Rapid diagnostic tests (RDTs)
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| OVERVIEW |
RDT tests use antibodies that recognise particular parasite proteins (antigens) allowing them to show whether malarial parasites are present in the blood of the individual being tested. There are a range of forms for these tests, but the simplest format (with a single test and control line) is shown below.
Tests are supplied as a kit with a lysis buffer that is introduced to one window of the test as shown. A sample of blood from the patient is introduced into a second window. The interaction between buffer and sample breaks down the red cells and any malaria parasites present (lysis); this "lysed" sample then diffuses along the strip where it mixes with labelled-antibodies that detect the presence of parasites, if parasite proteins are present then the labelled antibody/antigen complex is "captured" and forms a visible band in one or more test windows (T). Successful test performance will be shown by the appearance of a control band (C).
→ Click for more on how tests work
| TEST FORMATS |
Precise test formats can differ according to manufacturer and purpose, but all follow similar principles using either a plastic cassette or a folding card. Tests all have a control line (C) to confirm the test was correctly performed, in addition RDTs have one or more tests bands that allow detection of one or more parasite species (red).
The choice of best RDT format will depend on the particular parasites likely to be encountered, but also on local expertise (see additional information in link below).
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Single band test
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Multi-band test
→ Click for more detail on test formats
| HOW EFFECTIVE ARE RDT tests? |
RDTs provide a rapid result to indicate the possible presence or absence of malaria infection. In some circumstances they may be the only easily available test for malaria. However, RDT tests should never be considered a "gold standard"
The World Health Organisation requires that tests can identify at least 75% of cases which have a parasite concentration of 200 parasites/μl. However, fevers may be present at lower parasite levels (particularly in non-immune travellers). Therefore, for P.falciparum the detection limit of approved tests may not be sufficient to confidently exclude infection. For non-falciparum species tests are less reliable.
→ Click for a comparison of RDT tests and microscopy
| HOW WELL ARE DIFFERENT SPECIES DETECTED? |
The different malaria species are detected with different sensitivity. The table below provides an overall summary of the sensitivity that can be expected for RDT tests when suitable species-specific together with pan-malarial antibodies are used. The table is only a broad summary. Users are advised to understand the characteristics of the test they use including the merits of the antibodies employed in their test (see section below)
| P.falciparum1 |
|---|
| At high parasitaemia expect high sensitivity (90-95%). This declines when parasite levels are lower (45-70%)2 |
| P.vivax |
| Expect moderate to high sensitivity at high parasitaemia (70-90%). This declines markedly when parasite levels are lower (30-60%)2 |
| P.ovale & P.malarae |
| Generally sensitivity is poor (approximately half the sensitivity shown for P.vivax) |
| P.knowlesi3 |
| Reports suggest variable sensitivity that may depend on test used, RDTs are reported tp detect this species well when parasitaemia is high. |
Notes:
1 HRP2 mutation mean that in some geographical areas the diagnostic sensitivity may be significantly lower for HRP2-based tests (see below)
2 Low parasite number is considered as (<1000 parasites/μL) a level that may cause symptoms in children or non-immune travellers
3 There have been relatively few studies with this species so caution is required in RDT interpretation
| DOES THE ANTIGEN DETECTED BY THE RDT TEST MATTER? |
Yes, it is important to realise that the RDT tests produced by different manufacturers use antibodies that recognise recognise different malarial antigens. Different antigens have different strengths or drawbacks that may affect which test you select or how you use it.
It is important that you are aware of the benefits or drawbacks of the antigens used in the test you use.
→ Click stengths and drawbacks of different test antigens
| INTERPRETATION |
The possible patterns seen on RDT tests are relatively simple, and in most cases the most probable diagnosis is clear. However, there may be alternative interpretations of band patters. The guides below are for the most common test formats, and most frequent band patters.