What does a forced vital capacity (FVC) of 4.45 L indicate in an adult?

Interpretation of FVC 4.45 L

An FVC of 4.45 L is an absolute volume measurement that cannot be interpreted in isolation—it must be compared to predicted values based on the patient's age, sex, height, and ethnicity to determine if lung function is normal or abnormal. 1

Context Required for Interpretation

The raw FVC value of 4.45 L requires the following patient-specific information for proper interpretation:

• Height - The strongest predictor of lung volumes, as taller individuals have proportionally larger lung capacities 2, 3
• Age - Lung function declines with age in a nonlinear pattern, with accelerated decline after age 35 3
• Sex - Males typically have larger lung volumes than females of the same height and age 3
• Ethnicity - Different reference equations apply to different ethnic populations 1

Determining Normal vs. Abnormal

The key threshold is whether FVC falls below the 5th percentile (lower limit of normal, LLN) of predicted values for that individual. 1

Classification Framework:

• Normal: FVC ≥ 5th percentile of predicted (≥ LLN) 1
• Reduced: FVC < 5th percentile of predicted (< LLN) 1
• Borderline: FVC 86-94% of predicted 4
• Low: FVC ≤ 85% of predicted 5, 4

Clinical Significance by Predicted Percentage

For risk stratification purposes, specific thresholds have been validated: 5, 4

• FVC > 100% predicted (males) or > 85% predicted (females): Effectively rules out restrictive lung disease in non-obstructive patients 5
• FVC < 55% predicted (males) or < 40% predicted (females): Positively predicts reduced total lung capacity (TLC) in non-obstructive patients 5
• FVC ≤ 85% predicted: Associated with increased mortality risk, particularly in patients with intermediate cardiovascular risk 4

Common Pitfalls to Avoid

A reduced FVC does not automatically indicate restrictive lung disease—it requires confirmation with TLC measurement. 1

• In obstructive disease (FEV₁/FVC < LLN), FVC may be reduced due to air trapping rather than true restriction, and spirometry alone cannot reliably diagnose concomitant restrictive defects 1, 5
• Poor effort or technique can artificially lower FVC values, making test quality assessment critical 1
• Incomplete exhalation (failure to meet end-of-test criteria of ≥6 seconds or plateau) will underestimate true FVC 1

Quality Considerations

The FVC measurement must meet acceptability criteria to be interpretable: 1

• Good start with extrapolated volume < 5% of FVC or 0.15 L (whichever is greater) 1
• Free from artifacts including cough, glottic closure, or early termination 1
• Exhalation duration ≥ 6 seconds (3 seconds for children) or demonstrable plateau 1
• Repeatability: Two largest FVC values within 0.15 L of each other 1, 2

Next Steps for Clinical Assessment

If FVC is below predicted values, the diagnostic approach depends on the FEV₁/FVC ratio: 1

• FEV₁/FVC < LLN: Suggests obstructive pattern; measure lung volumes to assess for hyperinflation vs. concomitant restriction 1
• FEV₁/FVC normal or increased with low FVC: Suggests restrictive pattern; confirm with TLC measurement, as TLC < 5th percentile is required to diagnose true restriction 1
• Consider additional testing: Diffusing capacity (DLCO), gas exchange evaluation, or exercise testing if clinically indicated 1