AFB Antibiotic Susceptibility, 5 Drugs

What it is:
This test determines the antibiotic susceptibility of acid-fast bacilli (AFB), primarily Mycobacterium tuberculosis, to a standard set of five first-line anti-tuberculosis drugs. The test is performed on cultured isolates from patient samples.

Common Drugs Tested

1. Isoniazid (INH)

2. Rifampicin (RIF)

3. Ethambutol (EMB)

4. Pyrazinamide (PZA)

5. Streptomycin (SM)

Uses

• Guide treatment of tuberculosis (TB) by identifying drug resistance

• Detect multidrug-resistant TB (MDR-TB) (resistance to at least INH and RIF)

• Assist in selecting effective antibiotic regimens

• Monitor response and resistance during therapy

Symptoms That May Lead to the Test

• Confirmed or suspected tuberculosis that is not responding to standard therapy

• History of previous TB treatment or treatment failure

• Known exposure to drug-resistant TB

• Severe or recurrent TB cases

Abnormal Results

• Drug-sensitive (susceptible):

  • Bacteria are killed/inhibited by all five drugs → standard treatment effective

• Drug-resistant:

  • Resistance to one or more drugs → requires modified treatment plan

  • MDR-TB: Resistance to at least isoniazid and rifampicin

  • Possible XDR-TB (extensively drug-resistant): May require further extended susceptibility testing beyond these 5 drugs

Risks

• No direct risks from the test itself

• Risks relate to sample collection (sputum, fluid, or tissue): mild discomfort, rarely bleeding or infection