When should a Pulmonary Function Test (PFT) be obtained?

When Should a Pulmonary Function Test (PFT) Be Obtained?

PFTs should be obtained for diagnostic evaluation of unexplained respiratory symptoms (dyspnea, cough), baseline assessment before initiating treatment for known respiratory disease, monitoring disease progression at disease-specific intervals, and preoperative risk assessment for thoracic or upper abdominal surgery.

Diagnostic Indications

Symptomatic Patients Requiring Diagnosis

• Obtain PFT when patients present with unexplained dyspnea or chronic cough to differentiate between obstructive, restrictive, or mixed patterns and guide further workup 1, 2.
• For children with vague respiratory symptoms (cough, shortness of breath) and normal physical examination, PFT with bronchodilator responsiveness testing provides better diagnostic accuracy than baseline measurements alone, though sensitivity remains limited at 76% 1.
• Do NOT obtain PFT when a child presents with acute wheeze that responds to bronchodilator treatment, as clinical observation establishes the diagnosis of reversible airway disease without need for testing 1.

Baseline Assessment for Known Disease

• Obtain baseline PFT (including FVC, TLC, and DLCO) in all patients with newly identified interstitial lung abnormalities (ILAs) on imaging to establish a reference point for future comparison and help distinguish ILA from interstitial lung disease 1.
• For patients with neuromuscular disease (NMD), obtain baseline PFT measuring vital capacity, maximum inspiratory pressure (MIP), maximum expiratory pressure (MEP), sniff nasal inspiratory pressure (SNIP), and peak cough flow (PCF) at diagnosis 1.
• Obtain baseline spirometry before hematopoietic stem cell transplantation (HSCT) in all pediatric patients, with consideration for multiple breath washout testing where technical expertise exists 1.

Monitoring Established Disease

Neuromuscular Disease

• Perform PFT every 6 months in patients with neuromuscular disease as standard surveillance, recognizing this represents a consensus-based interval balancing detection of decline against testing burden 1, 3.
• In rapidly progressive conditions like ALS, consider testing every 3 months, as significant respiratory parameter changes can occur within this timeframe 1, 3.
• In stable or slowly progressing diseases like Duchenne muscular dystrophy, extend intervals to every 12 months 1.

COPD Management

• Perform annual PFT in clinically stable COPD patients to detect progressive lung function decline while avoiding unnecessary testing burden 4.
• For unstable COPD (more than 2 hospitalizations per year, worsening symptoms, or recent exacerbations), increase frequency to every 3 months until stability is demonstrated, then reduce to every 6 months 4, 3.
• For severe COPD (FEV1 <40% predicted), consider testing every 6-12 months even when stable, as these patients face higher risk for rapid deterioration 4.

Interstitial Lung Disease

• Perform PFT every 3-6 months for at least 1 year in patients with ILD to establish disease trajectory and determine treatment need 5.
• DLCO reduction is the earliest and most sensitive PFT abnormality in ILD, often preceding changes in lung volumes 5.

Post-HSCT Surveillance

• Begin spirometry at 3 months post-HSCT, then every 3 months until 12 months, every 3-6 months from 13-24 months, every 6 months from 24-36 months, then annually until 10 years post-HSCT 1.
• Obtain chest CT with inspiratory/expiratory views and perform bronchoscopy with BAL when surveillance PFT shows persistent obstructive lung disease over at least 2 weeks to evaluate for bronchiolitis obliterans syndrome 1.

Screening Considerations (When NOT to Obtain PFT)

Asymptomatic Patients

• Do NOT perform routine screening PFT in asymptomatic children or adults with sickle cell disease, as insufficient evidence exists that screening leads to management changes improving patient-important outcomes (pain, acute chest syndrome, mortality) 1.
• The absence of evidence for screening benefit does not prove screening is harmful, but the balance favors against routine testing in truly asymptomatic patients 1.

Preschool Children

• Recognize that baseline PFT has very poor diagnostic accuracy in preschool children due to substantial overlap between healthy children and those with mild asthma or isolated cough 1.
• Bronchodilator responsiveness testing (BDR ratio ≥1.22) provides better diagnostic profile than baseline measurements, but still carries 30% false-positive rate 1.

Critical Thresholds Triggering Intervention

Neuromuscular Disease

Initiate noninvasive ventilation when PFT shows:

• FVC <80% predicted with symptoms OR FVC <50% predicted without symptoms 1
• MIP <60 cm H₂O or MEP <40 cm H₂O 1
• PCF <270 L/min for age ≥12 years 1
• SNIP <70 cm H₂O in males or <60 cm H₂O in females for age ≥12 years 1

Sarcoidosis Treatment Decisions

• DLCO <60% or FVC <70% indicates clinically significant pulmonary sarcoidosis warranting treatment 6.
• During follow-up, a 5% decline in FVC from baseline or 10% decline in DLCO reflects disease progression 6.

Common Pitfalls to Avoid

• Do not wait for symptom development before repeating PFTs, as lung function decline can precede symptom worsening, particularly in patients who have reduced activity levels 4.
• Do not rely solely on baseline PFT in preschool children with suspected asthma; clinical judgment and atopic status testing must complement PFT interpretation given high false-positive rates 1.
• Recognize that bronchodilator response varies day-to-day and may not correlate with symptomatic benefit; interpret changes in context of clinical status 4.
• For patients unable to perform standard spirometry (e.g., bulbar dysfunction in NMD), consider alternative measurements like SNIP or home-based monitoring 1.

Components to Include

Standard PFT monitoring should include:

• Spirometry (FEV1, FVC, FEV1/FVC ratio) as primary measurement for all visits 4, 3
• DLCO to evaluate gas exchange capacity, particularly for detecting emphysema or ILD progression 4, 5
• Static lung volumes when restrictive pattern is suspected 1, 2
• Arterial blood gases in severe disease to identify persistent hypoxemia or hypercapnia guiding oxygen therapy or ventilation decisions 4