Acid-fasting is a characteristic of some bacterial cells, and AFB stands for Acid-fasting Bacillus. When someone is believed to have tuberculosis, an AFB Smear test is performed. Mycobacterium tuberculosis AFB is the culprit of tuberculosis infection. The test is the quickest and most accurate one now available to identify tuberculosis, and it is performed by obtaining a sample from the patient. The absence of the infection in the patient is indicated by the negative test results. Mycobacteria are present in the sample if the AFB test is positive. The sample that tested negative may have an infection, thus it is still let to develop in the culture.