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, and in some cases may be the only available test. However, they should never be considered to a "gold standard" test, their limitations should be appreciated.
In brief:
1. Tests vary in quality: both manufacturer and batch variation (see the World Health Organisation performance evaluation).
2. Practicalities are vital: incorrect storage, use or reading are major causes of test failure world-wide.
3. Performance varies between species and design: be aware of the limitations of your selected test..
A broad summary is given below and applies to species detection when suitable species-specific or pn-malaria antifens are tested
| 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 Plasmodium lactate dehydrogenase (PpanLDH): a form of LDH that is present in all malaria species Click for details |
| Pan-Plasmodium Aldolase Antigen (Aldolase): a malaria-specific form of aldolase that is present in all species Click for details |
| TEST FORMATS |
Single test-band RDT tests (these comprising a single test band and a control band)
(image)
- simplicity of interpretation
- lower production cost
- most applicable where there is a single dominant malaria species.
USE: The single species test for P.falciparum has been successfully applied in Africa areas where infections with this species account for more than 95% of infections (remainder are not vivax) and may be used where technical expertise is lower.
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Multi-band RDT tests The most frequent form of test outside of Africa comprising two (pr occasionally more) test bands together with a control band. Most often one band is species specific and identifies the dominant or most important species for the selected region, then a second band uses 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 and PROBLEMS |