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.
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.
Normal Values: No acid-fast bacteria are normally found on the stained specimen.
What abnormal results mean: Abnormal results may indicate an infection with an acid-fast bacteria. Acid-fast bacteria include those that cause tuberculosis, non-tuberculous infections, and nocardia.
What the risks are: The risks are minimal and are associated with the method of specimen collection.
Special considerations: If a tissue biopsy is required for the test, other diagnostic tests may be performed on the same specimen.
