03. Approach to PFTs

Resident Editor:  Alfredo Aguirre, M.D.

Faculty Editor:  Molly Cooke, M.D.


✔ Pulmonary function tests help diagnose obstructive and restrictive lung disease.

✔ Step 1:  Look at FEV1/FVC and flow-volume loop.  

✔ Step 2:  Look at FVC.

✔ Step 3:  Look at TLC.

✔ Step 4:  Look at diffusion capacity (DLCO).


  • 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 <5th 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.


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.


  • 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.

Test Ordered


Obstructive Pattern % Predicted

Restrictive Pattern % Predicted

Forced Vital Capacity (FVC)

2.0-3.0 Liters

1.5 (60%)

1.5 (60%)

Forced Expiratory Volume in 1 second (FEV1)

1.6-2.4 Liters

0.8 (38%)

1.2 (60%)

FEV1/FVC Ratio

63.4-95.2 %

50 %


Residual Volume (RV)

1.5-2.3 Liters

3.6 (191%)

1.5 (78%)

Total Lung Capacity (TLC)

3.6-5.4 Liters

5.2 (115%)

3.0 (66%)

Diffusing Capacity (DLCO)

17.6-26.5 mL/mn/mmHg

3.2 (14%)

7.0 (31%)

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


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