Resident Editor:  Alfredo Aguirre, M.D.

Faculty Editor:  Molly Cooke, M.D.

Indications

• Pulmonary function tests (PFTs) compare individual patient data with reference (predicted) values based on healthy subjects.
• There are 3 basic parts to the pulmonary function test:
  • Spirometry: measures changes in volume over time (e.g., forced expiratory maneuvers).
  • Lung volumes: measures static lung volumes, such as TLC and RV.
  • Diffusing capacity: assess diffusion defects at the alveolar-capillary level.
• Abnormal results are typically those in the <5^th percentile of reference spirometry data (aka, lower limit of normal, or LLN)
• Used to diagnose suspected pulmonary disease in patients with a suspicious history (chest pain, orthopnea, cough, dyspnea, wheeze), physical findings (chest wall abnormalities, cyanosis, diminished breath sounds, finger clubbing) and/or abnormal labs (blood gas, radiograph)
• To assess progression of known pulmonary disease.
• To assess effectiveness of treatments.
• To screen people at risk for lung disease including smokers and those at risk from occupational exposures or iatrogenic effects such as bleomycin toxicity.

Interpretation 

Step 1:  Look at FEV₁/FVC and flow-volume loop (spirometry).

• FEV₁/FVC ratio:
  • Consider obstructive defect if FEV₁/FVC <70%.
• Flow-volume loop:
  • Curvilinear flow-volume loop (with a concave shape to the expiratory loop) indicates reduced flow at lower lung volumes during expiration and suggests obstruction, even if spirometry values are normal.

Step 2: Look at FVC.

• Can help distinguish between restriction, obstruction and mixed defects.
• Flow-volume loop and spirometry may suggest restrictive defect, but must have low lung volumes to make diagnosis of restrictive lung disease.

Step 3: Look at TLC (lung volumes).

• TLC < LLN = restrictive disease
• TLC increased (>120%) = hyperinflation (air-trapping seen in emphysematous/obstructive disease)

Step 4: Look at Diffusion Capacity (DLCO)

• Look at adjustment for hemoglobin (anemia decreases DLCO) and carboxyhemoglobin (smokers may have high CO levels, which decreases DLCO).
• Adjustment for lung volume (DLCO/VA or DLCO/TLC) is controversial.
• Severity categories:
  • DLCO >60% and <LLN = Mildly decreased.
  • DLCO 40-60% predicted = Moderately decreased.
  • DLCO <40% predicted = Severely decreased.

Bronchodilator testing

• Conduct if obstructive pattern detected.
• If FEV1 increases by 12% and 200cc after bronchodilator therapy = bronchodilator responsive obstructive process, more likely asthma.

Bronchoprovocation:

• Some patients with asthma may not respond to bronchodilation, but if there is high suspicion for exercise-induced or allergic asthma, bronchoprovocation can be utilized for diagnosis.
  • Methacholine challenge: >20% decline in FEV1 in response to methacholine. High sensitivity, poor specificity for asthma.
  • Mannitol inhalation challenge: lower sensitivity but higher specificity.

Differential Diagnosis

• Figure adapted from Pellegrino, et al. 
• *seen in pure obstruction
• ¶ can be seen in obstruction when FEV₁ decreased in proportion to FVC  assess flow-volume loop in this case
• $ can be seen in emphysema with air trapping.
• Isolated low DLCO: consider anemia, pulm vascular disease, early ILD.
• Isolated high DLCO consider L-R intracardiac shunts, asthma, pulmonary hemorrhage, polycythemia.

What to order in clinic

• UCSF clinics:
  • Find “Pulmonary Function” in Medication/Orders tab.
  • There are 3 options for location (Mt. Zion, Parnassus and Parnassus drop-in). Parnassus drop-in only does spirometry.
  • Select desired procedures from drop-down menu (e.g., full PFTs, spirometry only, exercise, etc…)
• ZSFG clinics:
  • Submit eConsult for Pulmonary Function Testing (options include full PFTs, bronchodilator testing, etc…)
• VA clinics:
  • Outpatient consults -> Pulmonary consults -> PFT Formal Pulm Lab

Table 1: Sample PFTS.

This example is based on a 65 year old woman, Ht: 153 cm, Wt: 70 kgs.

Sample PFT norms:  http://www.cdc.gov/niosh/topics/spirometry/RefCalculator.html​

References

Universities Occupational Safety and Health Educational Resource Center and CDC National Institute for Occupational Safety and Health (2003). NIOSH Spirometry Training Guide. Morgantown; Centers for Disease Control and Prevention.

Johnson JD, Theurer WM. A stepwise approach to the interpretation of pulmonary function tests. Am Fam Physician. 2014 Mar 1;89(5):359-66.

Pellegrino R, et al. Series ‘‘ATS/ERS Task Force: Standardisation of lung function testing’’: Interpretative strategies for lung function tests. Eur Respir J 2005; 26: 948–968