What is the RK39 (Rapid K39) test used for?

From the Guidelines

The RK39 (Rapid K39) test is primarily used for diagnosing visceral leishmaniasis (VL), with a sensitivity of 94% and specificity of 91% in populations unlikely to have HIV/AIDS, as reported in a meta-analysis of 13 publications 1. The RK39 test is a point-of-care immunochromatographic test that detects antibodies against the K39 antigen, a protein found in Leishmania parasites, particularly Leishmania donovani and Leishmania infantum.

• The test requires only a small blood sample and provides results within 10-20 minutes, making it valuable for field settings in endemic regions.
• To perform the test, healthcare providers place a drop of blood or serum on the test strip and add buffer solution, then wait for the appearance of control and test lines.
• The RK39 test is particularly useful in resource-limited settings where more complex diagnostic methods like microscopy or PCR are unavailable.
• However, its sensitivity is lower in persons coinfected with HIV/AIDS, ranging from 51% to 84%, and it has poor sensitivity in persons with cutaneous leishmaniasis (CL), with a sensitivity of 10.2% in a cohort of otherwise healthy military personnel 1.
• The test works because patients with visceral leishmaniasis develop antibodies against the K39 antigen during infection, allowing for detection before symptoms become severe.
• It is worth noting that the sensitivity of the RK39 test can vary depending on the population being tested and the presence of coinfections, but it remains a valuable tool for the diagnosis of visceral leishmaniasis in endemic regions, as supported by the guidelines from the Infectious Diseases Society of America (IDSA) and the American Society of Tropical Medicine and Hygiene (ASTMH) 1.

From the Research

Overview of the RK39 Test

• The RK39 test, also known as the Rapid K39 test, is a diagnostic tool used to detect visceral leishmaniasis (VL), a neglected tropical disease caused by the Leishmania parasite 2, 3, 4, 5, 6.
• The test uses a recombinant antigen K39, which is a specific antigen produced by the Leishmania donovani complex, to detect the presence of antibodies against the parasite in a patient's blood or serum.

Purpose and Application of the RK39 Test

• The primary purpose of the RK39 test is to provide a rapid and accurate diagnosis of VL, allowing for early treatment and control of the disease 2, 4, 5.
• The test is particularly useful in endemic regions where VL is prevalent, and in areas where access to laboratory facilities and trained personnel may be limited 3, 6.
• The RK39 test can be used as a point-of-care diagnostic method, providing results in a short amount of time, typically within 10-20 minutes 2.

Performance and Accuracy of the RK39 Test

• Studies have shown that the RK39 test has high sensitivity and specificity, ranging from 89% to 94% and 81% to 100%, respectively 2, 4, 6.
• The test has been compared to other diagnostic methods, such as the direct agglutination test (DAT) and PCR, and has been found to be a reliable and accurate tool for diagnosing VL 2, 3, 5, 6.
• However, the test may not be suitable for diagnosing relapses, as antibodies against the parasite may remain positive for several months to years after cure 5.

Limitations and Future Directions

• Despite its high accuracy, the RK39 test may not be suitable for all populations, particularly in areas where the disease is not endemic 3.
• Further studies are needed to evaluate the performance of the RK39 test in different regions and populations, and to develop new diagnostic tools that can detect active VL infection 5, 6.
• The use of the RK39 test in combination with other diagnostic methods, such as PCR and DAT, may provide a more comprehensive and accurate diagnosis of VL 2, 3, 5, 6.