What is the Forced Expiratory Volume in 1 second Quotient (FEV1Q) and how is it calculated?

What is FEV1Q?

FEV1Q (Forced Expiratory Volume in 1 second Quotient) is a race-neutral method of interpreting spirometry that divides the measured FEV1 by a sex-specific lower boundary value: 0.4 L for females or 0.5 L for males. 1

Calculation Method

• FEV1Q is calculated as: FEV1 (in liters) ÷ 0.4 L for females, or FEV1 (in liters) ÷ 0.5 L for males 1
• These denominators represent the sex-specific first percentile (lower limit) of FEV1 values in the population 2, 3
• The result is expressed as a simple ratio or multiple of this lower boundary 2

Clinical Advantages Over Traditional Methods

FEV1Q is superior to traditional FEV1 % predicted for predicting mortality and clinical outcomes. The evidence strongly supports this:

• In a large study of over 26,967 patients, FEV1Q had a hazard ratio of 6.9 for the worst decile compared to only 4.1 for FEV1 % predicted on multivariate analysis 2
• On univariate analysis, the difference was even more striking: hazard ratio of 18.8 for FEV1Q versus 6.1 for FEV1 % predicted 2
• FEV1Q demonstrated the highest discriminative power (C-statistic) for predicting survival compared to alternative FEV1 approaches including % predicted, z-scores, and FEV1/height² in both Black and White individuals 1

Race-Neutral Interpretation

A critical advantage of FEV1Q is that it eliminates racial bias in spirometry interpretation:

• The lower boundary FEV1 values (first percentile) do not differ significantly between Black and White individuals (difference ≤0.01 L; p>0.05) 1
• Race-specific reference equations can misclassify lung function impairment, particularly in Black individuals—using Black-specific references identified 9.3% with impairment versus 36.9% using White references in the same Black population 4
• Black individuals with FEV1 ≥ lower limit of normal using Black references but < lower limit using White references had 48% increased breathlessness and nearly doubled mortality, suggesting race-specific references miss clinically significant impairment 4
• The association between decreasing FEV1Q and mortality was not confounded or modified by race 1

Clinical Application in Different Populations

In elderly populations with and without COPD, FEV1Q provides superior prognostic information:

• Among COPD patients, the worst versus best quintile of FEV1Q showed an incidence rate ratio of 4.65 at 5 years, compared to 2.98 for FEV1 % predicted 3
• In elderly controls without COPD, even moderate reductions in FEV1Q were associated with long-term mortality, while FEV1 % predicted showed no such association 3
• This suggests FEV1Q captures clinically meaningful impairment that traditional methods miss, particularly in populations where prediction equations may be less reliable 3

Practical Advantages

FEV1Q is simpler and more intuitive than traditional spirometry interpretation methods:

• It eliminates the need for complex prediction equations that account for age, height, sex, and race 1, 2
• The calculation requires only the measured FEV1 and knowledge of the patient's sex 1
• It avoids the assumption of proportional variability inherent in % predicted methods, which may be unrealistic especially in elderly populations 3

Common Pitfalls to Avoid

• Do not confuse FEV1Q with FEV1/FVC ratio—these are entirely different measurements serving different purposes 5
• FEV1Q still requires accurate FEV1 measurement following standard spirometry quality criteria (acceptable start, free from artifacts, reproducible within 0.150 L) 5, 6
• The sex-specific denominators (0.4 L for females, 0.5 L for males) must be applied correctly—using the wrong denominator will produce misleading results 1