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. 1

Step-by-Step Interpretation Algorithm

1. Assess Test Quality

• Review technical aspects of the test before relying on numerical results for clinical decision making 1
• Ensure proper technique was used and results meet acceptability and reproducibility criteria 1
• Tests that are less than optimal may still contain useful information, but limitations should be identified 1

2. Compare Results with Reference Values

• Compare patient results with predicted values based on age, sex, height, and ethnicity 1
• Use reference equations from studies of "normal" or "healthy" subjects with similar anthropometric characteristics 1
• Consider the impact of race and ethnicity on interpretation, as these factors may influence results in borderline cases but are less significant in clearly normal or abnormal results 1

3. Identify Ventilatory Pattern

• Obstructive Pattern:

  • Defined by reduced FEV1/VC ratio below the 5th percentile of predicted value 1
  • Characterized by disproportionate reduction of maximal airflow relative to maximal volume 1
  • Early changes may show concave shape on flow-volume curve and slowing in terminal portion of spirogram 1

• Restrictive Pattern:

  • Characterized by reduced TLC below the 5th percentile of predicted value 1
  • VC is often reduced in proportion to loss of functioning lung parenchyma 1
  • May show preserved or increased FEV1/FVC ratio 1

• Mixed Pattern:

  • Shows features of both obstruction and restriction 1
  • Requires measurement of lung volumes to confirm 1

• Normal Pattern:

  • All parameters within normal limits 1

4. Assess Severity of Abnormalities

• For obstructive, restrictive, and mixed defects, severity is primarily based on FEV1 % predicted 1:

  • Mild: >70%
  • Moderate: 60-69%
  • Moderately severe: 50-59%
  • Severe: 35-49%
  • Very severe: <35% 1

• For diffusing capacity (DLCO):

  • Low values (<60%) are associated with higher mortality and pulmonary morbidity 2
  • The lower limit of normal should be the 5th percentile of the reference population 2

5. Evaluate Bronchodilator Response (if performed)

• Significant response typically defined as ≥12% and ≥200 mL increase in FEV1 or FVC 1
• Response may indicate asthma but can also occur in other conditions 1

6. Compare with Previous Tests (if available)

• Evaluate for significant changes over time 3
• Changes may reflect disease progression or response to treatment 4

Clinical Application to Guide Treatment

Obstructive Diseases

• In COPD, FEV1 correlates with symptom severity and prognosis 1
• For patients with IPF requiring treatment with pirfenidone, baseline eligibility includes %FVC ≥50% and %DLCO ≥30-35% 5
• Treatment response in IPF can be monitored by tracking changes in FVC, with reduced decline indicating treatment efficacy 5

Restrictive Diseases

• VC may be only slightly impaired in diffuse interstitial diseases despite marked loss of diffusing capacity 1
• Both FEV1 and DLCO should be measured systematically in preoperative evaluation of patients with lung cancer 2
• A reduced DLCO (<60%) indicates higher risk (25% mortality rate and 40% pulmonary morbidity) in patients undergoing lung resection 2

Small Airway Disease

• May be identified by late expiratory flattening of the flow-volume loop 6
• FEF25-75% may be reduced out of proportion to FEV1 1
• However, these mid-range flow measurements are not specific for small airway disease in individual patients 1

Common Pitfalls and Caveats

• Relying solely on computer interpretations without reviewing test quality 1
• Using FEV1/VC ratio to determine severity of obstruction (should use FEV1 % predicted instead) 1
• Failing to measure lung volumes when a restrictive pattern is suspected based on spirometry alone 1
• Not adjusting DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for toxicity 2
• Overlooking that PFTs are only one of many tools available to the clinician and should be interpreted in clinical context 1
• Not recognizing that upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV1 % predicted 1

Special Considerations

• The majority of PFTs (53%) are performed to follow disease progress or response to treatment, emphasizing the importance of comparing to previous results 4
• In borderline cases, additional testing may be necessary to establish a diagnosis 1
• PFT interpretation 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