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.

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.

Infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

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.

Last Edited: Aug 17. 2005

Johns Hopkins patient information

Last revised: December 4, 2012
by Janet G. Derge, M.D.

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