Acid-fast stain

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Acid-fast stain

Definition:

The acid-fast stain is a test used to examine a specimen for the presence of microorganisms, specifically mycobacteria which include the bacteria that cause tuberculosis.

How the test is performed:

Specimens of blood, urine, stool, sputum, bone marrow, or tissue may be collected, depending on the location of the suspected infection.

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The specimen is then sent to a microbiology laboratory for processing. A small amount of the specimen is placed on a microscope slide. A dye is placed on the slide and heated. The cells retain the dye. The slide is then washed with an acid solution, and a counter-stain is applied.

The bacteria that retain the first dye are acid-fast because they resist the acid wash. Bacteria that wash free of the first dye and take the counterstain are nonacid-fast.

How to prepare for the test:

Preparation for the test depends on the sample to be collected. There may be special preparation for a blood, urine, or stool collection. A biopsy of an organ or tissue may be indicated for certain types of infection.

How the test will feel:

The amount of discomfort depends on the type of specimen to be collected. A tissue biopsy or aspiration with a needle may be necessary for some tests.

Why the test is performed:

The test detects infection caused by mycobacteria. The test is useful in detecting tuberculous infection.