Before measuring single-breath DLCO it is important to: (1) standardize carefully test technique and computation using current ATS guidelines; (2) confirm that computerized systems correctly perform the calculations; and (3) confirm that the reference values selected are consistent with the data produced by the device in the laboratory by comparing measured values with predicted values in several healthy subjects. Interpretation of the test should begin with the following steps: (1) confirmation that the test was properly performed; (2) identification of "noise" factors that may interfere with the interpretation (these may include age, body size, ethnic factors, Hb concentration, COHb, smoking, lung volume, PaO2, exercise, and hormonal effects); and (3) correlation of abnormal findings with clinical data to ensure that they are consistent with the clinical diagnosis.