AFB smear (ZN stain for TB)
Quick answer: An AFB smear is a stained slide of a sample checked under a microscope for acid-fast bacilli, the type of bacteria that cause tuberculosis (TB). It is a quick first test when TB is suspected, such as a cough lasting more than two weeks, and is used to follow treatment.
- SampleSputum, urine, pus or body fluid
- TakesAbout 5 to 10 minutes to give each sample
- Before the testFor sputum, give a deep-cough sample, often two samples including one early in the morning; rinse your mouth with plain water first.
How the test is done
- 1You give a sample, usually two sputum samples including one early morning
- 2The lab spreads a little of the sample on a glass slide
- 3The slide is stained using the Ziehl-Neelsen method
- 4Under the microscope, acid-fast bacilli show up red if present
What your results can mean
Reading your report
A negative smear does not rule out TB, so culture or molecular tests may be added; discuss the result with your doctor.
If the result is outside the usual range
A positive smear can point to TB or, less often, related bacteria, and suggests the person may be infectious. A negative smear does not rule out TB, as smears miss many cases.
Who usually has this test
People with a cough for two weeks or more, fever, night sweats or weight loss, and people in close contact with TB.
Risks and aftercare
There are no risks from giving the sample.
Names and options
Also called: AFB stain, sputum AFB, Ziehl-Neelsen stain, acid-fast bacilli test
Options
- Sputum
- Urine
- Pus
- Body fluid
Listed by the hospital as
- AFB STAINING, FLUID
- AFB, URINE
- PUS, AFB
- SPUTUM AFB
Questions about this test
Why are two sputum samples needed?
TB bacteria are not present in every sample. Testing two samples, including one first thing in the morning, increases the chance of finding them.
Does a negative smear mean I do not have TB?
No, smears miss many cases, especially early TB or TB outside the lungs. Doctors may use culture or rapid molecular tests too.
Am I infectious if my smear is positive?
A positive sputum smear suggests you may spread TB through coughing. Your doctor will advise on treatment and precautions, which reduce this risk quickly.
Related tests
Tamara Hospital & IVF Centre, Rajajinagar
34/3, 10th Cross Road, opp. Vardhaka Sardar Patel School, 1st N Block, 2nd Stage, Rajajinagar, Bengaluru 560010
The hospital is open 24 hours. Call before you come to check the timing for your scan or sample collection.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.