Can you interpret my pulmonary function test results?

Systematic Approach to Pulmonary Function Test Interpretation

Yes, I can interpret pulmonary function tests using a standardized four-step algorithm that prioritizes test quality assessment, appropriate reference value comparison, pattern recognition, and clinical correlation. 1

Step 1: Assess Technical Quality First

Before accepting any numerical results, you must review the raw data and technical parameters:

• Inspect the flow-volume curves for rounded peaks, abrupt terminations, or coughing artifacts that indicate submaximal effort 1
• Require ≥3 acceptable maneuvers with reproducibility ≤150 mL for both FEV₁ and FVC 1
• Verify forced expiration duration ≥6 seconds in adults to obtain reliable volumes 1
• Document any quality issues in your interpretation and state the likely direction and magnitude of measurement error, even when the test is suboptimal 1

Critical pitfall: Relying solely on computer-generated interpretations without reviewing test quality is the single most frequent error in PFT interpretation. 1

Step 2: Select and Apply Appropriate Reference Values

• Measure height with a calibrated stadiometer at the time of testing—never use self-reported height 1, 2
• Match reference equations to the patient's age, sex, measured height, and ethnicity 1, 2
• Use the 5th percentile (z-score of -1.64) as the lower limit of normal for all parameters—avoid fixed cut-offs like "80% predicted" 1, 3, 2
• **Never use FEV₁/FVC <0.70 as a fixed threshold** because it generates false-positive COPD diagnoses in men >40 years and women >50 years, especially elderly never-smokers 1
• Ensure all spirometric parameters (FVC, FEV₁, FEV₁/FVC) derive from the same reference source to maintain internal consistency 1, 3

Race/Ethnicity Adjustment Factors (when specific equations unavailable):

• Black patients: multiply FEV₁, FVC, and TLC by 0.88 (but NOT the FEV₁/FVC ratio) 1
• Asian-American patients: multiply by 0.94 (but NOT the FEV₁/FVC ratio) 1

Step 3: Identify the Physiological Pattern

Obstructive Pattern

• FEV₁/VC (or FEV₁/FVC) ratio below the 5th percentile defines obstruction and predicts morbidity and mortality even when absolute FEV₁ is within normal limits 1
• Measure total lung capacity (TLC) to assess hyperinflation; elevated TLC, RV, or RV/TLC ratio supports emphysema, asthma, or other obstructive disorders 1
• Severity is graded by FEV₁ % predicted: Mild >70%, Moderate 60-69%, Moderately severe 50-59%, Severe 35-49%, Very severe <35% 1

Restrictive Pattern

• TLC below the 5th percentile together with a normal FEV₁/VC ratio confirms true restrictive physiology 1
• Never confirm restriction without measuring TLC—reduced FVC on spirometry alone has poor positive predictive value for true restriction, as only about half of low-VC cases have low TLC 1, 3
• Suspect restriction when: VC is reduced, FEV₁/VC ratio is increased (>85-90%), and the flow-volume loop shows a convex shape 1

Critical pitfall: Single-breath alveolar volume (VA) from DLCO testing systematically underestimates TLC by up to ~3 L in severe obstruction, markedly increasing the risk of misclassifying restrictive disease. 1

Mixed Pattern

• Both FEV₁/VC ratio AND TLC fall below the 5th percentile of reference values 1
• 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 a superimposed restrictive component cannot be excluded without TLC assessment 1

Borderline/Normal Results

• Normal FEV₁/VC ratio with reduced FEV₁ and FVC may indicate suboptimal effort, patchy peripheral airway obstruction, or inability to sustain expiration long enough 1
• Incomplete inspiratory or expiratory effort produces simultaneous reduction in FEV₁ and FVC with normal FEV₁/FVC ratio—interpret as suboptimal test rather than true disease 1

Step 4: Interpret DLCO (Diffusing Capacity)

• Normal DLCO: above the 5th percentile (z-score ≥-1.64) using appropriate reference equations 2
• Adjust for hemoglobin concentration—anemia artificially reduces DLCO while polycythemia increases it 2
• Adjust for carboxyhemoglobin (elevated in smokers) and altitude when applicable 1, 2
• Low DLCO values (<60%) are associated with higher mortality (25% mortality rate) and pulmonary morbidity (40%) in patients undergoing lung resection 1
• Grade A DLCO measurements require: inspired volume >90% of vital capacity, breath-hold time 8-12 seconds, and sample collection time <4 seconds 1

Critical warning: Never interpret DLCO in isolation—always consider it in the context of spirometry, lung volumes, and clinical presentation. 2

Important note: Inter-laboratory variability for DLCO is substantially greater than for spirometry, making reference equation selection critical. 1, 2

Monitoring Strategy for Serial Testing

• >5% change in FEV₁ or FVC indicates significant variation 3
• >12% change in FEV₁ or >11% change in FVC from week to week suggests meaningful change 3
• >15% change in FEV₁ from year to year indicates clinically meaningful progression 3
• Maintain consistent interpretation strategy within your laboratory—changes in analytical approach alone can create the illusion of disease progression 1

Additional Critical Pitfalls to Avoid

• Never rely solely on FEV₁ to diagnose obstruction—always check the FEV₁/FVC ratio first 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 1
• Patients with values near diagnostic thresholds have the greatest risk of misclassification—consider additional testing (repeat PFTs, lung volumes, or diffusion studies) in borderline cases 1
• Upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV₁ % predicted 1