What are the appropriate next steps after completing pulmonary function testing?

Next Steps After Pulmonary Function Testing

The immediate next step after completing pulmonary function testing is to systematically interpret the results by comparing them to reference values, identifying any ventilatory impairments, assessing severity, and determining whether specialist referral or additional testing is warranted based on specific thresholds. 1, 2

Systematic Interpretation Framework

Step 1: Assess Technical Quality and Validity

• Verify test acceptability and reproducibility before proceeding with clinical interpretation, as poor-quality tests may reflect technical issues, equipment malfunction, or underlying disease rather than true pulmonary function 1
• Confirm accuracy of demographic data (age, height, sex, ethnicity) since these directly affect reference value selection and interpretation 1
• Consider repeating spirometry if results are not reproducible or quality is poor, which may itself indicate disease 1

Step 2: Compare Results to Reference Values

• Determine if FVC, FEV1, and FEV1/FVC ratio are normal, low, or high by comparing to healthy reference populations 2
• Use FEV1/FVC ratio <70% (or below 5th percentile) to identify obstructive defects in adults, or <85% in patients 5-18 years of age 3
• Use FVC below 5th percentile (or <80% predicted in ages 5-18) to identify restrictive patterns 3

Step 3: Identify Specific Ventilatory Impairments

For Obstructive Pattern (Low FEV1/FVC):

• Perform post-bronchodilator testing if not already done to determine reversibility: >12% and >200 mL increase in FEV1 in adults (or >12% in ages 5-18) indicates reversible disease like asthma 3
• A low DLCO with obstruction greatly increases probability of emphysema phenotype COPD, while normal DLCO makes chronic asthma more likely 4
• Post-bronchodilator FEV1 <80% predicted with FEV1/FVC <70% confirms COPD diagnosis per GOLD criteria, though this has 100% sensitivity but only 38% specificity for distinguishing COPD from asthma 5

For Restrictive Pattern (Low FVC with Normal FEV1/FVC):

• Order full pulmonary function tests including DLCO and lung volumes to confirm restrictive lung disease and establish differential diagnosis 3
• A low DLCO with restriction increases probability of interstitial lung disease, while normal DLCO makes chest wall restriction more likely 4
• Check total lung capacity (TLC) from single-breath helium dilution during DLCO testing—a normal TLC rules out true restriction without requiring body plethysmography 4

For Mixed Pattern:

• Both low FEV1/FVC ratio and low FVC indicate mixed defect requiring comprehensive evaluation 3

Step 4: Determine Disease Severity

• Use FEV1 percentage predicted as the primary severity metric: ≥80% (mild), 50-79% (moderate), <50% (severe) 6
• DLCO <40% predicted or decline >4 units is associated with increased morbidity and mortality and warrants aggressive management 4

Step 5: Specialist Referral Thresholds

Refer to pulmonologist when:

• FVC 60-80% predicted, FEV1/FVC 50-70%, or pulse oximetry 90-93% (yellow zone requiring specialist evaluation) 1
• FVC <60% predicted, FEV1/FVC <50%, or pulse oximetry <90% (red zone requiring urgent specialist referral) 1
• More than 2 pneumonia episodes treated with antibiotics in last 5 years, or 1 episode requiring hospitalization 1
• Low DLCO with normal spirometry (suggests pulmonary vascular disease or mild interstitial lung disease) 4
• Resting hypoxemia detected on pulse oximetry, as this may indicate hypercapnia from thoracic cage restriction 1

Step 6: Advanced Testing by Pulmonologist

When referred, the pulmonologist should perform:

• Chest X-ray when first seen to assess anatomy, restriction, and signs of chronic lung disease 1
• Peak cough flow (PCF): if <160 mmHg, further intervention needed for inadequate cough clearance 1
• Diffusing capacity (DLCO) after volume adjustment: if <90% predicted, further testing required 1
• Complete lung volumes (TLC, RV/TLC, ERV) by plethysmography or washout if spirometry is abnormal or restrictive lung function/hyperinflation suspected 1
• FEF25-75% to evaluate small airways function 1
• Maximum inspiratory and expiratory pressures to assess respiratory muscle strength 1

Step 7: Longitudinal Monitoring

For serial testing:

• Compare current results to previous baseline to detect disease progression 1
• Calculate FEV1 decline rate: typical decline is ~29 mL/year in nonsmokers; >60-90 mL/year indicates accelerated decline associated with increased morbidity and mortality 1
• A >15% decline in FEV1 after correcting for aging may indicate excessive decline requiring intervention 1
• Use linear regression of all FEV1 data over time for reliable decline estimation after 5-8 years of follow-up 1

Step 8: Consider Additional Clinical Context

Integrate PFT results with:

• Respiratory symptoms and their timing relative to work or exposures 1
• Smoking history and cessation status 1
• Weight changes, medication adherence, and inhaler technique 6
• Sleep-related symptoms: if ≥2 positive responses to sleep questionnaire (daytime sleepiness, restlessness, difficulty waking, morning headaches), recommend sleep study 1

Common Pitfalls to Avoid

• Do not proceed with forced expiratory maneuvers within 4-6 weeks of ocular surgery due to risk of increased intraocular pressure compromising surgical repair 7
• Do not interpret "normal" predicted values in patients with skeletal abnormalities (e.g., osteogenesis imperfecta) without considering that prediction equations may overestimate function 1
• Do not rely solely on spirometry for diagnosis—DLCO adds critical diagnostic information, particularly for distinguishing emphysema from asthma in obstructive disease and ILD from chest wall restriction in restrictive patterns 4
• Do not ignore borderline abnormal results (FVC 60-80%, FEV1/FVC 50-70%, oxygen saturation 90-93%) as these warrant specialist evaluation 1
• Maintain consistent testing conditions (same equipment, operator, time of day within 2 hours) for serial monitoring to ensure valid comparisons 1