Clinical Relevance of Fixed Ratio vs Lower Limit of Normal of FEV₁/FVC in COPD: Patient-Reported Outcomes From the CanCOLD Cohort

Diagnosing disease means taking decisions: it is absent or present. As we are taught to discriminate between absence of presence of disease, we also look upon measures in a black and white manner. If a person is above a certain threshold he is supposed to be disease-free, if he is below the threshold he may have the disease. The division line is thin and absolute: in this case a 0.71 FEV1/FVC means no COPD and one of 0.69 may indicate COPD. A z-score of -1.63 equals no disease and one of -.1.65 may lead to a disease diagnosis. The reader tastes a skeptic approach and will be not surprised that the frequent comparisons between FEV1/FVC <0.70 or <LLN is met with even more skepticism.

First of all (cross-sectional) spirometry measures only one aspect of COPD, the presence of obstruction and not necessarily that of emphysema. Emphysema can exist without obstruction being present, at that moment at least. COPD is characterized by an increased lung function decline, and please realize that 50% of all subjects start from a higher-than-predicted lung function. For these subjects the time needed to cross the threshold of disease is much longer compared to those starting from a lower baseline lung function. If in a smoker an increased decline is present, the FEV1/FVC can still remain above thresholds: COPD is present but hasn't manifested itself by a below-threshold function. Consecutive measurements are the only sensible option. The use of black-and-white thresholds is evidently dangerous and so are the comparisons between them.

Comparisons between diagnostics are only valid if a gold standard is available and if not, comparisons are not informative. The authors of this publication correctly state that the current debate is hampered by a lack of a gold standard, but still present a gold-standard-lacking comparison. They do correlate the thresholds to a range of parameters, but none of them can function as a COPD gold standard. The outcome is thus debatable and doesn't solve the problem.

Some advice to close with: never use thresholds as absolute divisors between yes/no disease. Use the FEV1/FVC 0.70 as a warning sign that something may be wrong and further analysis is warranted. E.g., try diffusion capacity measures as an additional diagnostic tool and follow the patient over time, which should give more insight.

Competing interests: None declared