COPD Diagnostic Tests: Pulmonary Function, Spirometry, and More
Routine lab tests are often done on individuals who have been diagnosed with COPD. These tests include what is known as the Complete Blood Count (CBC) and a basic chemistry profile. Other tests that are ordered for individuals with lung diseases are discussed below.
Arterial Blood Gases
An arterial blood gases (ABGs) test may be ordered to give your doctor more information about your lung health. ABGs determine how well your lungs are getting oxygen into your blood and carbon dioxide out of your blood. A sample of blood is drawn from an artery, most often near the wrist.
The needle stick usually hurts a bit but the information obtained can be very important. The most important measurements in the blood gas sample are the acid/base balance (pH), the carbon dioxide level (PaCO2), the oxygen level (PaO2) and oxygen saturation (SaO2).
- pH is a measurement of the acid/base balance of the body. Body fluids are mainly water, but they do contain a mixture of acids and bases. A pH below 7.35 indicates "acidosis" or too much acid. A pH above 7.45 indicates "alkalosis" or too much base. To function properly, your body tries to maintain a balanced pH that falls between these two values.
- PaCO2 measures the pressure of carbon dioxide in the arterial blood. It is measured by units called millimeters of mercury (mmHg). This pressure is related to the amount of carbon dioxide in the blood.
- PaO2 measures the pressure of oxygen in the arterial blood. It is also measured in millimeters of mercury (mmHg). The total amount of oxygen carried in the blood is a complex function. It is based on the PaO2, the number of red blood cells in your blood, the amount of hemoglobin in each red blood cell and the ability of hemoglobin to carry oxygen. Hemoglobin is a protein in the red blood cells that carries oxygen throughout the body. The PaO2 number decreases somewhat as we get older. A low number may mean abnormal lung function.
- SaO2 measures what percent of the hemoglobin in your red blood cells is carrying oxygen.
A pulse oximeter is a simple device used to estimate the percent of oxygen in your blood (SpO2). The oximeter can be clipped on your finger or an ear lobe and provides immediate results. Pulse oximetry is not as accurate as getting the oxygen saturation level from an arterial blood gas test. But it is a quick and easy test for getting oxygen levels. Most doctors try to keep oxygen saturations at 88 percent or better. It is normal for saturation values to vary during exercise, sleep and other activities.
The oximeter relies on the fact that hemoglobin that is carrying oxygen is a different color than hemoglobin that is not carrying oxygen. Blood traveling through a vein after it has delivered oxygen to the body is dark red. Blood in an artery that is still carrying oxygen is bright red. By shining a special light through the skin, the oximeter measures how much light is absorbed by the hemoglobin. It then determines what percent of the hemoglobin is carrying oxygen.