What parts of a pulmonary function test (PFT) should be completed to diagnose chronic bronchitis in a patient with a history of smoking or exposure to pollutants?

Pulmonary Function Testing for Chronic Bronchitis Diagnosis

Spirometry with post-bronchodilator testing is the mandatory cornerstone test to diagnose chronic bronchitis, and should be supplemented with static lung volumes and diffusing capacity for carbon monoxide (DLCO) to fully characterize the disease and exclude other pathology. 1

Essential PFT Components

Spirometry (Mandatory)

• Post-bronchodilator spirometry is required to establish the diagnosis - a post-bronchodilator FEV1/FVC ratio <0.70 confirms persistent airflow limitation and establishes COPD when chronic bronchitis symptoms are present 2
• Pre-bronchodilator spirometry can effectively rule out COPD if normal, but post-bronchodilator measurements must be used to confirm the diagnosis 2
• Measure FEV1 percentage predicted to determine severity classification: Mild (FEV1 ≥80% predicted), Moderate (FEV1 50-79% predicted), Severe (FEV1 30-49% predicted), Very Severe (FEV1 <30% predicted) 1, 3
• Assess bronchodilator response: ≥12% and ≥200 mL improvement in FEV1 suggests a reversible component, though this does not exclude chronic bronchitis 1

Static Lung Volumes (Strongly Recommended)

• Measure residual volume (RV) and total lung capacity (TLC) to detect gas trapping, which is common in chronic bronchitis 2
• An elevated RV/TLC ratio indicates air trapping and hyperinflation, even when FEV1/FVC may appear falsely normal due to reduced FVC from gas trapping 2, 4
• This is critical because "volume responders" may have pre-bronchodilator FEV1/FVC ≥0.7 due to gas trapping, but post-bronchodilator testing reveals FEV1/FVC <0.7 as FVC improves more than FEV1 2

Diffusing Capacity for Carbon Monoxide (DLCO) (Strongly Recommended)

• DLCO measurement is critical for distinguishing chronic bronchitis from emphysema - chronic bronchitis typically has preserved or mildly reduced DLCO, while emphysema shows marked DLCO reduction 1
• Low DLCO in smokers with normal spirometry identifies those at high risk (22% vs 3%) of developing COPD with obstruction within 3-4 years 5
• DLCO correlates with small airway dysfunction and can be reduced even when spirometry appears normal 6

Algorithmic Approach to Interpretation

Step 1: Confirm Clinical Diagnosis

• Verify chronic cough and sputum production occurring on most days for at least 3 months per year for at least 2 consecutive years 2
• Exclude other respiratory or cardiac causes of chronic productive cough 2
• Document exposure history: smoking (>10 pack-years), occupational dusts, biomass fuels, or environmental pollutants 2

Step 2: Perform Post-Bronchodilator Spirometry

• Administer 400 mcg albuterol (or equivalent) and wait 15 minutes 3
• If post-BD FEV1/FVC <0.70: COPD confirmed, proceed to severity classification 2
• If post-BD FEV1/FVC ≥0.70 but FEV1 <80% predicted: This is PRISm (Preserved Ratio Impaired Spirometry), proceed to Step 3 4, 7
• If both ratios are normal: COPD effectively excluded, but proceed to Step 3 if symptoms persist 2

Step 3: Add Lung Volumes and DLCO

• Measure RV/TLC: If elevated with normal FEV1/FVC, suggests gas trapping from small airway disease 2, 4
• Measure DLCO: If reduced with normal spirometry, indicates emphysematous changes or small airway disease and predicts future COPD development 1, 5
• Assess small airway function parameters: MEF25%, MEF50%, MMEF25-75% are typically reduced in chronic bronchitis even with preserved FEV1/FVC 4

Step 4: Classify and Monitor

• For confirmed COPD (FEV1/FVC <0.70): Classify severity by FEV1% predicted and initiate treatment 1, 3
• For PRISm pattern: These patients have small airway dysfunction and require close monitoring as they may progress to obstructive disease 4, 7
• For normal spirometry with low DLCO: Monitor closely with repeat spirometry every 6-12 months, as 22% will develop obstruction within 3-4 years 5

Critical Pitfalls to Avoid

Do Not Rely on Clinical Diagnosis Alone

• Spirometry is mandatory - clinical symptoms alone lead to overdiagnosis, with 88.4% of self-reported or physician-diagnosed chronic bronchitis cases not meeting standard criteria 2
• Physical examination is rarely diagnostic and cannot detect airflow limitation until lung function is significantly impaired 2

Do Not Skip Post-Bronchodilator Testing

• Pre-bronchodilator testing alone misses "volume responders" who have gas trapping causing falsely normal FEV1/FVC ratios that only become abnormal after bronchodilation 2
• Post-bronchodilator measurements reduce COPD overdiagnosis by identifying "flow responders" whose obstruction reverses with bronchodilators 2

Do Not Omit DLCO Measurement

• DLCO is essential to distinguish chronic bronchitis (preserved DLCO) from emphysema (reduced DLCO), which have different prognoses and may influence treatment decisions 1
• Normal spirometry with reduced DLCO identifies a high-risk group requiring intensive monitoring and smoking cessation efforts 5

Do Not Assume All Chronic Cough is Chronic Bronchitis

• Approximately one-third of patients with chronic cough actually have asthma, which requires different management 1
• Other causes include congestive heart failure, bronchiectasis, tuberculosis, and gastroesophageal reflux 2

Special Considerations

Occupational Chronic Bronchitis

• The same PFT battery (spirometry, lung volumes, DLCO) should be used for patients with occupational dust exposures (cotton, coal, silica, welding fumes, organic dusts) 2, 1
• Document temporal relationship between workplace exposure and symptom onset 2

Repeat Testing for Borderline Results

• When post-bronchodilator FEV1/FVC is close to 0.70 threshold, repeat testing is essential to ensure correct diagnosis 2
• Serial spirometry over time helps identify "flow responders" who may develop persistent obstruction during follow-up 2