Guide to Lung Function Tests: Types, Procedures, and Indications for Diagnosis, Monitoring, and Assessment of Respiratory Conditions

What are lung function tests?

Lung function tests are medical tests that are used to assess whether the lungs are functioning properly. These tests measure a range of values to gauge different lung functions. Commonly, these tests analyze lung functions by measuring lung volumes, air flow rates, and gaseous exchange occurring in the lungs.

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Why are lung function tests done?

Lung function tests are typically done to diagnose respiratory conditions like obstructive lung diseases such as asthma and chronic obstructive pulmonary disease (COPD) and restrictive diseases like pulmonary fibrosis. These tests can be used to understand lung functions before major surgery or t

Patient Doing Spirometry

Patient Doing Spirometry. Credit: Jmarchn

o monitor treatment response.

There are many types of lung function tests. They are mentioned below:


It is the most common lung function test performed to assess the volume of air in the lungs during the inspiratory and expiratory phases of respiration. It is also used to measure air flow rate during the normal respiration process.

Indications for spirometry

Spirometry can be used for diagnosing conditions and for monitoring treatment response.

Generally, it is used to diagnose diseases like asthma and COPD, find the cause of breathing problems, and monitor the treatment response.

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The procedure of spirometry

During this test, the patient sits in a chair, and a small clip is placed on the patient’s nose to make it easy for them to breathe through the mouth.

The patient is also provided with a mouthpiece. The mouthpiece is attached to an electronic machine called a spirometer. The patient is asked to breathe in and out by an instructor in different ways.

The spirometer records the patient’s breaths to measure the airflow rate and amount of air.



Plethysmography. Credit: Finchbook01

This is another test that is used to assess lung functions. It is mainly performed to measure the total volume of air in the lungs and the volume of air remaining in the lung after a complete exhalation.

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Indications for plethysmography

Plethysmography is best for diagnosing obstructive lung conditions like asthma and chronic obstructive pulmonary disorder (COPD). This test accurately measures the total lung capacity, which can be challenging to measure using spirometry. It is the most accurate test used to measure volume and pressure inside the lungs.

The procedure of plethysmography

The person sits in an airtight box resembling a telephone booth. A nose clip is placed on the patient’s nose, and the patient is asked to put their lips around a mouthpiece. An instructor will provide directions on when to breathe in and when to breathe out. The connected electronic machine will measure the pressure changes in the airtight box due to breathing and display results about the lung volumes.

Diffusion capacity for carbon monoxide (CO)

This test is used to measure gaseous exchange in the lungs. It informs about the effectiveness of the ability of the lung to transport the inhaled gas from the alveoli into the blood.

What does this test reveal?

The diffusion test for CO reveals information about the functions of the lungs in achieving the optimum levels of gaseous exchange. Many conditions such as anemia and pulmonary fibrosis (hardening of the lungs), can decrease the amount of CO being exchanged. On the other hand, conditions like asthma and polycythemia (increased number of red blood cells) can increase the lung’s diffusion capacity for carbon monoxide.

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The procedure

It is measured by the single-breath method. During this test, the patient inhales a diluted mixture of carbon monoxide (CO) and holds their breath for ten seconds. After this, the patient breathes air into a sensor attached to an infrared analyzer. The analyzer measures the amount of exhaled CO.

Stress exercise test

It is used to assess lung functions during exercise. This test provides information about how lungs function during stressful conditions.

Indications for stress exercise test

It is typically done to assess the causes of shortness of breath in a patient. This test can also measure lung functions in patients with known pulmonary or cardiovascular diseases.

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The procedure

In this test, the patient rides a stationary bicycle that is connected to various types of analyzers. Patient’s blood pressure, pulse rate, and blood oxygen are measured and displayed on many monitors attached to analyzers.

Some other tests such as inert gas dilution and nitrogen washout are also used to understand lung functions.

References (Accessed April 28, 2023) (Accessed April 28, 2023)



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