What is the normal range for forced expiratory volume (FEV) on spirometry?

Normal Range for Forced Expiratory Volume on Spirometry

The normal range for forced expiratory volume in 1 second (FEV1) is considered to be above 80% of predicted values or above the lower limit of normal (LLN), with the FEV1/FVC ratio typically above 0.70 or above the LLN. 1

Understanding Spirometry Parameters and Normal Ranges

Key Spirometry Measurements

• FEV1 (Forced Expiratory Volume in 1 second):

  • Normal: >80% of predicted value or above LLN 2, 1
  • Measures volume of air exhaled in the first second of forced expiration

• FVC (Forced Vital Capacity):

  • Normal: >80% of predicted value or above LLN 2
  • Measures total volume of air that can be forcefully exhaled

• FEV1/FVC ratio:

  • Normal: >0.70 or above LLN (5th percentile of predicted value) 2, 3
  • A more accurate LLN can be determined by subtracting 0.10 from the age-specific predicted FEV1/FVC 3

• Other Parameters:

  • PEF (Peak Expiratory Flow): Maximum flow achieved during forced expiration
  • FEF25-75%: Mean flow during middle portion of expiration (small airway function)

Reference Values and Interpretation

Reference values for spirometry are based on several factors:

• Age
• Sex
• Height
• Ethnicity
• Weight/BMI

The interpretation of spirometry results should use the LLN rather than fixed percentages whenever possible 1. The LLN is typically defined as the 5th percentile of a healthy, non-smoking population 3.

Factors Affecting Spirometry Measurements

Technical Factors

• Equipment calibration (requires daily volume checks with a 3-L syringe) 2, 1
• Testing procedure standardization
• Technician skill and experience (can affect measurements) 4

Patient Factors

• Height (increases FEV1 and FVC by approximately 40-43 mL per 10 cm) 4
• BMI (affects within-subject variability) 4
• Age (FEV1/FVC ratio decreases with age) 3
• Effort and cooperation during testing

Quality Assessment of Spirometry

For reliable interpretation, spirometry measurements must meet quality criteria:

• Back-extrapolated volume <5% of FVC or <0.150 L 2
• Free from artifacts 2
• No cough during first second of exhalation (for FEV1) 2
• No early termination or cutoff 2
• Repeatability: The two largest values of FEV1 and FVC should be within 0.150 L of each other 2

Common Pitfalls in Interpretation

• Using fixed cutoffs (like FEV1/FVC <0.70) without considering age can lead to:

  • Under-diagnosis in younger individuals
  • Over-diagnosis in older individuals 3

• Relying solely on FEV1 without examining FVC and FEV1/FVC ratio may miss important abnormalities 1

• Failure to consider the pre-test probability of disease in the interpretation 3

• Not accounting for variability in measurements (within-subject standard deviation for FEV1 is approximately 102 mL) 4

By understanding these normal ranges and interpretation principles, clinicians can more accurately assess respiratory function and identify potential abnormalities that may require further evaluation.