What Does Acid-Fast Bacilli (AFB) Mean on a Lab Report?

What it means

Some bacteria have a waxy outer coat. When a lab stains them with a red dye and then washes the slide with acid, they hold on to the colour, which is why they are called "acid-fast". "Bacilli" means rod-shaped bacteria. The best-known acid-fast germ is the one that causes tuberculosis (TB), but there are others, including germs related to TB that live in soil and water.

An AFB test is usually done on sputum, the phlegm coughed up from deep in the chest, but it can be done on other samples too. The scientist looks under a microscope and reports whether acid-fast bacilli were seen and roughly how many, often using a grading of plus signs. Samples are often collected on two or three separate days because the bacteria may not show up in every sample. A culture or a molecular test is usually done at the same time to confirm which germ it is.

An AFB smear is not the same as a GeneXpert test or an AFB culture. The smear is a quick look under the microscope, GeneXpert looks for TB genetic material, and the culture grows the germ over several weeks.

Where you may see it

• Sputum test results when TB is being checked
• TB clinic records during treatment, to see whether smears are turning negative
• Microbiology reports for lymph node, urine or other samples
• Reports listing results like "AFB not seen" or "AFB seen (2+)"