DefinitionThe 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 and other illnesses.
How the test is performedYour health care provider will collect a sample of blood, urine, stool, sputum, bone marrow, or tissue, depending on the location of the suspected infection.The sample is then sent to a laboratory, where a small amount is placed on a glass slide, stained, and heated. The cells in the sample hold onto the dye. The lab team member washes the slide with an acid solution and applies a different stain.The bacteria that hold onto the first dye are considered "acid-fast" because they resist the acid wash. This type of bacteria is associated with tuberculosis and other infections.
How to prepare for the testPreparation depends on how the sample is collected. Your health care provider will tell you how to prepare.
How the test will feelThe amount of discomfort depends on how the sample is collected.
Why the test is performedThe test can tell if you have come into contact with the bacteria that causes tuberculosis and related infections.
Normal ValuesA 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 results.
What abnormal results meanAbnormal results may be due to: Tuberculosis and related infectionsNocardia infections
What the risks areRisks depend on how the sample is collected. Ask your health provider to explain the risks and benefits of the medical procedure.
ReferencesSeptimus EJ. Pleural effusion and empyema.In: Mandell GL, Bennett JE, Dolin R, eds.Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 65.Murray PR, Witebsky FG. The clinician and the microbiology laboratory. In: Mandell GL, Bennett JE, Dolin R, eds.Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 17.