The acid-fast stain is a laboratory test that determines if a sample of tissue, blood, or other body substance is infected with the bacteria that causes tuberculosis (TB) and other illnesses.
Your health care provider will collect a sample of urine, stool, sputum, bone marrow, or tissue, depending on the location of the suspected infection.
The sample is then sent to a laboratory. Some of the sample is placed on a glass slide, stained, and heated. The cells in the sample hold onto the dye. The slide is then washed with an acid solution and a different stain is applied.
Bacteria that hold onto the first dye are considered "acid-fast" because they resist the acid wash. These types of bacteria are associated with TB and other infections.
Preparation depends on how the sample is collected. Your provider will tell you how to prepare.
The amount of discomfort depends on how the sample is collected. Your provider will discuss this with you.
The test can tell if you are likely infected with the bacteria that cause TB and related infections.
A normal result means no acid-fast bacteria were found on the stained sample.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test result.
Abnormal results may be due to:
Risks depend on how the sample is collected. Ask your provider to explain the risks and benefits of the medical procedure.
Reviewed By: Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School; Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.