Interpretation of Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) Values

Definition of Normality

The European Respiratory Society defines normal DLCO values as those above the lower limit of normality (LIN), established at a z-score of -1.64 or above the 5th percentile of the reference population, using appropriate reference equations for the patient's age, sex, height, and ethnicity 1.
The American Thoracic Society recommends reporting DLCO values as absolute values (mL/min/mmHg), z-scores, and percentage of predicted, using the GLI 2017 reference equations when available 2.
Values below the 5th percentile of the frequency distribution measured in the reference population are considered below the normal expected range 1.

The European Respiratory Society suggests that predicted values should be obtained from studies in "normal" or "healthy" subjects with the same anthropometric (sex, age, height) and ethnic characteristics as the patient being evaluated 1.
The concentration of hemoglobin should be measured and reported, as anemia artificially reduces DLCO while polycythemia increases it, requiring adjustment of predicted values 2.
Reference values for lung volumes in people of black ethnicity are, on average, 12% lower than in Caucasians, and this difference should be considered when selecting reference equations 1.

The selection of reference values for DLCO is more problematic than for spirometry because inter-laboratory differences are much greater for DLCO 4, 5.
Predicted values for alveolar volume (VA), inspiratory volume (VI), DLCO, and KCO should be derived from the same reference source to maintain internal consistency 4.
Values should be adjusted for hemoglobin concentration, carboxyhemoglobin, and altitude when applicable 4, 2.

In Europe, the combined ERS equations from 1993 are frequently used for adults aged 18-70 years, although a single set of equations for all of Europe cannot currently be recommended 1, 4.
Laboratory directors should select reference values that match the numbers produced in their own laboratories, ideally by measuring DLCO in a sample of healthy subjects and comparing with several reference equations 4.

The European Respiratory Society recommends never interpreting DLCO in isolation: it should always be considered in the context of spirometry, lung volumes, and clinical presentation 2.
Height and weight should be measured for each patient at the time of testing, rather than using self-reported values 1.
Extrapolation beyond the size and age of the subjects investigated in the selected reference equations should be avoided 1.