Diffusing capacity: how to get it right.

Abstract

The carbon monoxide diffusing capacity test (D(LCO)) is a commonly performed pulmonary function test that requires technical expertise and attention to detail to get acceptable results. With the advent of automated devices and powerful computer programs, D(LCO) measurement has rapidly gained wide clinical acceptance. But there are many subtle aspects to performing the test that can diminish its accuracy and repeatability. The clinician must ensure: that the D(LCO) instrument is correctly calibrated; that inhalation is least 90% of the largest previously measured vital capacity; that the patient executes a quick, smooth inhalation within 2 seconds; that the breath-hold is 9-11 seconds; that the breath-hold is without straining (no Valsalva or Müller maneuvers); that exhalation is quick and smooth; that a representative gas sample is obtained from the correct portion of the exhalation; and that at least 5 minutes elapse between D(LCO) tests. At least 2 but no more than 5 D(LCO) tests should be conducted, and testing is complete when 2 tests are within 10% or 3 D(LCO) units (mL CO/min/mm Hg) of each other. The reported D(LCO) value is the average of the first 2 tests that meet the reproducibility criteria, but if 5 tests are performed and no 2 meet the reproducibility criteria, the reported value is the average of the 2 tests with the highest inspiratory volumes. These quality controls will help laboratories achieve consistent high D(LCO) accuracy.