• Clinical science

Pulmonary function testing (Lung function testing)


Pulmonary function tests (PFTs) measure different lung volumes and other functional metrics of pulmonary function. They can be used to diagnose ventilatory disorders and differentiate between obstructive and restrictive lung diseases. The most common PFT is spirometry, which involves a cooperative patient breathing actively through his or her mouth into an external device. This simple and cost-effective test measures both dynamic and static lung volumes (with the exception of residual volume and total lung capacity), as well as airflow rates. Full-body plethysmography is an additional PFT that is able to estimate both residual volume and total lung capacity and is performed with the patient in a closed space. Lastly, single-breath diffusing capacity helps determine if the alveolar membrane is thickened (e.g., pulmonary fibrosis) or destroyed (e.g., emphysema), or if the pulmonary vasculature is affected (e.g., pulmonary hypertension).


This convenient pulmonary function test measures breath volume and airflow rates and is routinely performed with the help of a spirometer.

Diagnostically relevant spirometric values
Parameter Definition Normal finding
Peak expiratory flow (PEF)
  • The maximum airflow rate attained during forced expiration (in L/s)
  • ≥ 80% of the predicted average value based on race, height, gender, and age

Forced expiratory volume in 1 second (FEV1)

  • The maximum volume of air that can be forcefully expired (forced expiratory volume) within 1 second after maximal inspiration
  • ≥ 80% of the predicted average value based on height, gender, and age (or >75% of vital capacity)

Vital capacity

  • The difference between the volume of air in the lungs after maximal inspiration and after maximal expiration
  • VC can be measured through:
  • Among young individuals IVC, EVC, and FVC have nearly the same value. However among patients with obstructive lung disease: IVC > EVC > FVC.
  • Depends on race, height, age, and gender; approximately 4.5–5 L in healthy young adults

FEV1/FVC (Tiffeneau-Pinelli index, relative FEV1)

  • ≥ 70%
Forced expiratory flow rate at 75%, 50%, and 25% of vital capacity (FEF75%, FEF50%, FEF25%)
  • Average airflow rates observed during forced expiration when 75%, 50%, and 25% of the vital capacity remains in the lungs.
  • ≥ 65% of the predicted average value based on race, height, gender, and age


Obstructive and restrictive lung diseases

Type Obstructive lung disease Restrictive lung disease
  • Increased resistance to air flow caused by narrowing of airways
Spirometric findings FEV1 Normal or ↓
FEV1/FVC Normal or
Vital capacity
Residual volume Normal or
Total lung capacity Normal or ↑
Resistance to air flow Normal
Lung compliance Normal Normal (extrinsic causes) or ↓ (intrinsic causes)
Spirometer tracing

Air trapping: a "scalloping" of the expiratory limb in conditions such as emphysema or in patients who have undergone a pneumectomy.

In restrictive lung disease, alterations in blood gas analysis are seen much later in the course of the disease!

Bronchial challenge tests

The following tests help distinguish bronchial asthma from other causes of obstructive lung disease.

Because the methacholine challenge test can trigger a life-threatening asthma attack, medications that reverse bronchospasm (e.g., epinephrine, atropine) should be kept at hand during the test!


Single-breath diffusing capacity

Single breath diffusing capacity Spirometric findings
Restrictive lung disease (normal or FEV1/FVC) Obstructive lung disease (FEV1/FVC < 70%) Normal


Normal DLCO
  • Healthy findings
  • Position and/or certain maneuvers also affect results.
    • Exercise will increase DLCO through recruitment of additional lung zones and increased capillary transit time


Respiratory muscle function

The following non-invasive tests are used to diagnose and monitor patients with respiratory muscle weakness:

  • Test of inspiratory muscle function (e.g., diaphragm): maximal inspiratory pressure (MIP); , sniff nasal inspiratory pressure (SNIP)
  • Test of expiratory muscle function: : maximal expiratory pressure (MEP)

Patients with respiratory muscle weakness show spirometric findings of restrictive lung disease!

Clinical features of respiratory muscle weakness do not manifest until diaphragmatic strength is reduced to a quarter of its normal strength! (Unilateral diaphragmatic paralysis decreases ventilatory capacity by only 20%.)

Lung volumes

Lung volumes depend on age, height, and gender. The values that are listed below are for a healthy young adult.

Lung volume Definition Normal range
Total lung capacity (TC,TLC) Volume of air in the lungs after maximal inhalation 6–6.5 L
Vital capacity (VC) Difference in lung volume between maximal exhalation and maximal inhalation 4.5–5 L
Residual volume (RV) Volume of air that remains in the lungs after maximal exhalation 1–1.5 L
Tidal volume (TV) Volume of air that is inhaled and exhaled in a normal breath at rest ∼ 500 mL or 7 mL/kg
Inspiratory reserve volume (IRV) Maximum volume of air that can still be forcibly inhaled following the inhalation of a normal TV 3–3.5 L
Inspiratory capacity (IC) Maximum volume of air that can be inhaled after the exhalation of a normal TV 3.5–4 L
Expiratory reserve volume (ERV) Maximum volume of air that can still be forcibly exhaled after the exhalation of a normal TV 1.5 L
Expiratory capacity (EC): Maximum volume of air that can be exhaled after the inspiration of a normal TV 2 L
Functional residual capacity (FRC) Volume of air that remains in the lungs after the exhalation of a normal TV 2.5–3 L


  • 1. Constán E, Medina J, Silvestre A, Alvarez I, Olivas R. Difference Between The Slow Vital Capacity And Forced Vital Capacity: Predictor Of Hyperinflation In Patients With Airflow Obstruction. nternet Journal of Pulmonary Medicine. 2004; 4(2). url: http://ispub.com/IJPM/4/2/5845.
  • 2. Philip H. Quanjer. Lung Function Indices. http://spirxpert.ers-education.org/en/spirometry/lung-function-indices/spirographic-indices-an-overview/. Updated February 19, 2017. Accessed February 19, 2017.
  • 3. Ciprandi G, Capasso M, Tosca M, et al. A forced expiratory flow at 25-75% value <65% of predicted should be considered abnormal: a real-world, cross-sectional study. Allergy Asthma Proc. 2012; 33(1): pp. e5–8. doi: 10.2500/aap.2012.33.3524.
  • 4. Barreiro TJ, Perillo I. An Approach to Interpreting Spirometry. Am Fam Physician. 2004; 69(5): pp. 1107–1115. url: http://www.aafp.org/afp/2004/0301/p1107.html.
  • 5. National Institute for Occupational Safety and Health. NIOSH Spirometry Training Guide 6: Compairing observed to predicted normal Values. url: https://www.cdc.gov/niosh/docs/2004-154c/pdfs/2004-154c-ch6.pdf Accessed February 19, 2017.
  • 6. Culver BH. How should the lower limit of the normal range be defined?. Respir Care. 2012; 57(1): pp. 136–145. doi: 10.4187/respcare.01427.
  • 7. Caronia JR. Restrictive Lung Disease. In: Restrictive Lung Disease. New York, NY: WebMD. http://emedicine.medscape.com/article/301760-overview. Updated October 31, 2016. Accessed February 19, 2017.
  • 8. University of Granada. Lung Diseases: Restrictive and Obstructive. http://www.ugr.es/~jhuertas/EvaluacionFisiologica/Espirometria/restobst.htm. Updated February 19, 2017. Accessed February 19, 2017.
  • 9. Crapo RO, Casaburi R, Coates AL, et al. Guidelines for methacholine and exercise challenge testing-1999: This official statement of the American Thoracic Society was adopted by the ATS Board of Directors, July 1999. Am J Respir Crit Care Med. 2000; 161(1): pp. 309–329. doi: 10.1164/ajrccm.161.1.ats11-99.
  • 10. CDC staff. Respiratory Health Bronchodilator Procedures Manual. https://www.cdc.gov/nchs/data/nhanes/nhanes_07_08/Bronchodilator.pdf. Updated January 1, 2008. Accessed June 22, 2018.
  • 11. McCarthy K. Pulmonary Function Testing . In: Pulmonary Function Testing . New York, NY: WebMD. http://emedicine.medscape.com/article/303239-overview. Updated October 17, 2016. Accessed February 19, 2017.
  • 12. McCormack MC. Diffusing capacity for carbon monoxide. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/diffusing-capacity-for-carbon-monoxide. Last updated February 10, 2016. Accessed February 19, 2017.
  • 13. Puri S, Baker BL, Dutka DP, Oakley CM, Hughes JMB, Cleland JGF. Reduced Alveolar–Capillary Membrane Diffusing Capacity in Chronic Heart Failure. Circulation. 1995; 91(2769-2774). doi: 10.1161/01.CIR.91.11.2769.
  • 14. Wilson JS, Galvin JR. Normal diffusing capacity in patients with PiZ alpha(1)-antitrypsin deficiency, severe airflow obstruction, and significant radiographic emphysema. Chest. 2000; 118(3): pp. 867–871. pmid: 10988219.
  • 15. Espiritu JD, Ruppel G, Shrestha Y, Kleinhenz ME. The diffusing capacity in adult cystic fibrosis. Respir Med. 2003; 97(6): pp. 606–611. doi: 10.1053/rmed.2003.1487.
  • 16. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • 17. Walker BR, Colledge NR, Raston SR, Penman ID. Davidson's Principles and Practice of Medicine. Elsevier; 2013.
  • 18. Moxham J. Tests of respiratory muscle strength. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. http://www.uptodate.com/contents/tests-of-respiratory-muscle-strength. Last updated April 12, 2016. Accessed February 19, 2017.
  • 19. Polkey MI, Green M, Moxham J. Measurement of respiratory muscle strength. Thorax. 1995; 50(11): pp. 1131–1135. url: http://thorax.bmj.com/content/thoraxjnl/50/11/1131.full.pdf.
  • 20. Le T, Bhushan V, Sochat M, Chavda Y. First Aid for the USMLE Step 1 2017. McGraw-Hill Education; 2017.
  • Herold G. Internal Medicine. Cologne, Germany: Herold G; 2014.
  • Criée CP, Sorichter S, Smith HJ, et al. Body plethysmography – Its principles and clinical use. Respir Med. 2011; 105(7): pp. 959–971. doi: 10.1016/j.rmed.2011.02.006.
last updated 08/17/2020
{{uncollapseSections(['gnbFs8', 'ECX8FZ0', 'CCXq8Z0', 's2ctQb0', 'wCXh8Z0', 'rfcfnb0', 'xCXE8Z0'])}}