What does a spirometer result of 5.1 indicate?

Interpreting a Spirometer Result of 5.1

A single numerical value of "5.1" from spirometry cannot be interpreted without knowing which specific parameter was measured (FEV1, FVC, or another value), the patient's predicted values based on age, height, sex, and ethnicity, and whether this represents liters or a percentage of predicted. 1

Essential Information Needed for Interpretation

To properly interpret any spirometry result, you must have the following components 1:

• Which parameter: FEV1 (forced expiratory volume in one second), FVC (forced vital capacity), or vital capacity (VC) 1
• Units: Whether 5.1 represents liters (L) or percentage of predicted (%) 1
• Predicted values: Based on the patient's age, height, sex, and ethnicity 1
• FEV1/FVC ratio: Essential for determining obstructive vs. restrictive patterns 1

Stepwise Approach to Spirometry Interpretation

Step 1: Verify Test Quality

Before interpreting any value, confirm the test was performed correctly 1:

• At least three acceptable maneuvers were obtained 1
• The two highest values for FEV1 and FVC are within 0.15 L of each other 2
• Visual inspection of flow-volume and volume-time curves shows proper technique 1

Step 2: Identify the Ventilatory Pattern

Use FEV1/FVC ratio and FVC together to classify the pattern 1:

• Obstruction: FEV1/VC ratio below the 5th percentile of predicted 1
• Restriction: Total lung capacity (TLC) below the 5th percentile of predicted 1
• Mixed defect: Both FEV1/VC and TLC below the 5th percentile 1

Important caveat: A reduced FVC alone does not prove restriction—it may suggest restriction only when FEV1/FVC is normal or increased 1. Confirmation requires measurement of lung volumes 1.

Step 3: Assess Severity

Once the pattern is identified, FEV1 as a percentage of predicted determines severity 3:

• The specific severity thresholds were arbitrarily determined and are not based on patient outcome evidence 3
• This applies to both obstructive and restrictive patterns 3

Step 4: Test for Reversibility (if obstructive)

Administer a short-acting bronchodilator and repeat spirometry 3:

• Significant reversibility in adults: Improvement of FEV1 or FVC by ≥12% AND ≥0.2 L 3
• Significant reversibility in children/adolescents (5-18 years): Improvement >12% 3

Common Pitfalls to Avoid

• Do not use a fixed FEV1/FVC ratio of 0.7 as the sole criterion for obstruction, as this overestimates ventilatory defects in older adults 1
• Use VC (vital capacity) rather than just FVC when calculating the ratio, as FVC is more dependent on flow and volume histories and may miss obstructive patterns 1
• Do not interpret TLC at the lower limit of normal as definitively restrictive if the patient has a disease expected to cause restriction (e.g., lung resection), as the defect may be difficult to prove if TLC remains above the 5th percentile 1
• Ensure proper BTPS correction (body temperature and ambient pressure saturated with water vapor), as errors up to 10% can occur without proper temperature measurement 1

When Additional Testing Is Needed

If spirometry values are near the upper or lower limits of normal, additional studies should be performed 1:

• Bronchodilator response testing 1
• Diffusing capacity for carbon monoxide (DLCO) 1
• Gas exchange evaluation 1
• Respiratory muscle strength testing 1
• Exercise testing 1

For patients with FEV1 <70% of predicted and suspected asthma, a therapeutic trial of a short-acting bronchodilator may be tried instead of formal bronchoprovocation testing 3.