How to determine if a Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) report is acceptable in a patient with a known history of respiratory disease, such as Chronic Obstructive Pulmonary Disease (COPD) or interstitial lung disease?

How to Determine if a DLCO Report is Acceptable

A DLCO report is acceptable when it contains at least two Grade A maneuvers that are repeatable within 2 ml/min/mm Hg (or 0.67 mmol/min/kPa), with the average of these maneuvers reported as the final result. 1

Quality Grading Criteria for DLCO Maneuvers

The American Thoracic Society established a standardized grading system based on three critical technical parameters 1:

Grade A (Highest Quality)

• Inspired volume (VI) >90% of the largest vital capacity (VC) in the same test session 1
• Breath-hold time: 8-12 seconds 1
• Sample collection time: <4 seconds 1

Alternative Grade A criteria: VI/VC >85% AND alveolar volume (VA) within 0.200 L or 5% of VA from another acceptable maneuver 1

Grade B

• VI/VC >85% 1
• Breath-hold time: 8-12 seconds 1
• Sample collection time: <4 seconds 1

Grade C

• VI/VC >80% 1
• Breath-hold time: 8-12 seconds 1
• Sample collection time: <5 seconds 1

Grade D

• VI/VC >80% 1
• Breath-hold time: <8 or >12 seconds 1
• Sample collection time: <5 seconds 1

Grade F (Unacceptable)

• Any test not meeting Grade A, B, C, or D criteria 1
• Grade F maneuvers are not usable and should not be reported 1

Reporting Standards for Acceptable Tests

Optimal Reporting (Grade A Maneuvers)

• Report the average DLCO value from at least two Grade A maneuvers that are repeatable within 2 ml/min/mm Hg 1
• If only one Grade A maneuver is obtained, report the DLCO value from that single maneuver 1

Suboptimal but Usable (Grades B-D)

• If only Grades B-D maneuvers are available, report the average of the two best-graded maneuvers 1
• A cautionary comment MUST be included to alert the interpreter that acceptability criteria were not fully met 1
• These results may still have clinical utility despite technical limitations 1

Essential Components That Must Be Present

Beyond maneuver quality grading, an acceptable DLCO report must include 2:

• Absolute values in mL/min/mmHg 2
• Z-scores using GLI 2017 reference equations 2
• Percent predicted values 2
• Measured hemoglobin concentration with DLCO adjusted accordingly 2
• Alveolar volume (VA) measured during the maneuver 2
• Correction to standard barometric pressure (particularly critical at altitude) 2

Critical Pitfalls to Avoid

Technical Errors That Invalidate Results

• Inadequate inspired volume (<80% of VC) renders the test unacceptable 1
• Breath-hold time outside 8-12 seconds significantly affects accuracy 1, 2
• Sample collection time >5 seconds compromises measurement validity 1

Interpretation Errors

• Never interpret DLCO in isolation—always review in context of spirometry, lung volumes, and clinical presentation 2
• Failure to adjust for hemoglobin leads to misinterpretation, as anemia artificially lowers DLCO while polycythemia increases it 2
• The lower limit of normal (LLN) is at z-score of -1.64, not arbitrary cutoffs like 80% predicted 2

Common Reporting Mistakes

• Using the term "DLCO/VA ratio" instead of the preferred "KCO" (carbon monoxide transfer coefficient) 2
• Reporting Grade F maneuvers—these should be excluded entirely 1
• Failing to include cautionary comments when only Grades B-D are available 1

Quality Control Verification Steps

When reviewing a DLCO report, systematically verify 1, 2:

1. Maneuver quality grade is documented (A, B, C, D, or F)
2. At least two acceptable maneuvers were performed (Grade A preferred)
3. Repeatability is within 2 ml/min/mm Hg between maneuvers
4. Hemoglobin value is reported and adjustment applied
5. Reference equations are specified (GLI 2017 recommended)
6. Cautionary comments are present if Grades B-D were used