Malaria: Diagnosis, antigen detection

Several antigen detection tests have been developed (Parasight®, Malaquick®, ICT Malaria Pf® [Immunochromatographic Test]). The material for the test kit consists of a simple strip (similar to urine dipsticks) and some dropper bottles with reagents. These tests detect the histidine-rich protein, the PfHRP-II antigen. The abbreviation stands for Plasmodium falciparum Histidine Rich Protein. This protein is a constituent of the nodules on the membranes of red blood cells which are infected by P. falciparum. The test makes use of two antibodies which are specific for the PfHRP-II antigen. Both are attached to a paper strip. One of the antibodies is coupled to colloidal gold and applied to the place where the blood sample is to be applied. The second antibody is fixed elsewhere on the strip, in a band where the test result is read. Approximately 10 µl blood is applied (a droplet from a finger prick. Some blood may also be applied via a capillary tube containing EDTA). The red blood cells are lysed. If there is PfHRP-II antigen in the blood, this binds to the antibodies labelled with gold. After administration of a buffer the gold-labelled antibodies migrate with the capillary flow along the test strip and then cross the band containing the second antibody. If the blood sample is positive the antigen antibody complex labelled with PfHRP-II binds to the second antibody and a clear purple band is produced. This does not occur with a negative sample. A control band must always be visible.

The test is quite quick and simple to carry out and needs no technical apparatus. The sensitivity is 90-95% for parasitaemia of more than 100 parasites/µl. Low parasitaemia is thus often missed. The test remains positive so long as there is still antigen in the blood (that is even if the parasites have already disappeared due to adequate therapy). The ICT Malaria Pf® test can only detect P. falciparum. The presence of rheumatoid factor may lead to a false positive result (a problem with Parasight®). P. vivax does not cause cross reactivity. The chief problem, however, is the high price. A curiosity: using Parasight® malaria antigen was found in Egyptian mummies, which is an argument for the presence of this infection in antiquity.

An antigen detection test has also been developed for P. vivax. Using the latter test both P. falciparum and P. vivax can be detected, as well as mixed infections, on the same paper strip (ICT Malaria P.f/P.v®). The result is the presence of one or more horizontal stripes on the strip (as with a urine dipstick). In mixed infections the presence of P. vivax may not be discovered. In view of the simplicity of the test, this technique should in future be of benefit to frequent travellers in isolated tropical regions. It should be noted that these tests do not produce any quantitative information (including no parasitaemia). This is important in regions where chronic carriers are common.

The test "OptiMAL" is based on the detection of parasitic lactate dehydrogenase (pLDH). This quick test (10 minutes) consists of a dipstick coated with monoclonal antibodies to pLDH. Differentiation of the parasite species is based on antigenic differences between various pLDH isoforms. The pLDH is only produced by live parasites (also by gametocytes). The specificity and sensitivity of the test varies from study to study. The test may be positive due to the presence of circulating gametocytes when these people have been clinically cured and no longer have trophozoites or schizonts.

Category: Medicine Notes