AFB DRUG SUSCEPTIBILITY PANEL 1ST AND 2ND LINE PANEL 10 DRUGS

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Culture-based panel to check TB resistance against 10 drugs.

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AFB Drug Susceptibility Panel 1st and 2nd Line Panel 10 Drugs

Why is it done?
- Detects resistance patterns of Mycobacterium tuberculosis to first-line and second-line anti-tuberculosis medications, enabling personalized treatment strategies for drug-resistant tuberculosis (DR-TB)
- Identifies susceptibility to 10 specific drugs across both first-line agents (isoniazid, rifampicin, ethambutol, pyrazinamide) and second-line agents (fluoroquinolones, injectable agents, and other newer drugs)
- Ordered when tuberculosis infection is confirmed or suspected, particularly in patients with treatment failure, relapse, or epidemiological risk factors for resistant TB
- Performed on AFB (Acid-Fast Bacilli) positive sputum, respiratory specimens, or tissue samples to guide appropriate multi-drug therapy for drug-resistant tuberculosis management
- Essential for patients with MDR-TB (multidrug-resistant tuberculosis) or XDR-TB (extensively drug-resistant tuberculosis) to optimize treatment outcomes and prevent further resistance development
Normal Range
- Result Interpretation:
- Susceptible (S): Organism demonstrates susceptibility to the drug; MIC (minimum inhibitory concentration) is at or below the critical concentration breakpoint. The standard first-line or second-line drug is expected to be effective.
- Resistant (R): Organism demonstrates resistance to the drug; MIC is above the critical concentration breakpoint. The organism will not respond to this drug at standard therapeutic doses.
- The 10 drugs typically included in comprehensive panels are:
- First-line drugs (4): Isoniazid (INH), Rifampicin (RIF), Ethambutol (EMB), Pyrazinamide (PZA)
- Second-line drugs (6): Fluoroquinolones (levofloxacin/moxifloxacin), Injectable agents (amikacin, capreomycin, streptomycin), Linezolid, Clofazimine, or other WHO-recommended agents
- Normal Result: Susceptible (S) to all tested drugs - indicates fully drug-susceptible TB (DS-TB) treatable with standard first-line therapy
Interpretation
- Fully Drug-Susceptible TB (DS-TB): Susceptible to all first-line drugs. Patient should respond to standard 6-month therapy (2 months intensive phase with isoniazid, rifampicin, ethambutol, pyrazinamide followed by 4 months continuation phase with isoniazid and rifampicin)
- Multidrug-Resistant TB (MDR-TB): Resistant to at least isoniazid AND rifampicin. Requires prolonged second-line therapy (18-20 months), typically combining fluoroquinolone, injectable second-line drug, and additional oral agents
- Extensively Drug-Resistant TB (XDR-TB): MDR-TB plus resistance to any fluoroquinolone AND at least one second-line injectable drug. Requires newer agents (bedaquiline, linezolid, clofazimine, delamanid) and may require 20+ months of therapy
- Isoniazid Mono-Resistance: Resistant to isoniazid only. Requires longer rifampicin-based therapy (9 months) or alternative regimens with fluoroquinolone
- Rifampicin Mono-Resistance: Resistant to rifampicin only. Requires second-line therapy similar to MDR-TB due to treatment difficulty and high risk of acquiring further resistance
- Poly-Drug Resistant TB (Other Patterns): Resistance to multiple drugs but not meeting MDR definition (e.g., resistant to INH and EMB but susceptible to RIF). Treatment decisions based on specific resistance pattern
- Factors Affecting Results:
- Specimen quality (adequate AFB load required for reliable results), prior antibiotic exposure increasing likelihood of resistance, specimen contamination, prior TB treatment history, patient adherence to therapy, genetic mutations in M. tuberculosis conferring resistance
Associated Organs
- Primary Organ System: Respiratory System - primarily lungs, though M. tuberculosis can disseminate to other organs
- Pulmonary Tuberculosis: Most common form affecting lungs; drug-resistant strains complicate treatment and increase mortality risk
- Extrapulmonary Tuberculosis: Can affect lymph nodes, pleura, meninges (CNS TB), bones, kidneys, adrenal glands, and other organs; drug resistance patterns critical for treatment success in disseminated disease
- Associated Complications of Drug-Resistant TB:
- Treatment failure and prolonged infectivity leading to community transmission
- Chronic lung damage, bronchiectasis, and progressive pulmonary deterioration
- Development of cavitary disease with increased transmissibility
- Hemoptysis and respiratory failure in advanced cases
- High mortality rates, particularly in XDR-TB and in immunocompromised patients (HIV co-infection)
- Adverse effects from prolonged second-line drug therapy (ototoxicity, nephrotoxicity, neuropathy, hepatotoxicity)
- TB-HIV co-infection complications with accelerated immune decline
Follow-up Tests
- Recommended follow-up testing based on AFB drug susceptibility results:
- Liquid Culture (MGIT or other): Confirms diagnosis and provides additional organism for targeted testing; may be repeated during treatment
- Molecular Line Probe Assay (LPA) or Gene Sequencing: Provides rapid detection of genetic mutations associated with drug resistance; complements phenotypic DST results
- Extended Drug Susceptibility Panel (If Initial Panel Shows Resistance): Testing to newer drugs (bedaquiline, linezolid, clofazimine, delamanid, meropenem/clavulanate) for XDR-TB patients
- HIV Testing: Performed in all TB patients to identify co-infection; critical for treatment decisions and prognosis in drug-resistant cases
- Sputum AFB Smear Microscopy: Repeated monthly during treatment to monitor bacterial load decrease and treatment response
- Repeat Sputum Culture: Performed at 2-3 months of treatment to confirm treatment efficacy; if still positive, suggests treatment failure
- Chest X-ray: Initial baseline and periodic monitoring (6 months, end of treatment) to assess radiological improvement and detect complications
- Hepatic Function Tests (ALT, AST, Bilirubin): Baseline and periodic monitoring during treatment as second-line drugs (fluoroquinolones, linezolid) can cause hepatotoxicity
- Renal Function Tests (Creatinine, BUN): Essential with injectable second-line drugs (aminoglycosides, capreomycin) due to nephrotoxicity risk
- Audiometry: Baseline and during treatment monitoring when using ototoxic drugs (aminoglycosides, fluoroquinolones, capreomycin)
- Thyroid Function Tests: Baseline and periodic monitoring as para-aminosalicylic acid (PAS) and other second-line agents can cause hypothyroidism
- Contact Investigation (Rapid Molecular Testing): Xpert MTB/RIF Ultra on contacts to identify active cases, particularly important given drug-resistant TB transmissibility
Fasting Required?
- Fasting: NO - Fasting is not required for this test
- Specimen Collection Requirements:
- Early morning sputum specimen is preferred (first sputum of the day has higher AFB yield)
- Sputum volume should be at least 5-10 mL for optimal recovery and testing
- Specimen must be genuine sputum, not saliva; patient should be instructed to cough deeply from lungs
- Collect in sterile, leak-proof container with biohazard label
- Specimen should be transported immediately to laboratory (within 2 hours); if delay expected, refrigerate at 2-8°C
- Special Instructions:
- No special dietary restrictions - eat and drink normally
- No medications need to be held or avoided for specimen collection
- Existing TB medications should continue as prescribed; do not alter therapy to accommodate testing
- Patient should be instructed on proper coughing technique to produce adequate sputum sample
- Airborne precautions should be maintained during specimen collection in healthcare settings
- Multiple specimens (2-3 samples on consecutive days) may be requested for accurate diagnosis, particularly if initial culture is negative

How our test process works!