What is the preferred initial diagnostic test, spirometry or pulmonary function test (PFT), for a pregnant patient with asthma?

Spirometry is the Preferred Initial Diagnostic Test for Asthma in Pregnancy

Spirometry should be used as the preferred initial diagnostic test for pregnant patients with suspected asthma, rather than relying on peak flow meters or other pulmonary function tests. 1, 2

Why Spirometry is Essential

Spirometry is an essential objective measure to establish the diagnosis of asthma because medical history and physical examination alone are not reliable means of excluding other diagnoses or accurately assessing lung status. 1 This is particularly critical in pregnancy where accurate diagnosis directly impacts both maternal and fetal outcomes. 2

Key Diagnostic Criteria Using Spirometry

• Spirometry demonstrates airflow obstruction and assesses reversibility, which is the cornerstone of asthma diagnosis. 1
• Reversibility is confirmed by an increase in FEV1 of >200 mL AND 12% from baseline after inhaling a short-acting beta-agonist (SABA). 1
• Some evidence suggests that a 10% increase in predicted FEV1 after SABA may better differentiate asthma from COPD, though the 12% threshold remains standard. 1

Why NOT Peak Flow Meters

Peak flow meters are explicitly NOT recommended for diagnosis because of wide variability in devices and reference values—they are designed for monitoring established asthma, not as diagnostic tools. 1 This distinction is crucial: while PEF monitoring may help track control in diagnosed patients, it lacks the diagnostic precision needed for initial evaluation. 3

Pregnancy-Specific Considerations

• Monthly monitoring of asthma symptoms and lung function throughout pregnancy is recommended, as asthma worsens in one-third of pregnant women and improves in one-third. 1, 3, 2
• Spirometry should be performed between 21-27 weeks of gestation when asthma exacerbations are most likely to worsen. 4
• Advancing gestation negatively affects FVC% and FEV1% due to extrapulmonary restriction from the enlarging uterus, but optimal asthma management can oppose these negative effects. 5

When Additional PFTs May Be Needed

Additional pulmonary function studies beyond basic spirometry are NOT routinely necessary but may be useful when considering alternative diagnoses: 1

• Diffusing capacity (DLCO) testing if COPD is suspected 1
• Lung volume measurements if a restrictive defect is suspected 1
• Inspiratory flow-volume loop evaluation if vocal cord dysfunction (VCD) is suspected 1
• Bronchoprovocation testing with methacholine may be useful when asthma is suspected but spirometry is normal or near-normal, though this should only be performed by trained personnel for safety reasons 1

Common Pitfalls to Avoid

• Do not rely on clinical symptoms alone or delay objective testing—patients' perceptions of airflow obstruction are highly variable. 1
• Do not use peak flow meters for initial diagnosis, even though they may seem more convenient. 1
• Do not withhold spirometry testing due to pregnancy concerns—it is safe and essential for proper diagnosis and management. 2
• Do not assume normal spirometry rules out asthma in pregnant patients with classic symptoms; consider bronchoprovocation testing if clinical suspicion remains high. 1

Clinical Algorithm

1. Obtain detailed medical history focusing on episodic symptoms of airflow obstruction (cough worse at night, recurrent wheeze, difficulty breathing, chest tightness). 1
2. Perform spirometry to measure FEV1, FVC, and FEV1/FVC ratio. 1, 2
3. If obstruction is present (decreased FEV1/FVC ratio), assess reversibility with SABA and repeat spirometry. 1
4. If spirometry is normal but clinical suspicion remains high, consider bronchoprovocation testing. 1
5. Reserve additional PFTs (DLCO, lung volumes) only for cases where alternative diagnoses need exclusion. 1