When to Order Pulmonary Function Testing: A Practical Guide for the Internist

Dr Neeraj Manikath , claude.ai

Abstract

Pulmonary function testing (PFT) remains an essential diagnostic tool in internal medicine, yet its appropriate utilization continues to challenge practitioners. This review provides evidence-based guidance on indications for PFT ordering, interpretation pearls, and common pitfalls. Understanding when to order—and when not to order—PFTs optimizes patient care while avoiding unnecessary testing.

Introduction

Pulmonary function testing encompasses spirometry, lung volumes, diffusing capacity, and additional specialized testing that quantifies respiratory system mechanics and gas exchange. Despite widespread availability, PFTs are frequently underutilized in appropriate clinical scenarios and overordered in others. The American Thoracic Society (ATS) and European Respiratory Society (ERS) have established standardized guidelines for PFT performance and interpretation, yet clinical application requires nuanced judgment.

This review addresses practical scenarios internists encounter, focusing on when PFTs add diagnostic value versus when clinical assessment or alternative testing suffices.

Technical Foundations: What Are We Actually Measuring?

Spirometry measures forced expiratory volume in one second (FEV₁), forced vital capacity (FVC), and their ratio. This effort-dependent test requires patient cooperation and proper technique.

Lung volumes via body plethysmography or gas dilution methods measure total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC), distinguishing restrictive from obstructive patterns when spirometry alone is ambiguous.

Diffusing capacity for carbon monoxide (DLCO) assesses gas transfer across the alveolar-capillary membrane, providing insights into parenchymal lung disease, pulmonary vascular disorders, and anemia effects.

Pearl #1: Spirometry alone misses approximately 30% of restrictive lung disease cases. If clinical suspicion for restriction exists despite normal spirometry, order full PFTs with lung volumes.

Established Indications for PFT Ordering

1. Evaluation of Unexplained Dyspnea

Dyspnea evaluation represents the most common indication for PFTs. However, the key word is "unexplained."

Order PFTs when:

• Dyspnea persists despite treatment of obvious causes (anemia, heart failure, deconditioning)
• Physical examination or chest imaging suggest pulmonary pathology
• Differentiating cardiac versus pulmonary dyspnea remains unclear after initial workup
• Assessing whether dyspnea is disproportionate to known disease severity

Hack: In patients with both cardiac and pulmonary disease, a reduced DLCO <60% predicted with normal or near-normal lung volumes suggests significant pulmonary vascular component, potentially warranting right heart catheterization consideration.

Studies demonstrate that PFTs alter management in 45-60% of patients with unexplained dyspnea, making this a high-yield indication.

2. Chronic Cough Evaluation

The American College of Chest Physicians recommends PFTs in chronic cough (>8 weeks duration) evaluation, particularly when:

• Chest radiograph is normal
• Cough persists despite empiric treatment for common causes
• Assessing for bronchial hyperreactivity or occult airflow obstruction

Pearl #2: Approximately 25% of patients with chronic cough have undiagnosed asthma. Bronchoprovocation testing (methacholine challenge) should be considered when spirometry is normal but asthma remains suspected.

3. Asthma Diagnosis and Management

Initial diagnosis: Spirometry demonstrating reversible airflow obstruction (FEV₁ improvement ≥12% and ≥200 mL post-bronchodilator) confirms asthma diagnosis. However, normal spirometry doesn't exclude asthma.

Oyster: Patients often present when relatively asymptomatic. If clinical suspicion remains high despite normal spirometry, consider:

• Repeating spirometry during symptomatic periods
• Bronchoprovocation testing
• Peak flow monitoring with variability >20%

Monitoring: Annual spirometry in asthmatics identifies those with progressive airflow obstruction who may benefit from intensified therapy. The GOAL study demonstrated that objective monitoring with spirometry improved asthma control scores compared to symptom-based management alone.

4. COPD Diagnosis, Staging, and Prognostication

PFTs are mandatory for COPD diagnosis per GOLD guidelines. Post-bronchodilator FEV₁/FVC <0.70 confirms airflow obstruction.

Hack: FEV₁ alone determines COPD severity staging and correlates with mortality, but don't overlook:

• DLCO predicts mortality better than FEV₁ in some COPD cohorts
• Combined FEV₁ <50% predicted plus DLCO <50% predicted identifies very high-risk patients
• TLC and RV help assess hyperinflation, guiding decisions about lung volume reduction procedures

The Copenhagen City Heart Study showed that DLCO decline predicted mortality independent of FEV₁, emphasizing comprehensive PFT's prognostic value.

5. Interstitial Lung Disease (ILD) Evaluation and Monitoring

Diagnostic phase: Full PFTs with DLCO are essential when ILD is suspected based on imaging or clinical presentation. Classic patterns include:

• Reduced TLC and DLCO
• Preserved or increased FEV₁/FVC ratio
• DLCO often decreases before lung volumes decline

Oyster: Early ILD may present with isolated DLCO reduction and normal lung volumes. In patients with systemic sclerosis, rheumatoid arthritis, or drug exposures (amiodarone, methotrexate, nitrofurantoin), obtain baseline PFTs even with normal chest imaging.

Monitoring: Serial PFTs every 3-6 months guide treatment decisions. A ≥10% decline in FVC or ≥15% decline in DLCO indicates disease progression warranting treatment escalation.

Pearl #3: In ILD monitoring, absolute FVC decline (in mL) matters more than percentage predicted changes. A 150-200 mL FVC decline over 6-12 months predicts mortality and should trigger therapeutic intervention.

6. Preoperative Risk Assessment

Preoperative PFTs are indicated for:

• Thoracic surgery candidates: FEV₁ and DLCO predict postoperative complications and help calculate predicted postoperative lung function
• Lung resection candidates: FEV₁ and DLCO both >60% predicted generally allow pneumonectomy; values 30-60% require split-lung function studies
• Abdominal surgery in patients with known or suspected lung disease: Controversial, but reasonable when lung disease is moderate-severe

Hack: Don't order preoperative PFTs "just because" in patients with mild lung disease undergoing non-thoracic surgery. Clinical assessment, functional capacity, and perioperative optimization matter more than PFT numbers.

Studies show routine preoperative spirometry in unselected surgical candidates doesn't improve outcomes and increases costs without benefit.

7. Occupational and Environmental Lung Disease

PFTs establish baseline function for workers with exposures (asbestos, silica, coal dust, organic dusts) and document disease progression. Serial testing at 1-3 year intervals depending on exposure intensity is standard practice.

Medicolegal consideration: Well-documented PFTs are often crucial for disability and workers' compensation determinations.

8. Sarcoidosis Evaluation and Monitoring

While diagnosis requires tissue confirmation, PFTs serve multiple roles:

• Baseline assessment of physiologic impairment
• Monitoring treatment response
• Detecting late fibrotic complications

Pearl #4: Sarcoidosis can cause obstruction (endobronchial disease), restriction (parenchymal involvement), or isolated DLCO reduction (granulomatous infiltration). Don't assume sarcoidosis always causes restriction.

9. Unexplained Hypoxemia

When arterial blood gas demonstrates hypoxemia and chest imaging is unrevealing, PFTs with DLCO help differentiate:

• Parenchymal disease (reduced DLCO with restrictive pattern)
• Pulmonary vascular disease (reduced DLCO with normal/near-normal volumes)
• Shunt physiology (normal PFTs, prompting contrast echocardiography)

10. Neuromuscular Disease Monitoring

Patients with myasthenia gravis, muscular dystrophies, ALS, or diaphragm paralysis benefit from:

• Supine and upright spirometry: >20% FVC decline when supine suggests diaphragm weakness
• Maximal inspiratory and expiratory pressures (MIP/MEP): Detect respiratory muscle weakness before FVC declines
• Serial monitoring guides non-invasive ventilation initiation

Hack: In suspected diaphragm paralysis, order upright and supine FVC. A supine FVC decline ≥25% is highly suggestive, potentially avoiding the need for fluoroscopic sniff testing.

When NOT to Order PFTs

Understanding when PFTs add limited value prevents unnecessary testing:

1. Acute Illness

PFTs during acute exacerbations of asthma, COPD, or pneumonia are unreliable and rarely alter immediate management. Exception: Severe asthma exacerbations where objective measurement guides admission decisions.

2. Well-Controlled Single Disease

Stable, well-controlled asthmatics or COPD patients without symptom changes don't require frequent PFTs. Annual spirometry in moderate-severe disease is reasonable; more frequent testing lacks evidence unless symptoms change.

3. Terminal Illness or When Results Won't Alter Management

If a patient has end-stage disease and treatment goals are purely symptomatic, PFTs add burden without benefit.

4. Inability to Cooperate

Cognitive impairment, language barriers without interpretation, or physical inability to perform maneuvers makes PFTs unreliable. Don't waste resources on poor-quality studies.

Oyster: Sometimes attempting PFTs in seemingly uncooperative patients succeeds with patience and coaching. But after two failed attempts, stop.

Interpretation Pearls and Common Pitfalls

Pearl #5: The "rule of thirds" for DLCO interpretation:

• Reduced DLCO with normal spirometry/volumes: Think pulmonary vascular disease, early ILD, anemia, or pulmonary hemorrhage
• Reduced DLCO proportionate to volume loss: Consistent with restriction
• Reduced DLCO out of proportion to obstruction: Suggests emphysema over chronic bronchitis

Pearl #6: Fixed airflow obstruction isn't always COPD. Consider:

• Bronchiolitis obliterans (post-transplant, toxin exposure)
• Tracheal stenosis (prior intubation, vasculitis)
• Bronchiectasis

Pearl #7: Race-specific reference equations matter. Using inappropriate reference values leads to misdiagnosis, particularly affecting Black and Asian patients.

Oyster: A "restrictive pattern" on spirometry (reduced FVC with normal FEV₁/FVC) isn't true restriction unless lung volumes confirm reduced TLC. Up to 30% have pseudorestriction from incomplete exhalation or obesity without true restriction.

Hack: When spirometry shows mild reduction in FEV₁ and FVC with preserved ratio, and you're uncertain about restriction versus poor effort, check the flow-volume loop. A "bent" or "scooped" configuration suggests early obstruction despite a numerically preserved ratio.

Special Populations

Obesity

Obesity reduces FRC and ERV but often preserves TLC and DLCO. Obese patients may have "restrictive" spirometry without true restriction. Don't assume dyspnea in obesity is purely mechanical without checking for coexisting asthma (common) or obesity hypoventilation syndrome.

Elderly Patients

Age-related decreases in FEV₁, FVC, and DLCO occur normally. Ensure proper reference equations are used. Lower limit of normal (LLN) methodology is superior to fixed ratios (like FEV₁/FVC <0.70) in elderly patients to avoid COPD overdiagnosis.

Heart Failure

Patients with heart failure may have reduced DLCO from interstitial edema or pulmonary vascular changes. PFTs help when:

• Dyspnea seems disproportionate to cardiac function
• Distinguishing primary lung disease from cardiac contribution
• Pre-transplant evaluation

The Role of Exercise Testing

While not standard PFTs, cardiopulmonary exercise testing (CPET) deserves mention. When resting PFTs are normal but unexplained dyspnea persists, CPET can:

• Identify exercise-induced bronchospasm
• Detect pulmonary vascular limitation
• Quantify deconditioning versus organ system limitation
• Assess preoperative risk in selected patients

Cost-Effectiveness Considerations

A complete PFT battery costs approximately $200-400 in most U.S. healthcare systems. Given this expense:

• Start with spirometry alone when assessing for simple obstruction
• Add lung volumes and DLCO when spirometry is abnormal or clinical scenario demands (ILD, unexplained dyspnea, preoperative thoracic surgery)
• Reserve bronchoprovocation for specific indications (suspected asthma with normal spirometry)

High-quality studies demonstrate that appropriate PFT utilization reduces downstream costs by guiding accurate diagnosis and avoiding unnecessary treatments.

Future Directions

Emerging technologies including:

• Home spirometry with smartphone integration
• Oscillometry for patients unable to perform forced maneuvers
• Impulse oscillometry detecting small airway disease
• Machine learning algorithms integrating PFT data with clinical variables for enhanced prediction

These may change future practice, but currently lack sufficient validation for routine use.

Conclusion: A Practical Algorithm

Order PFTs when:

1. Unexplained dyspnea after initial evaluation
2. Chronic cough with normal chest X-ray
3. Confirming asthma or COPD diagnosis
4. Suspected or known ILD (diagnosis and monitoring)
5. Preoperative assessment for thoracic surgery or lung resection
6. Neuromuscular disease with respiratory symptoms
7. Occupational lung disease screening/monitoring
8. Systemic diseases with pulmonary manifestations (scleroderma, RA, sarcoidosis)

Include full PFTs (spirometry + volumes + DLCO) when:

• ILD is suspected or confirmed
• Spirometry shows restriction needing confirmation
• Preoperative thoracic surgery evaluation
• COPD with disproportionate symptoms or hypoxemia
• Pulmonary vascular disease suspected

Remember: PFTs are tools, not diagnoses. Integrate results with history, examination, imaging, and clinical context. When in doubt about whether to order PFTs, ask: "Will the results change my management?" If yes, order them. If no, reconsider.

References

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