Acid-Fast Bacteria Smear
Does this test have other names?
AFB smear microscopy, AFB smear, mycobacterial smear
What is this test?
This test looks for a type of bacteria called acid-fast bacillus in your sputum. Tuberculosis
is the most common infection from this type of bacteria.
Your sputum sample is collected from mucus coughed up from your lungs. The sample
is "smeared" on a glass slide and treated with a special acid-fast stain to look at
under a microscope.
Why do I need this test?
You may need this test if your healthcare provider suspects that tuberculosis is causing
your symptoms. The most common symptom of tuberculosis is a chronic cough that produces
mucus and sometimes contains blood streaks. Other symptoms of tuberculosis include:
If the infection travels beyond the lungs, it can cause severe symptoms in other parts
of the body such as kidney and bone.
What other tests might I have along with this test?
Your healthcare provider will also order a culture test, which is always done at the
same time as the smear test. A culture test attempts to grow the bacteria in a lab
from the culture sample. The smear test provides quicker results but isn't as accurate.
The culture result can take several weeks, but it can definitively diagnose tuberculosis.
Your healthcare provider may also order a nucleic acid amplification test, or NAA,
done on your sputum sample. This is another rapid response test for tuberculosis similar
to a smear test.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
A normal result for an acid-fast bacteria smear is negative, meaning no bacteria were
found in the sputum sample. A positive result means that bacteria were found and that
you may have an infection.
The smear is treated with a special acid-fast stain that can provide a preliminary
test result in 24 hours. At the same time, another sample of sputum is also tested
as a culture. This means the sample will be used to grow the bacteria in a lab if
they are present. The culture provides a more definite result, but it can take several
weeks to determine a positive or negative diagnosis.
How is this test done?
The test requires a sample of sputum, which is made by coughing up mucus from your
Does this test pose any risks?
This test poses no known risks.
What might affect my test results?
You may get a false-positive result because this test isn't as accurate as the AFB
How do I get ready for this test?
You don't need to prepare for this test. But be sure your healthcare provider knows
about all medicines, herbs, vitamins, and supplements you are taking. This includes
medicines that don't need a prescription and any illicit drugs you may use.