What does a Forced Expiratory Volume (FEV) 1% over 100 indicate?

Understanding FEV1% Over 100

An FEV1% (Forced Expiratory Volume in 1 second) value over 100% indicates that the patient's lung function exceeds the predicted normal value for someone of their age, height, sex, and ethnicity, and generally represents excellent lung function.

What FEV1% Represents

FEV1% refers to the percentage of predicted normal value for forced expiratory volume in the first second of exhalation. This measurement is calculated by:

• Taking the patient's actual measured FEV1 value
• Dividing it by the predicted normal value for someone of the same age, height, sex, and ethnicity
• Multiplying by 100 to get a percentage

When this value exceeds 100%, it means the patient's lung function is better than what would be expected for their demographic characteristics.

Clinical Significance of FEV1% > 100

Normal Variation

• Values above 100% often represent normal physiological variation
• Most reference equations consider values between 80-120% of predicted as within normal range 1, 2
• Some individuals naturally have larger lungs or better respiratory mechanics than predicted

Possible Interpretations

1. Superior lung function: May be seen in athletes or individuals with excellent respiratory health
2. Reference equation mismatch: The reference equations used may not be optimal for the specific population being tested 3
3. Technical considerations: Exceptionally good effort during the spirometry test

Diagnostic Considerations

Obstructive Patterns

• Even with FEV1% > 100, an obstructive pattern may still be present if the FEV1/FVC ratio is below the lower limit of normal 2
• A normal FEV1 with decreased FEV1/FVC ratio may indicate early or mild obstructive airway disease, even when asymptomatic 2

Evaluation Algorithm

1. Check FEV1/FVC ratio:

   • If normal: Confirms excellent lung function
   • If below lower limit of normal: May indicate early obstructive disease despite good FEV1 2, 4

2. Consider bronchodilator testing:

   • Significant reversibility (increase in FEV1 ≥12% and ≥200mL) suggests asthma 2
   • Research shows 28% of patients with FEV1 > 90% predicted but obstructive indices may have underlying airway hyperresponsiveness 4

3. Evaluate lung volumes if clinically indicated:

   • To assess for hyperinflation or air trapping
   • Particularly if symptoms don't match spirometry results 2

Common Pitfalls in Interpretation

1. Assuming normal lung function solely based on FEV1% > 100

   • Always check the FEV1/FVC ratio as well 2
   • Research shows that normal FEV1 with obstructive indices may not represent a normal physiological variant 4

2. Using fixed FEV1/FVC ratio cutoffs across all ages

   • Using a fixed ratio of 0.70 can lead to false-positive diagnoses in older adults and false-negative diagnoses in younger adults 2, 5
   • Consider using lower limit of normal based on the 5th percentile instead 2

3. Overlooking reference equation limitations

   • Different reference equations may yield different predicted values 3, 6
   • Some equations may be less appropriate for certain populations 3

Clinical Implications

• FEV1% > 100 generally indicates good lung function, but doesn't rule out early obstructive disease
• In symptomatic patients with FEV1% > 100, consider additional testing such as methacholine challenge if obstructive indices are present 4
• For patients with severe airflow obstruction, FVC responsiveness may be more relevant than FEV1 responsiveness 7

In conclusion, while an FEV1% over 100 typically represents excellent lung function, it should be interpreted in the context of other spirometric parameters, particularly the FEV1/FVC ratio, to ensure early obstructive disease is not missed.