How do you interpret Pulmonary Function Tests (PFTs) results in patients with suspected respiratory diseases?

How to Interpret Pulmonary Function Tests

Interpret PFTs using a systematic, stepwise approach: first assess test quality, then compare results to reference values, identify ventilatory patterns using FEV1/VC ratio as the primary discriminator, confirm patterns with additional measurements, assess severity, and integrate clinical context. 1, 2

Step 1: Assess Test Quality First

Before interpreting any numerical values, review test quality and identify technical errors—relying solely on computer-generated interpretations without quality review is a common and critical mistake. 1 Document the nature and extent of unacceptable maneuvers, as tests that are less than optimal may still contain useful information but require acknowledgment of potential errors. 1

Step 2: Use the Core Parameters

Limit primary interpretation to VC (or FVC), FEV1, and FEV1/VC ratio to avoid false-positive results. 1 When examining 14 different spirometric measurements, the rate of finding at least one "abnormal" test increases to 24% even in healthy subjects, compared to only 10% when using just these three core parameters. 1

• Use the largest available VC from any maneuver (inspiratory, slow expiratory, or forced expiratory), as FVC is often reduced more than IVC or SVC in airflow obstruction. 1
• FEV6 may substitute for VC if appropriate lower limit of normal (LLN) for FEV1/FEV6 is used. 1

Step 3: Identify the Ventilatory Pattern

Obstructive Pattern

An obstructive defect is defined by FEV1/VC ratio below the 5th percentile of predicted value (or <0.70). 1, 2

• Look for a concave shape on the flow-volume curve, which reflects slowing of expiratory flow and is the hallmark of airflow obstruction. 1, 2
• FEV1 is proportionally more reduced than VC in obstructive patterns. 1, 2
• Additional supportive findings include proportionally greater reduction in FEF75% or FEF25-75% compared to FEV1, though these are not specific for small airway disease in individual patients. 1

Restrictive Pattern

A normal or increased FEV1/VC ratio (>85-90%) with reduced VC suggests restriction, but TLC <5th percentile is required to confirm true restriction. 2

• Only 50% of cases with low VC actually have low TLC, so reduced VC alone does not prove restriction. 2
• A convex pattern on the flow-volume curve is characteristic of restrictive patterns. 2

Mixed or Indeterminate Patterns

• A normal FEV1/VC ratio with low values for both FEV1 and VC may indicate poor effort or a mixed obstructive-restrictive pattern. 2
• When FEV1/VC is borderline, late expiratory flow measurements (FEF25-75%) may help suggest airway obstruction. 1

Step 4: Assess Severity

Use FEV1 % predicted to categorize severity into mild, moderate, and severe categories, though recognize that FEV1 correlates poorly with symptoms in individual patients. 1, 2 The severity classification is based on studies relating pulmonary function to work ability, daily function, morbidity, and prognosis. 1

Step 5: Evaluate Diffusing Capacity (DLCO)

Low DLCO indicates parenchymal disease, emphysema, or vascular involvement and is an important predictor of mortality. 2

• Adjust DLCO interpretation for hemoglobin value and altitude. 2
• In obstructive patterns, TLC measurement helps differentiate emphysema (increased TLC indicating hyperinflation) from asthma. 2

Step 6: Check Bronchodilator Response

A significant bronchodilator response is defined as an increase in FEV1 or FVC of >12% AND >200 mL. 2

• The absence of a response does not rule out clinical benefit from bronchodilator therapy. 2
• This is relevant primarily for obstructive patterns.

Step 7: Integrate Clinical Context

Record respiratory symptoms (cough, phlegm, wheezing, dyspnea), smoking status, and recent bronchodilator use to make interpretation clinically meaningful. 1, 2

• Consider chest radiograph findings, hemoglobin value, and suspicion of neuromuscular disease or upper airway obstruction. 1, 2
• Ask ordering physicians to state the clinical question and ask patients why they were sent for testing. 1

Common Pitfalls to Avoid

• Do not examine multiple flow parameters simultaneously (beyond the core three), as this dramatically increases false-positive rates. 1
• Do not use FVC alone in obstructive disease—it underestimates true VC compared to IVC or SVC. 1
• Do not diagnose restriction without measuring TLC, as half of patients with low VC have normal TLC. 2
• Do not rely on MVV routinely for diagnosis, as it correlates well with FEV1, though it may help identify neuromuscular disorders (disproportionate decrease relative to FEV1) or upper airway obstruction. 1
• Consider upper airway obstruction when FEV1/PEF ratio >8 or when there is a flattened inspiratory flow loop with normal expiratory findings. 3