Sputum stain for mycobacteria

Definition

Sputum stain for mycobacteria is a test to check for a type of bacteria that cause tuberculosis and other infections.

Alternative Names

Acid fast bacilli stain; AFB stain; Tuberculosis smear; TB smear

How the Test is Performed

This test requires a sample of sputum. 

• You will be asked to cough deeply and spit any substance that comes up from your lungs (sputum) into a special container.
• You may be asked to breathe in a mist of salty steam. This makes you cough more deeply and produce sputum.
• If you still do not produce enough sputum, you might have a procedure called bronchoscopy.
• To increase the accuracy, this test is sometimes done 3 times, often 3 days in a row.

The test sample is examined under a microscope. Another test, called a culture, is done to confirm the results. A culture test takes a few days to get results. This sputum test can give your doctor a quick answer.

How to Prepare for the Test

Drinking fluids the night before the test helps your lungs produce phlegm. It makes the test more accurate if it is done first thing in the morning.

If you are having a bronchoscopy, follow your health care provider's instructions on how to prepare for the procedure.

How the Test will Feel

There is no discomfort, unless a bronchoscopy needs to be performed.

Why the Test is Performed

The test is performed when the doctor suspects tuberculosis or other mycobacterium infection.

Normal Results

Results are normal when no mycobacterial organisms are found.

What Abnormal Results Mean

Abnormal results show that the stain is positive for:

• Mycobacterium tuberculosis
• Mycobacterium avium-intracellular
• Other mycobacteria or acid-fast bacteria

Risks

There are no risks with this test, unless bronchoscopy is performed.

References

Hopewell PC, Kato-Maeda M, Ernst JD. Tuberculosis. In: Broaddus VC, Mason RJ, Ernst JD, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 35.

Woods GL. Mycobacteria. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 61.