How do you interpret Pulmonary Function Tests (PFTs) results to guide treatment?

Interpreting Pulmonary Function Tests to Guide Treatment

Pulmonary function tests (PFTs) should be interpreted using a systematic, algorithmic approach that evaluates test quality first, then compares results with reference values, identifies specific physiological patterns, and finally answers the clinical question that prompted the test.

Step-by-Step Interpretation Algorithm

• Begin by reviewing technical quality of the test, ensuring proper technique was used and results meet acceptability and reproducibility criteria before relying on numerical results 1, 2
• Compare patient results with predicted values based on age, sex, height, and ethnicity, using appropriate reference equations 1
• Identify the pattern of abnormality (obstructive, restrictive, mixed, or normal) based on specific parameters 1, 2
• Assess severity of the abnormality to guide treatment decisions 2, 3
• Consider the clinical context and question that prompted the test 2, 4

Pattern Recognition and Classification

Obstructive Pattern

• Characterized by reduced FEV1/VC ratio below the 5th percentile of predicted value 1
• Flow is less than expected over the entire volume range 1
• TLC may be normal or increased, and RV is often increased 1
• Typical examples include asthma, COPD, and bronchiectasis 1, 2
• Assess for reversibility (improvement of FEV1 or FVC by at least 12% and 0.2L in adults after bronchodilator) 3

Restrictive Pattern

• Characterized by reduced TLC below the 5th percentile of predicted value with normal FEV1/VC ratio 1
• Flow is higher than expected at a given lung volume 1
• VC is typically reduced, but a reduced VC alone does not prove restriction 1
• Requires measurement of lung volumes to confirm diagnosis 1, 2
• Examples include interstitial lung disease, chest wall disorders, and neuromuscular diseases 1, 5

Mixed Pattern

• Shows features of both obstruction and restriction 1
• Characterized by low TLC and low FEV1/VC ratio 1
• Examples include combined pulmonary fibrosis and emphysema 1, 5

Special Consideration: Normal Spirometry with Air Trapping

• May indicate early small airway disease despite normal FEV1/FVC ratio 6
• Characterized by concave appearance toward the end of expiratory flow-volume loop 6
• Normal TLC with increased RV and RV/TLC ratio 6
• Consider measuring slow vital capacity (SVC) for more accurate assessment 6

Assessing Severity of Abnormalities

• For obstructive, restrictive, and mixed defects, severity is primarily based on FEV1 % predicted 2:

  • Mild: >70%
  • Moderate: 60-69%
  • Moderately severe: 50-59%
  • Severe: 35-49%
  • Very severe: <35%

• Low diffusing capacity (DLCO) values (<60%) are associated with higher mortality and pulmonary morbidity 2

Clinical Application to Guide Treatment

Obstructive Diseases

• In asthma, monitor FEV1 and FVC to assess response to bronchodilator and controller medications 7
• In COPD, FEV1 correlates with symptom severity and prognosis 2
• Consider bronchodilator reversibility testing to guide therapy choices 3
• For patients with FEV1 <70% predicted, a therapeutic trial of bronchodilator may be appropriate 3

Restrictive Diseases

• In interstitial lung disease, monitor both FVC and DLCO to assess disease progression 2
• VC may be only slightly impaired despite marked loss of diffusing capacity 2
• Both FEV1 and DLCO should be measured systematically in preoperative evaluation of patients with lung cancer 2

Mixed Patterns

• Require comprehensive assessment of both components 1
• Treatment should target both the obstructive and restrictive elements 1, 2

Quality Control Considerations

• A quality-control program with continuous monitoring of technician performance is critical 1
• Feedback to technicians should include information about unacceptable maneuvers, corrective actions, and positive reinforcement 1
• Manufacturers should include quality-control aids in their software packages 1
• For DLCO measurements, a grading scheme based on maneuver acceptability should be used 1

Common Pitfalls and Caveats

• Relying solely on computer interpretations without reviewing test quality 2
• Using FEV1/VC ratio to determine severity of obstruction instead of FEV1 % predicted 2
• Failing to measure lung volumes when a restrictive pattern is suspected based on spirometry alone 2
• Not adjusting DLCO for hemoglobin and carboxyhemoglobin 2
• Special attention must be paid when FEV1 and FVC are concomitantly decreased with normal FEV1/FVC ratio, which may reflect incomplete exhalation or early airway collapse 1
• A reduced VC with normal FEV1/VC ratio does not prove restriction without confirming reduced TLC 1
• Single-breath VA from DLCO test should not be interpreted as demonstrating restriction, as it may underestimate TLC, especially in airflow obstruction 1

Race and Ethnicity Considerations

• Consider the impact of race and ethnicity on interpretation, especially in borderline cases 1, 2
• Statistically significant differences in FEV1 and FVC (but not FEV1/FVC ratio) have been observed between racial/ethnic groups 1
• The interpretation strategy should be consistent within a laboratory to avoid inferring a change in patient condition when it is actually due to a change in interpretation approach 1