Skip to 0 minutes and 15 seconds VANESSA YARDLEY: There are more than one rK39 rapid diagnostic tests available for the detection of visceral leishmaniasis. This video uses the InBios kala-azar kit to demonstrate a typical dipstick rK39 test to detect VL antibodies. First of all, you will need to gather all the necessary equipment and reagents before performing the test. You will need gloves and a suitable marker pen, the rK39 kit. The kit contains the test, the chase or buffer solution and instructions for use. Things to check before using the RDT. Has the shelf life of the kit expired? Check for an expiry date on the package. Does the package have a hologram? This is one way to check that the kit is not a fake.
Skip to 1 minute and 5 seconds The sealed kit containing the test strip along with the buffer is designed to be stored at room temperature for the duration of its shelf life. Exposure to temperatures over 30 degrees C can have an impact on the performance of the test, and so should be avoided. The strips must not be frozen. The test must be used within one hour after removal from the kit to prevent exposure to humidity. Before starting, it is important to remember that you will be handling human samples and hazardous chemicals. Wear protective clothing, such as the lab coat and disposable gloves, and remember to dispose of the samples and materials in a suitable biohazard waste container. Moving on to carrying out the test.
Skip to 1 minute and 50 seconds Unwrap the kit and write the patient ID reference clearly on the strip. If you are using a serum sample, add 20 microliters to the absorbent area of the strip below the arrow. If you are not using a serum sample, you will be performing a finger prick to collect blood. Make sure the patient is comfortable and sterilise the fingertip area. A wipe of alcohol will suffice. Then stab the skin boldly with a sterile lancet. You may need to gently squeeze the finger to encourage blood to flow. Now place one drop of blood on the absorbent pad at the bottom of the strip. If necessary, blot the finger where it has been pricked with clean cotton wool.
Skip to 2 minutes and 36 seconds Once you have added the sample, lie the strip flat on a clean, absorbent surface, or stand it in a test tube. Then add two to three drops, about 150 microliters, of the chase buffer solution provided within the kit, and leave for 10 minutes. Allow the mixture to migrate up the strip by capillary action. If this does not start within 10 to 15 seconds, a new kit must be used.
Skip to 3 minutes and 5 seconds After 10 minutes, read the result. Any longer than this and the result could be misleading.
Skip to 3 minutes and 22 seconds There are three possible outcomes.
Skip to 3 minutes and 26 seconds The test is positive when a control line and a test line appear in the areas as shown. A positive result indicates that the dipstick detected antibodies to the parasites of the L. donovani complex. A faint line is considered a positive result. As a guide for interpretation, the red colour in the test region will vary depending upon the concentration of anti-Leishmanial antibodies present. ‘Weakly positive’ samples are those with low affinity or low levels of antibodies for the antigen in the test. A negative result is when there is no line in the test area of the strip, but a line does appear in the control area. This means that no antibodies to Leishmania were detected in the sample.
Skip to 4 minutes and 12 seconds If no line appears in the control area of the strip, the test is invalid and has not worked. Even if a line is seen in the test area of the strip, it is still invalid. You will need to re-test with a new sample and the new kit. If the result is inconclusive, repeat the test procedure. Test results of the patients must be recorded in a laboratory record book, and care must be taken to make the patient ID clear at all times. The kit and materials must be disposed of correctly, ideally treated as clinical waste.
RDT demonstration video
Rapid rK39 antigen-based tests (RDTs) are based on the antigen rK39, a recombinant product of a 39 amino acid antigen encoded by a kinesin-like gene of the L. donovani complex. Originally an ELISA method, it has been adapted to a rapid strip test for use in basic field conditions and is now very widely used in VL endemic countries.
The RDTs are available from several manufacturers. Each brand should be validated for each endemic region. Most rK39 RDTs are highly sensitive and specific in South Asia, but to date only the Bio-Rad IT-LEISHⓇ showed satisfactory sensitivity (85-90%) and specificity (90-99%) in immunocompetent patients in East Africa.
The kits are heat sensitive and can give misleading results if not stored correctly. All RDTs have a cut-off point that corresponds to a high antibody titre and so are unsuitable for detecting sub-clinical infections. They cannot be used to diagnose relapse, given that antibodies can be present in healthy patients for years after VL cure.
DISCLAIMER: LSHTM and partners collaborating in this video can take no legal responsibility for the use of any information contained. Users implementing the techniques shown do so at their own risk and must take responsibility for local verification/ validation of these techniques before they are used for the diagnosis of patients.
© London School of Hygiene and Tropical Medicine 2018