Acid-fast stain

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.

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

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.

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.

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.

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

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