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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 health care 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:

  • Chest pain

  • Weakness

  • Chills

  • Fever

  • Weight loss

If the infection travels beyond the lungs, it can cause severe symptoms in other parts of the body.

What other tests might I have along with this test?

Your doctor 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 doctor 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 health care provider.

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 lungs.

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 culture test.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor 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.