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 and other illnesses.

Your 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.

Preparation depends on how the sample is collected. Your health care provider will tell you how to prepare.

The amount of discomfort depends on how the sample is collected.

The test can tell if you are infected with mycobacteria, the organism that causes tuberculosis and related infections.

A normal result means no acid-fast bacteria were found on the stained sample.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Abnormal results may indicate an infection with an acid-fast bacteria. Acid-fast bacteria include those that cause tuberculosis, nontuberculous infections, and nocardia infections.

Risks depend on how the sample is collected. Ask your health provider to explain the risks and benefits of the medical procedure.