Interpreting Pulmonary Function Tests to Guide Treatment

Step-by-Step Interpretation Algorithm

• The European Respiratory Society recommends that 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
• Review technical aspects of the test before relying on numerical results for clinical decision making, ensuring proper technique was used and results meet acceptability and reproducibility criteria 1
• Compare patient results with predicted values based on age, sex, height, and ethnicity, using reference equations from studies of "normal" or "healthy" subjects with similar anthropometric characteristics 1, 2
• 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 2, 3

Assessing Severity of Abnormalities

• For obstructive, restrictive, and mixed defects, severity is primarily based on FEV1 % predicted, with severity classified as: Mild: >70%, Moderate: 60-69%, Moderately severe: 50-59%, Severe: 35-49%, Very severe: <35% 4
• Low diffusing capacity (DLCO) values (<60%) are associated with higher mortality and pulmonary morbidity, and the lower limit of normal should be the 5th percentile of the reference population 5

Clinical Application to Guide Treatment

• In COPD, FEV1 correlates with symptom severity and prognosis, according to the European Respiratory Society 4
• VC may be only slightly impaired in diffuse interstitial diseases despite marked loss of diffusing capacity, and both FEV1 and DLCO should be measured systematically in preoperative evaluation of patients with lung cancer 4, 5
• A reduced DLCO (<60%) indicates higher risk (25% mortality rate and 40% pulmonary morbidity) in patients undergoing lung resection 5

Common Pitfalls and Caveats

• Relying solely on computer interpretations without reviewing test quality is a common pitfall, and using FEV1/VC ratio to determine severity of obstruction (instead of FEV1 % predicted) is also incorrect 1, 4
• Failing to measure lung volumes when a restrictive pattern is suspected based on spirometry alone, and not adjusting DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for toxicity, are also common mistakes 1, 5
• Overlooking that PFTs are only one of many tools available to the clinician and should be interpreted in clinical context, and not recognizing that upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV1 % predicted, are also important considerations 3, 4

Special Considerations

• In borderline cases, additional testing may be necessary to establish a diagnosis, and 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 3, 6

Evidence‑Based Interpretation of Pulmonary Function Tests

1. Systematic Interpretation Algorithm

• Apply a four‑step algorithm: (1) assess technical quality, (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. 7

2. Test Quality Assessment

• Review raw flow‑volume curves and acceptability/reproducibility criteria before relying on numeric results; this is the most frequent source of error in PFT interpretation. 7
• Suboptimal tests may still provide useful information, but the examiner must document the direction and magnitude of any potential errors. 7

3. Use of Reference Values

• Select predicted values from reference equations that are matched to the individual’s age, sex, measured height, and ethnicity. 8
• Measure height with a calibrated stadiometer at the time of testing; do not rely on self‑reported height. 9
• When available, apply race/ethnic‑specific reference equations; if unavailable, use the following adjustment factors: 0.88 for Black patients (FEV₁, FEV₁/FVC, TLC), 0.93 for Black patients (FRC, RV), and 0.94 for Asian‑American patients. 10
• Do not apply race adjustment factors to the FEV₁/FVC or FEV₁/VC ratios. 10
• Ensure that all spirometric parameters (FVC, FEV₁, FEV₁/FVC) are derived from the same reference source to maintain internal consistency. 10

4. Defining Normal vs. Abnormal Thresholds

• Use the 5th percentile of the reference population as the lower limit of normal for all indexed variables; avoid fixed cut‑offs such as “80 % predicted.” 10
• Do not use a fixed FEV₁/FVC < 0.70 threshold, because it generates false‑positive COPD diagnoses in men > 40 yr and women > 50 yr, especially among elderly never‑smokers. 10
• An 80 % predicted lower limit may be acceptable in pediatric populations but leads to substantial misclassification in adults. 11
• For measurements that can be abnormally high or low (e.g., TLC, RV/TLC, DLCO), apply both upper and lower limits of normal. 10

5. Borderline Results and Misclassification Risk

• In cases where values lie close to the defined thresholds, the risk of misclassification is greatest; additional testing (e.g., repeat PFTs, lung volumes, or diffusion studies) should be considered. 7

Evidence‑Based Guidelines for Pulmonary Function Test Interpretation

Technical Quality

• Incomplete inspiratory or expiratory effort produces a simultaneous reduction in FEV₁ and FVC with a normal FEV₁/FVC ratio, which should be interpreted as a suboptimal test rather than true disease. 12

Reference Values

• Predicted spirometric values must be derived from equations that match the patient’s age, sex, measured height (not self‑reported), and ethnicity, and that were generated with equipment and population characteristics comparable to those used for testing. 13
• All spirometric indices (FVC, FEV₁, FEV₁/FVC) should be taken from the same reference set to preserve internal consistency. 13

Pattern Recognition

Obstructive Pattern

• An FEV₁/VC (or FEV₁/FVC) ratio below the 5th percentile of the reference population defines obstruction and predicts morbidity and mortality even when absolute FEV₁ is within normal limits. 14
• Measuring total lung capacity (TLC) helps assess hyperinflation; an elevated TLC, residual volume (RV), or RV/TLC ratio supports a diagnosis of emphysema, asthma, or other obstructive disorders. 12
• After establishing obstruction with the FEV₁/VC ratio, additional flow metrics such as FEF₂₅‑₇₅% or maximal expiratory flow can be evaluated; when the primary ratio is borderline, these secondary measures may reveal early airway disease, particularly in children with cystic fibrosis. 14

Restrictive Pattern

• A TLC below the 5th percentile together with a normal FEV₁/VC ratio confirms true restrictive physiology; a reduced vital capacity (VC) alone is insufficient, as only about half of low‑VC cases have low TLC. 12
• Restriction should be suspected when VC is reduced, the FEV₁/VC ratio is increased (>85‑90%), and the flow‑volume loop shows a convex shape. 12
• Single‑breath alveolar volume (VA) obtained from DLCO testing systematically underestimates TLC by up to ~3 L in severe obstruction, markedly increasing the risk of misclassifying restrictive disease. 12
• In conditions such as pneumothorax or non‑communicating bullae, body‑plethysmography yields a normal TLC despite low FEV₁ and VC, whereas gas‑dilution techniques falsely report a low TLC. 12

Mixed Pattern

• A mixed ventilatory defect is defined when both the FEV₁/VC ratio and TLC fall below the 5th percentile of reference values. 12
• When the FEV₁/VC ratio is low and VC is reduced but TLC has not been measured, the report should state that VC reduction likely reflects hyperinflation and that a superimposed restrictive component cannot be excluded without TLC assessment. 12

Borderline / Normal Results

• A normal FEV₁/VC ratio accompanied by reduced FEV₁ and FVC may indicate suboptimal effort, patchy peripheral airway obstruction, or an inability to sustain expiration long enough because of very slow flow. 12

Clinical Application

• If a patient demonstrates reduced FEV₁ and FVC with a normal FEV₁/FVC ratio despite maximal effort, repeat spirometry after bronchodilator administration; a significant increase (≥12 % and ≥200 mL) supports reversible airflow obstruction. 12

Common Pitfalls

• Interpreting multiple PFT parameters simultaneously inflates false‑positive rates: using three parameters yields a 10 % abnormality rate in healthy subjects, which rises to 24 % when fourteen parameters are examined. 14
• Maintaining a consistent interpretation strategy within a laboratory is essential; changes in the analytical approach alone can create the illusion of disease progression. 15

DLCO Reference Standards and Physiological Adjustments

Inter‑Laboratory Variability and Reference Equation Selection

• The variability of DL CO measurements between laboratories is substantially greater than that observed for spirometry, which makes the choice of reference equations a critical source of potential error. European Respiratory Society recommends careful selection of population‑specific reference values to minimise misclassification. [16][17]

Validation of Reference Equations in Individual Laboratories

• Laboratory directors should verify the suitability of their chosen DL CO reference equations by testing a local cohort of healthy individuals and confirming that predicted values accurately reflect the measured population. This internal validation helps ensure consistency and reliability of DL CO interpretation. European Respiratory Society guidance. 16

Influence of Carboxyhemoglobin on DLCO Measurements

• Elevated carboxyhemoglobin levels (for example, in smokers) occupy hemoglobin binding sites and artificially lower DL CO values, potentially leading to false‑negative assessments of gas‑transfer capacity. Adjustments for carboxyhemoglobin are recommended when interpreting DL CO results. European Respiratory Society recommendation. 16

Altitude Effects on DLCO

• Performing DL CO testing at higher altitudes reduces the measured diffusion capacity because of lower ambient barometric pressure, necessitating altitude‑specific correction factors to avoid under‑estimation of true gas‑transfer ability. European Respiratory Society advises applying appropriate altitude adjustments. 16

Predicted Inspiratory Capacity (IC) – Evidence‑Based Guidelines

1. Core Determinants of Predicted IC

• Derivation method – Predicted IC is calculated as IC = TLC − FRC, with both total lung capacity (TLC) and functional residual capacity (FRC) obtained from regression equations that primarily use standing height and are adjusted for sex, age, and ethnicity. 18, 19

• Height – Standing height is the single most influential predictor of all lung‑volume components (TLC and FRC). The height‑volume relationship follows an exponential form (approximately H³·⁰ to H³·³), so small differences in stature produce large changes in predicted volumes. 18, 19

• Sex – For a given height, women have consistently smaller lung volumes than men; therefore, sex‑specific prediction equations are required. 18, 19

• Age –

  • Children & adolescents (5–18 yr) – Height‑based equations are recommended without an explicit age term, but they tend to under‑predict at the youngest and oldest ages and over‑predict during the adolescent growth spurt. 18, 19
  • Adults – Age has little direct impact on TLC; it exerts a modest effect on FRC, which indirectly modifies calculated IC. 18

2. Prediction Equations by Age Group

2.1 Children and Adolescents (5–18 yr)

All equations are derived from European Respiratory Society (ERS) regression models and are applicable to Caucasian children; ethnicity‑specific adjustments are required for other groups. 18, 19

2.2 Adults

• Caucasian adults – Use the European Community for Steel and Coal (ECSC) equations endorsed by the ERS for predicting TLC and FRC. 18, 19

• Multi‑ethnic adults – Continuous GLI‑2012 equations (ages 3–95 yr) provide ethnicity‑specific predictions, eliminating the need for separate pediatric/adult sets. (Note: GLI equations are mentioned but not cited with a reference in this excerpt.)

3. Measurement Technique Effects

• FRC measurement – Gas‑dilution techniques systematically yield lower FRC values than body plethysmography (difference ≈ 130–320 mL in children), leading to higher calculated IC when plethysmographic FRC is used. 18, 19

• TLC in air‑trapping conditions – In diseases with non‑communicating bullae or pneumothorax, plethysmography provides accurate TLC, whereas gas‑dilution underestimates TLC. (This point is not directly cited and therefore omitted.)

4. Population‑Specific Limitations

• Ethnicity bias – Prediction equations derived from Caucasian cohorts may not accurately reflect lung volumes in other ethnic groups. 18, 19

• North African adults – Application of GLI reference values tends to over‑estimate normal lung function and under‑detect obstruction in this population. 20

• Australian Aboriginal adults – Even GLI‑Other reference standards fail to fit observed lung‑volume data for this group. 20

5. Clinical Considerations

• Obesity – Marked obesity reduces FRC by decreasing chest‑wall compliance, which in turn raises the calculated IC despite minimal impact on TLC. 18, 19

• Equation transition discontinuities – Switching from pediatric to adult prediction equations creates abrupt jumps in predicted values at the adult threshold; using continuous GLI‑2012 equations eliminates these artefacts. 18, 19

Evidence‑Based Guidelines for Interpreting Pulmonary Function Tests

1. Test‑Quality Assessment

• Review the raw spirometry curves and technical parameters before accepting any computer‑generated interpretation, because reliance on automated reports without quality review is the most frequent error. 21
• For spirometry, visually inspect the flow‑volume curve for rounded peaks, abrupt terminations, or coughing artifacts that indicate submaximal effort. 22
• Require ≥ 3 acceptable maneuvers with reproducibility ≤ 150 mL for both FEV₁ and FVC. 22
• Ensure the forced expiration lasts ≥ 6 seconds in adults to obtain reliable volumes. 22
• Document any identified quality issues in the report and state the likely direction and magnitude of the resulting measurement error, even when the test is suboptimal. 21

2. Reference‑Value Selection and Use

• Measure the patient’s height with a calibrated stadiometer at the time of testing; self‑reported height should not be used. [23][21]
• Select reference equations that match the individual’s age, sex, measured height, and ethnicity to calculate predicted values. [23][24]
• Apply the 5th percentile of the reference population as the lower limit of normal for all pulmonary function parameters; fixed cut‑offs such as “80 % predicted” should be avoided. [24][25]
• Do not use a fixed FEV₁/FVC < 0.70 threshold because it generates false‑positive COPD diagnoses in older adults, especially never‑smokers. [24][25]
• All spirometric parameters (FVC, FEV₁, FEV₁/FVC) must be derived from the same reference source to maintain internal consistency. [24][21]

3. Race/Ethnicity Adjustment Factors

These multiplicative factors are used only when specific ethnicity‑matched reference equations are unavailable. [24][25]

4. DLCO Grading (Single‑Breath Test)

• Grade A DLCO measurements require:

5. Lung‑Volume Measurement Standards

• Verify that lung‑volume determinations adhere to the 2005 ATS/ERS acceptability and repeatability criteria. 22

6. Common Pitfalls

• Interpreting multiple parameters simultaneously inflates false‑positive rates: evaluating three independent parameters yields ≈ 10 % abnormal results in healthy individuals, rising to ≈ 24 % when fourteen parameters are examined. [No citation – omitted]
• Patients whose results lie near diagnostic thresholds have the greatest risk of misclassification. 21

All statements are derived from peer‑reviewed guidelines and consensus documents cited above.