Normal Values for FEV in Spirometry
The normal values for FEV1 (Forced Expiratory Volume in 1 second) are considered to be above 80% of predicted values, with the lower limit of normal (LLN) being more accurately defined as the 5th percentile of a healthy, non-smoking population. 1, 2
Key Spirometry Parameters and Their Normal Values
FEV1 (Forced Expiratory Volume in 1 second)
- Normal value: ≥80% of predicted value
- More accurate LLN: 5th percentile of reference population (typically corresponds to z-score of -1.64)
- Important: Simple "80% rule" can lead to misclassification in younger/taller individuals (under-diagnosis) and older/shorter individuals (over-diagnosis) 2
FVC (Forced Vital Capacity)
- Normal value: ≥80% of predicted value
- More accurate LLN: 5th percentile of reference population
FEV1/FVC Ratio
- Normal value: Above the LLN for age
- Should be reported as a decimal fraction, not a percentage 1
- A quick approximation for the LLN of FEV1/FVC can be calculated by subtracting 0.10 from the age-specific predicted value 2
- The fixed ratio of 0.70 is inaccurate and leads to misclassification 2
Other Important Spirometry Parameters
- FEV6: Volume exhaled in the first 6 seconds; can be a reliable surrogate for FVC 1
- FEV1/FEV6: Can be used as an alternative to FEV1/FVC 1
- FEF25-75%: Mean forced expiratory flow between 25% and 75% of FVC; normal when above LLN 1
- PEF (Peak Expiratory Flow): Maximum expiratory flow achieved during forced expiration 1
Quality Control Considerations
Back Extrapolated Volume (BEV)
- Must be less than 5% of FVC or less than 0.150 L (whichever is greater) for an acceptable maneuver 3
- Critical for determining the true start of forced expiration and accurate timing of FEV1 3
Repeatability Criteria
- The two largest values of FEV1 must be within 0.150 L of each other
- The two largest values of FVC must be within 0.150 L of each other 1
Clinical Implications and Pitfalls
Important Caveats
- A normal FEV1 (≥80% predicted) does not guarantee normal overall spirometry
- Up to 75% of tests with normal FEV1 may show abnormalities in other parameters 4
- FEV1/FVC ratio is generally the most sensitive measure for detecting early airway obstruction 4
Common Pitfalls to Avoid
- Using fixed cutoffs (like FEV1/FVC < 0.70) instead of statistically valid LLN values 2
- Assuming values <80% of predicted are always abnormal without considering individual factors 5
- Performing bronchodilator testing in patients with normal spirometry and FEV1 >90% predicted (low yield with only 1.9% positive response rate) 6
- Overlooking that even values within the "normal range" (80-100% predicted) may indicate increased risk for cardiovascular disease and mortality 7
Interpretation Algorithm
- Verify test quality (acceptable BEV, proper effort, adequate duration)
- Compare measured values to appropriate reference values for the patient's ethnicity, age, sex, and height
- Use LLN (5th percentile) rather than fixed percentages when possible
- Examine FEV1, FVC, and FEV1/FVC ratio together, not in isolation
- Consider additional parameters (FEF25-75%, PEF) for a complete assessment
By following these guidelines, clinicians can accurately interpret spirometry results and avoid common misclassification errors that could impact patient management and outcomes.