Sputum stain for mycobacteria
Acid fast bacilli stain; AFB stain; Tuberculosis smear; TB smear
Sputum stain for mycobacteria is a test to check for a type of bacteria that cause tuberculosis and other infections.
A sputum sample is obtained by coughing deeply and expelling the material that comes from the lungs into a sterile cup. The sample is taken to a laboratory and placed in a medium under conditions that allow the organisms to grow. A positive culture may identify disease-producing organisms that may help diagnose bronchitis, tuberculosis, a lung abscess, or pneumonia.
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. This sputum stain test can give your doctor a quick answer. Another test, called a culture, is done to confirm the results. A culture test may take several weeks (6 to 10 weeks) to get results.
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.
Considerations
References
Rodino KG, Woods GL, Wengenack NL. Mycobacteria. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 59.
Shete PB, Cattamanchi A, Yoon C. Tuberculosis: clinical manifestations and diagnosis. In: Broaddus VC, Ernst JD, King TE, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 7th ed. Philadelphia, PA: Elsevier; 2022:chap 53.
Version Info
Last reviewed on: 12/4/2022
Reviewed by: Jatin M. Vyas, MD, PhD, Associate Professor in Medicine, Harvard Medical School; Associate in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
