sensitivities and specificities for history of COPD or FET. The sensitivities, specificities, and likelihood ratios could be calculated from a 2 × 2 table using spirometry positive (disease) and spirometry negative (no disease) with index test positive (combination of findings) and index test negative (2/3 positive, 1/3 positive, or 0/3 positive) categories. Finally, I find it odd that the sensitivity for wheezing is 35% and the “estimated” sensitivity for the combination is 70% based on the LR −0.30. In order to obtain an LR +59, the false-positive rate must be slightly higher than 1%. As a result, LR− is represented by 30%/99% or 0.30. Intuitively, combinations of findings are much more likely to have lower sensitivities than single findings, whereas the specificity would be expected to be higher with combinations than with one finding.2David A. Nardone, MD,Hillsboro, Ore, VHA Medical Center, Professor Emeritus, Oregon Health Sciences University.

REFERENCES

  1. Top of page
  2. REFERENCES