What does a pulmonary function test (PFT) entail and how are the results interpreted?

Pulmonary Function Test (PFT) Examination: Components and Interpretation

What a PFT Entails

A pulmonary function test consists of three core components: spirometry (measuring airflow and lung volumes during forced breathing), static lung volume measurement (determining total lung capacity and its subdivisions), and diffusing capacity testing (assessing gas transfer across the alveolar-capillary membrane). 1

Core Test Components

• Spirometry measures forced expiratory volume in 1 second (FEV₁), forced vital capacity (FVC), and the FEV₁/FVC ratio during maximal forced exhalation 1
• Lung volume measurement uses either body plethysmography or gas dilution techniques to determine total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV) 1, 2
• Diffusing capacity (DLCO) assesses the lung's ability to transfer carbon monoxide from alveolar gas to pulmonary capillary blood, requiring inspired volume >90% of vital capacity and breath-hold time of 8-12 seconds 2

Patient Requirements

• Height must be measured with a calibrated stadiometer at the time of testing—self-reported height should never be used 1, 2
• Essential demographic data includes age, sex, measured height, weight, and ethnicity for proper reference value selection 1
• Patients should withhold short-acting bronchodilators for 8 hours, long-acting for 24-48 hours, and be free of respiratory infections for at least 3 weeks before testing 1

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How Results Are Interpreted

Interpretation follows a systematic four-step algorithm: (1) assess technical quality first, (2) compare results to appropriate reference values using the 5th percentile as the lower limit of normal, (3) identify the physiological pattern (obstruction, restriction, or mixed), and (4) answer the specific clinical question that prompted testing. 1, 2, 3

Step 1: Quality Assessment

• Review raw flow-volume curves before accepting any computer-generated interpretation—this is the single most frequent source of error 2, 3
• Spirometry requires ≥3 acceptable maneuvers with reproducibility ≤150 mL for both FEV₁ and FVC, and forced expiration lasting ≥6 seconds in adults 2
• Grade spirometry quality from A (best) to F (no acceptable tests) based on acceptability and repeatability criteria 1
• Document any quality issues and state the likely direction and magnitude of measurement error, even when tests are suboptimal 2

Step 2: Reference Value Selection

• Use the 5th percentile of the reference population as the lower limit of normal for all parameters—never use fixed cutoffs like "80% predicted" or FEV₁/FVC <0.70 1, 2, 3
• Select reference equations matched to the patient's age, sex, measured height, and ethnicity 1, 2
• The Global Lung Initiative (GLI-2012) equations are recommended for spirometry in patients aged 3-95 years, providing ethnicity-specific predictions for whites, African Americans, and Asian populations 1
• All spirometric parameters (FVC, FEV₁, FEV₁/FVC) must come from the same reference source to maintain internal consistency 1, 2
• For DLCO, the European Respiratory Society recommends using recently published GLI equations (2017) for ages 5-85 years 1

Critical caveat: Fixed cutoffs like FEV₁/FVC <0.70 generate false-positive COPD diagnoses in men >40 years and women >50 years, especially among elderly never-smokers 1, 2

Step 3: Pattern Recognition

Obstructive Pattern

• An FEV₁/FVC (or FEV₁/VC) ratio below the 5th percentile defines obstruction, even when absolute FEV₁ is within normal limits 1, 2, 3
• The flow-volume curve shows a characteristic concave shape during exhalation 1, 3
• Measure TLC to assess hyperinflation: elevated TLC, RV, or RV/TLC ratio supports emphysema or asthma 2, 3
• Bronchodilator response is significant when FEV₁ or FVC increases by ≥12% AND ≥200 mL in adults (≥12% alone in children 5-18 years) 3, 4

Restrictive Pattern

• A TLC below the 5th percentile together with a normal FEV₁/VC ratio confirms true restrictive physiology—reduced vital capacity alone is insufficient, as only 50% of low-VC cases have low TLC 2, 3
• The flow-volume curve shows a convex shape 3
• Suspect restriction when VC is reduced, FEV₁/VC ratio is increased (>85-90%), and the flow-volume loop is convex 2, 3

Common pitfall: Never confirm restrictive disease without measuring TLC, as spirometry alone has poor positive predictive value for restriction 4, 5

Mixed Pattern

• Defined when both FEV₁/VC ratio and TLC fall below the 5th percentile 2
• When FEV₁/VC ratio is low and VC is reduced but TLC has not been measured, state that VC reduction likely reflects hyperinflation and that superimposed restriction cannot be excluded without TLC 2

Step 4: Severity Grading

• Severity is based on FEV₁ % predicted, NOT the FEV₁/FVC ratio 1, 2, 4
• Classification: Mild (>70%), Moderate (60-69%), Moderately severe (50-59%), Severe (35-49%), Very severe (<35%) 2, 4
• FEV₁ correlates poorly with individual symptoms and should not be used alone to predict clinical severity 1, 4

DLCO Interpretation

• Low DLCO (<60% predicted) indicates parenchymal disease, emphysema, or vascular involvement and is associated with higher mortality 1, 2, 3
• Adjust DLCO for hemoglobin and carboxyhemoglobin levels—elevated carboxyhemoglobin (e.g., in smokers) artificially lowers DLCO 1, 2
• Apply altitude-specific correction factors when testing at higher elevations 2
• Inter-laboratory variability for DLCO is substantially greater than for spirometry, making reference equation selection critical 2

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Common Pitfalls and How to Avoid Them

• Relying on computer interpretations without reviewing test quality is the most frequent error—always inspect raw curves 2, 3
• Interpreting multiple parameters simultaneously inflates false-positive rates: examining three parameters yields ~10% abnormal results in healthy individuals, rising to ~24% when fourteen parameters are examined 2
• Using FEV₁/VC ratio to determine severity of obstruction (instead of FEV₁ % predicted) is incorrect 2
• Failing to measure lung volumes when restrictive pattern is suspected based on spirometry alone leads to misdiagnosis 2, 4
• Not adjusting DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for toxicity 2
• Incomplete inspiratory or expiratory effort produces simultaneous reduction in FEV₁ and FVC with normal FEV₁/FVC ratio—this represents suboptimal effort, not true disease 2

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Clinical Context Integration

• PFT interpretation should address relevant clinical diagnoses, chest radiograph appearance, most recent hemoglobin value, and any suspicion of neuromuscular disease or upper airway obstruction 1, 3
• Record respiratory symptoms (cough, phlegm, wheezing, dyspnea), smoking status, and recent bronchodilator use 1, 3
• Results near diagnostic thresholds carry the greatest risk of misclassification—consider additional testing (repeat PFTs, lung volumes, or diffusion studies) in borderline cases 2
• Maintain consistent interpretation strategy within a laboratory to avoid inferring disease progression when changes result only from altered analytical approach 2

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Special Populations

• In preschool children (ages 2-7), modified criteria apply: repeatability within 0.150 L for ages 2-6, and forced expiration time may be <6 seconds if a 1-second plateau is achieved 1
• Younger children or those with developmental delay may be unable to perform some maneuvers, limiting test robustness 1
• For pediatric patients, use GLI reference values without race-specific adjustments, as per recent ATS statement 1
• In patients with marked obesity, reduced FRC from decreased chest-wall compliance raises calculated inspiratory capacity despite minimal TLC impact 2