What this test measures
Lateral-flow immunoassay detecting IgG antibodies against the rK39 antigen (a 39-amino-acid repeat of a kinesin-related protein in L. donovani). High sensitivity (~95–100%) and specificity (~95%) in Indian L. donovani VL. WHO and the National Kala-Azar Elimination Programme use rK39 ICT as the first-line diagnostic — bone marrow / splenic aspirate microscopy is reserved for diagnostic failures.
Why it matters
India was historically the largest VL endemic region globally (Bihar, Jharkhand, West Bengal, UP). The National Kala-Azar Elimination Programme (target < 1 case per 10,000 population at block level) relies heavily on rK39 ICT for active and passive case detection. A positive rK39 in a febrile patient with splenomegaly from an endemic region is sufficient to start treatment (single-dose liposomal amphotericin B is the first-line drug in India). rK39 remains positive for years after cure, so cannot be used to monitor treatment response or for endemic-area screening.
How to prepare
No fasting required. Whole blood, plasma, or serum — fingerstick is acceptable. Disclose endemic-area travel/residence, HIV status (VL-HIV coinfection alters sensitivity), and prior VL treatment (test stays positive for years after cure).
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Anti-rK39 IgG (reactive / non-reactive)[1][2] | Non-reactive | Non-reactive in a febrile patient with splenomegaly from endemic area — VL unlikely; pursue alternative diagnoses (typhoid, TB, malaria, brucellosis) but consider bone marrow if clinical suspicion remains high. In HIV+ VL, sensitivity drops to 75%. | Reactive — VL highly likely if clinical features present. Start treatment per NCDC guideline (single-dose liposomal amphotericin B 10 mg/kg). In asymptomatic endemic-area residents, positive rK39 indicates past exposure (not necessarily active disease) — clinical correlation essential. |
rK39 in VL workup
| Clinical setting | rK39 result | Action |
|---|---|---|
| Symptomatic (fever + splenomegaly + cytopenia) | Positive | Treat as VL |
| Symptomatic | Negative + high suspicion | Bone marrow / splenic aspirate |
| HIV+ + symptoms | Negative | Bone marrow (rK39 sensitivity drops in HIV) |
| Asymptomatic endemic resident | Positive | Past exposure; monitor; do not treat |
| Post-treatment | Persistently positive | Expected; cannot track cure |
Frequently asked questions
What is kala azar?
Kala-azar (visceral leishmaniasis) is a parasitic infection transmitted by the sand fly. Classic features: persistent fever > 2 weeks, gross splenomegaly, hepatomegaly, weight loss, and pancytopenia. Endemic in Bihar, Jharkhand, eastern UP, West Bengal.
How accurate is rK39 in India?
In Indian L. donovani VL: sensitivity ~95–100%, specificity ~95%. Less accurate in East African VL (different Leishmania subspecies).
I had kala azar 10 years ago — will the test still be positive?
Yes — rK39 IgG persists for years (often lifelong) after cure. It cannot be used to diagnose relapse alone; clinical features + cytology are needed.
My result is positive but I feel fine — what does that mean?
In endemic areas, asymptomatic past exposure is common. A positive rK39 without clinical features does NOT need treatment. Discuss with a doctor who knows the local epidemiology.
What is the treatment for VL?
India's standard first-line is single-dose liposomal amphotericin B (10 mg/kg IV). Alternative: miltefosine 100 mg/day for 28 days (oral). Older drugs (pentavalent antimony, paromomycin) have higher toxicity but are still used in some settings.
Is VL fatal if untreated?
Yes — VL has > 95% mortality if left untreated, but is highly curable with prompt treatment.
Related Infectious Disease tests
Tests commonly ordered alongside KALA AZAR ANTIBODY RAPID TEST, or that help interpret an unexpected result.
Sources & references
- WHO — Visceral Leishmaniasis Guidelines · accessed 2026-05-30T00:00:00.000Z
- NCDC India — Kala Azar Elimination Programme · accessed 2026-05-30T00:00:00.000Z
- ICMR-RMRIMS Patna — Kala-azar Research · accessed 2026-05-30T00:00:00.000Z
- CDC — Leishmaniasis · accessed 2026-05-30T00:00:00.000Z
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