Lung Function Blood Tests

What are lung function blood tests?1

Blood tests can help find out if a person has chronic obstructive pulmonary disease (COPD). The blood tests are used to measure how well a person’s lungs are able to carry out two important jobs:

  • Absorbing oxygen from the air we inhale and moving it into the bloodstream
  • Removing carbon dioxide from the bloodstream and releasing it into the air we exhale from the lungs

There are millions of tiny air sacs in the lungs where this transfer of oxygen and carbon dioxide between blood and air takes place. Oxygen provides the fuel the body needs to function, and carbon dioxide is a waste product of the body’s functions. Having too little oxygen or too much carbon dioxide in the blood can cause serious problems.

People with COPD have lung damage that keeps this transfer from working as well as it should. The lung function blood tests are used to show if a person has this kind of lung damage, and if so, how much damage there is.

There are two main types of blood tests for COPD patients, or people who might have COPD. The tests measure lung function by measuring the amount of oxygen and carbon dioxide in a person’s blood. They are called:

  • Pulse oximetry test
  • Arterial blood gas test

There are also special kinds of exercise tests that monitor a person’s blood before, during, and after exercise. They can also be used to show how much oxygen a person’s body needs while exercising and while at rest.

What is a pulse oximetry test?1,2

A pulse oximetry test uses a small device called a “pulse oximeter” to measure the amount of oxygen in a person’s bloodstream. The test can be used to see if a person’s oxygen level is lower than the body needs to function well.

The pulse oximeter clips onto a finger, toe, or earlobe. The oximeter sends out a beam of light that passes painlessly through the finger, toe or earlobe. This beam of light enables the device to show how much oxygen is actually in the person’s blood. The measurement is given as a percentage of the largest amount of oxygen the person’s blood is possibly able to carry. For a healthy person, that percentage is usually about 90% or higher. If the person’s level is lower than that, then the blood does not contain enough oxygen. This can be a symptom of COPD or other kinds of respiratory diseases. If a person’s oxygen level is very low, oxygen therapy might be needed to help raise the level to a healthier range.

What is an arterial blood gas test?1,2

An arterial blood gas test measures the levels of both oxygen and carbon dioxide in a person’s blood. Another name for an arterial blood gas test is a “blood gas analysis.”

For this kind of test, a patient’s blood is drawn and sent to a lab to measure the exact levels of oxygen and carbon dioxide. The blood sample is taken with a needle through a blood vessel called an “artery.” This test often uses the artery in a person’s wrist.

An arterial blood gas test is usually a little more accurate than pulse oximetry. Pulse oximetry uses a beam of light to make a good guess about the level of oxygen in the blood, but arterial blood gas testing measures the levels in the blood directly. It also gives information about the amount of carbon dioxide in the blood, while pulse oximetry does not.

What are exercise tests?3

Exercise tests can measure the levels of oxygen and carbon dioxide in a person’s blood before, during, and after exercising. They can also show how much oxygen a person’s body requires while exercising versus rest.

Two common types of exercise tests for people with COPD are:

  • Exercise tolerance test
  • Exercise for desaturation test

During the exercise tests, special patches called “electrodes” are put on the chest to keep track of the patient’s heart rate. A pulse oximeter is clipped on to measure the level of oxygen in the blood. In some cases, patients might have small blood samples drawn for arterial blood gas testing.

During an exercise tolerance test, the patient may be asked to do some kind of exercise for a certain amount of time, such as riding a stationary bike or walking on a treadmill. During an exercise for desaturation test, the patient might be asked to do several periods of treadmill walking. For instance, this could involve walking for 6 minutes, and then resting before walking for another 6-minute period. This sequence might be repeated several times.

The exercise tolerance test shows how well the patient’s lungs can work during different levels of activities. The exercise for desaturation test shows how much oxygen the patient’s body needs during different kinds of conditions. These tests can help to diagnose a person with COPD, to find out the amount of lung damage there is, and to design the best kind of treatment plan.

Written by: Anna Nicholson | Last reviewed: July 2015.
View References
  1. National Heart, Lung and Blood Institute. “What Are Lung Function Tests?” Available at: http://www.nhlbi.nih.gov/health/health-topics/topics/lft/ [Accessed 16 January 2015.]
  2. American Thoracic Society Patient Information Series. “Pulse Oximetry.” Available at: http://patients.thoracic.org/information-series/en/resources/ats-patient-ed-pulse-oximetry.pdf [Accessed 6 February 2015.]
  3. National Jewish Health. “Exercise Tolerance Test or Exercise for Desaturation Test.” Available at: http://www.nationaljewish.org/programs/tests/pulmonary-physiology/exercise/exercise-tolerance/ [Accessed 6 February 2015.]