Assessment of Diffusing Capacity for Gas Exchange Impairment

Primary Testing Method

The American Thoracic Society recommends that the single-breath diffusing capacity of the lung for carbon monoxide (DLCO) be measured alongside spirometry and lung volumes, with results reported as absolute values, z-scores, and percent predicted using current reference equations (GLI 2017), adjusted for hemoglobin when measured 1, 2
The test must include measurement of alveolar volume (VA) during the maneuver, with results corrected to standard barometric pressure (particularly important at altitude) 1, 3
Hemoglobin concentration must be measured and reported, with predicted DLCO values adjusted accordingly, as anemia artificially lowers DLCO while polycythemia increases it 1, 2

The American Thoracic Society recommends reporting DLCO in absolute values (mL/min/mmHg), z-scores, and percent predicted using GLI 2017 reference equations, with the lower limit of normal (LLN) at z-score of -1.64 1, 2
The carbon monoxide transfer coefficient (KCO) may be reported optionally, but avoid using the term "DLCO/VA ratio" as it is commonly misunderstood 1, 3, 4

The European Respiratory Society recommends using the rebreathing DLCO technique when patients cannot perform single-breath maneuvers due to severe neuromuscular dysfunction or coordination problems 5, 6

The American Thoracic Society recommends always measuring DLCO alongside complete pulmonary function testing, including pre- and post-bronchodilator spirometry, static lung volumes (by body plethysmography or helium dilution), and arterial blood gas analysis 7
DLCO and FEV1 correlate poorly—reduction in DLCO can occur even with normal spirometry, and both should be assessed independently when evaluating disease severity 7

Reduced DLCO with normal lung volumes suggests early interstitial lung disease, pulmonary vascular disease (including pulmonary hypertension), early emphysema, or anemia 8
Reduced DLCO with restrictive pattern (low TLC) indicates interstitial lung disease, with severity correlating with extent of fibrosis on high-resolution CT 9, 8

The American Thoracic Society recommends repeating DLCO at yearly intervals for monitoring disease progression in chronic conditions like alpha-1 antitrypsin deficiency, connective tissue disease-associated ILD, and other progressive lung diseases 7, 9
Progressive pulmonary fibrosis requires documentation of physiological decline: ≥10% relative decline in FVC or ≥15% relative decline in DLCO within one year (along with worsening symptoms and/or radiological progression) 9

The American Thoracic Society recommends never interpreting DLCO in isolation—always consider in context of spirometry, lung volumes, and clinical presentation 8
Ensure proper breath-hold time (approximately 10 seconds) and adequate inspired volume (≥85% of VC) for valid single-breath measurements 1

The American Thoracic Society established a standardized grading system based on three critical technical parameters, including inspired volume, breath-hold time, and sample collection time 10
A DLCO maneuver is considered Grade A (Highest Quality) if the inspired volume is greater than 90% of the largest vital capacity, breath-hold time is between 8-12 seconds, and sample collection time is less than 4 seconds 10
Alternative Grade A criteria include an inspired volume to vital capacity ratio greater than 85% and an alveolar volume within 0.200 L or 5% of the alveolar volume from another acceptable maneuver 10

The American Thoracic Society recommends reporting the average DLCO value from at least two Grade A maneuvers that are repeatable within 2 ml/min/mm Hg 10
If only one Grade A maneuver is obtained, the DLCO value from that single maneuver should be reported 10
For suboptimal but usable tests (Grades B-D), the average of the two best-graded maneuvers should be reported, along with a cautionary comment to alert the interpreter that acceptability criteria were not fully met 10

Inadequate inspired volume (less than 80% of vital capacity) renders the test unacceptable 10
Breath-hold time outside 8-12 seconds significantly affects accuracy 10
Sample collection time greater than 5 seconds compromises measurement validity 10
Reporting Grade F maneuvers is not recommended, as these should be excluded entirely 10
Failing to include cautionary comments when only Grades B-D are available is a common reporting mistake 10