Rapid diagnostic tests (RDTs)
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| OVERVIEW |
RDTs detect malarial parasite antigens in the blood of infected individuals. Precise test formats can differ according to manufacturer and purpose, but follow similar principles using either a plastic cassette or a folding card. The simplest format (with a single test and control line) is shown below.
Essentially, a lysis buffer is introduced to one window and a sample of blood into the second window. The interaction between buffer and sample breaks down the red cells and any malaria parasites present; the "lysed" sample then diffuses along the strip encountering labelled antibodies that detect the presence of parasites, forming a visible band in one or more test windows (T). Successful test performance will be shown by the appearance of a control band (C).
| THE BASICS |
NOTE RDTs provide a rapid means to indicate the possible presence or absence of malaria infection. In some circumstances RDTs may be the only easily available test for malaria. However, RDT tests should never be considered a "gold standard" test and where ever possible results should be confirmed by microscopy wherever this is possible. It is important that their limitations should be appreciated.
In brief:
1. Practicalities are vital: test selection, storage, use and reading are major causes of test failure world-wide.click for more
2. Performance varies between species and design: be aware of the limitations of your selected test..
The broad summary below applies when suitable species-specific together with pan-malarial antibodies (as appropriate) are used
| 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 (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
| THE ANTIGENS |
Tests from different manufacturers recognise different malarial antigens. You should be aware of benefits or drawbacks of the antigens used in your test.
| 1. Species-specific antigens: these detect individual malaria species |
|---|
| histidine-rich protein 2 (HRP2): specific for P.falciparum - Click for details |
| Species-specific forms of lactate dehydrogenase: principally either P.falciparum or P.vivax Click for details |
| 2. Pan-specific antigens: these detect the presence of any malaria species |
| Pan-specific antigens: Plasmodium lactate dehydrogenase or Plasmodium aldolase Click for details |
| TEST FORMATS |
Single test-band RDT tests
This test format usually uses the HRP2 antigen to detect P.falciparum and offers particular advantages:
- simplicity of interpretation
- lower production cost
- stability for storage and transport
- valuable where a single malaria species dominates.
Use: The single species test for P.falciparum is very useful in Africa areas where infections with this species account for more than 95% of infections and may be used where technical expertise is less available. There are however existing and emerging problems with this test.
Issues with the single band test format
Multi-band RDT tests
These use two (or occasionally more) test bands together with a control band.
USE Most often one band is a species-specific antibody that identifies the dominant (or most important) species for the selected region. The second band generally employs a pan-malaria antibody that may detect other species.
The most frequent format is P.falciparum-specific combined with a Pan-malaria-specific
Test selection
| INTERPRETATION |
The possible patterns seen on RDT tests are relatively simple and the most probable diagnosis is generally clear. However, there are often alternative possible interpretations in many cases. The guides below are for the most common test formats, but may need to be adapted if you use a less common format.
Interpretation of tests
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Interpretation guide: MULTI BAND tests