Intersession variability in single-breath diffusing capacity in diabetics without overt lung disease.

RATIONALE

American Thoracic Society guidelines state that a 10% or greater intersession change in diffusing capacity of the lung (DL(CO)) should be considered clinically significant. However, little is known about the short-term intersession variability in DL(CO) in untrained subjects or how variability is affected by rigorous external quality control.

OBJECTIVES

To characterize the intersession variability of DL(CO) and the effect of different quality control methods in untrained individuals without significant lung disease.

METHODS

Data were pooled from the comparator arms of 14 preregistration trials of inhaled insulin that included nonsmoking diabetic patients without significant lung disease. A total of 699 participants performed repeated DL(CO) measurements using a highly standardized technique. A total of 948 participants performed repeated measurements using routine clinical testing.

MEASUREMENTS AND MAIN RESULTS

The mean intersession absolute change in DL(CO) using the highly standardized method was 1.45 ml/minute/mm Hg (5.64%) compared with 2.49 ml/minute/mm Hg (9.52%) in the routine testing group (P < 0.0001 for both absolute and percent difference). The variability in absolute intersession change in DL(CO) increased with increasing baseline DL(CO) values, whereas the absolute percentage of intersession change was stable across baseline values. Depending on the method, 15.5 to 35.5% of participants had an intersession change of 10% or greater. A 20% or greater threshold would reduce this percentage of patients to 1 to 10%.

CONCLUSIONS

Intersession variability in DL(CO) measurement is dependent on the method of testing used and baseline DL(CO). Using a more liberal threshold to define meaningful intersession change may reduce the misclassification of normal variation as abnormal change.