Diffusing Capacity: How to Get It Right

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.