What is the treatment for Acid Fast Bacilli (AFB)?

Treatment for Acid-Fast Bacilli (AFB)

Immediate Empiric Four-Drug Regimen

Initiate treatment immediately with isoniazid, rifampin, pyrazinamide, and ethambutol for all patients with AFB detected on smear or culture, without waiting for species identification or drug susceptibility results. 1, 2

This aggressive approach is critical because:

• AFB positivity strongly suggests active mycobacterial infection requiring urgent treatment 1
• Delaying therapy while awaiting culture results (which can take weeks) risks disease progression and transmission 2
• The fourth drug (ethambutol) protects against unrecognized isoniazid resistance until susceptibility results are available 2

Standard Dosing Regimen (for average 70-75 kg adult)

Initial Intensive Phase (First 2 Months - Daily Dosing):

• Isoniazid: 300 mg daily 3
• Rifampin: 600 mg daily 1, 3
• Pyrazinamide: 1500-2000 mg daily (15-30 mg/kg) 2
• Ethambutol: 1100 mg daily (15 mg/kg) 1, 2

Continuation Phase (Months 3-6):

• Isoniazid: 300 mg daily 3
• Rifampin: 600 mg daily 3

Total treatment duration: 6 months minimum 4, 1, 5

Critical Pre-Treatment Assessments

Before or immediately upon initiating therapy, obtain: 2, 5

• Three sputum specimens for AFB smear, mycobacterial culture, and drug susceptibility testing 2
• HIV testing with CD4 count if positive (affects dosing frequency and duration) 1, 2
• Baseline liver function tests (AST, ALT, bilirubin, alkaline phosphatase) 2
• Complete blood count with platelets 2
• Serum creatinine 2
• Visual acuity and red-green color discrimination testing (mandatory before ethambutol - do not use in young children whose vision cannot be monitored) 2
• Chest radiograph to assess cavitation and disease extent 5

Treatment Duration Modifications Based on Response

Extend to 9 months total if: 4, 2

• Cavitation present on initial chest radiograph AND sputum culture remains positive at 2 months 4, 2

Shorten to 4 months total if: 4, 5

• Culture-negative pulmonary TB with clinical/radiographic response at 2 months 4, 5

Monitoring Strategy

Monthly assessments must include: 2, 5

• Sputum AFB smear and culture until conversion to negative 5
• Clinical evaluation for hepatotoxicity symptoms (nausea, vomiting, abdominal pain, jaundice) 2
• Visual disturbance screening (with ethambutol use) 2
• Treatment adherence assessment 2

At 2 months: 4, 2

• Repeat sputum culture to assess microbiologic response 4, 2
• Most patients show >99% bacterial load reduction by this point 2

Repeat drug susceptibility testing if: 5

• Cultures remain positive after 3 months of treatment 5
• Cultures revert to positive after initial conversion 5

Modification Based on Species Identification

If Mycobacterium tuberculosis confirmed with drug susceptibility: 1, 5

• Continue standard four-drug regimen for 2 months, then isoniazid and rifampin for 4 additional months (total 6 months) 1, 5

If Nontuberculous Mycobacteria (NTM) identified (e.g., M. avium complex): 4, 1, 2

• Switch to macrolide-based regimen: clarithromycin 500 mg twice daily OR azithromycin 250-500 mg daily 1, 2
• Plus ethambutol 15 mg/kg daily 1, 2
• Plus rifampin 600 mg daily 1, 2

Special Population Considerations

HIV-Infected Patients (especially CD4 <100 cells/μL): 1, 2, 5

• Never use twice-weekly dosing - use daily or three-times-weekly therapy throughout treatment 1, 2, 5
• Screen for malabsorption and consider drug level monitoring 3
• Consider extending continuation phase to 7 months (total 9 months) 4

Suspected Drug Resistance (prior TB treatment failure/relapse, contact with drug-resistant case, origin from high-resistance area): 2

• Maintain all four drugs and consult infectious disease specialist immediately 2
• Do not reduce to two-drug therapy until susceptibility confirmed 2

Pregnant Women: 5

• Baseline liver function testing mandatory 5
• Standard four-drug regimen is safe (pyrazinamide now considered safe in pregnancy) 5

Critical Pitfalls to Avoid

Never use single or dual-drug therapy initially - this rapidly leads to drug resistance 5, 3

Never add a single drug to a failing regimen - this creates resistance to the added drug 5

Never use macrolides (clarithromycin/azithromycin) empirically for AFB-positive specimens until tuberculosis is definitively excluded, as these are not part of standard TB regimens and lead to inadequate treatment 1, 2

Never delay treatment in seriously ill patients - initiate therapy before AFB smear results if clinical suspicion is high 4

Do not assume treatment failure based on smear positivity alone after 5 months - 80% of such cases represent nonviable bacilli or NTM rather than viable M. tuberculosis, and culture confirmation is essential before changing to second-line regimens 6

Directly Observed Therapy (DOT)

All twice-weekly or three-times-weekly regimens must be administered by directly observed therapy 3

For patients at risk for nonadherence (e.g., intravenous drug users), supervised chemotherapy may be required even for daily regimens 4