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 will be introduced to the test through one window and a sample of blood is introduced into the second window. The interaction between buffer and sample will breakdown the red cells and any malaria parasites; this lysed sample they will diffuse along the strip where labelled antibodies will be used to 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 |
It is essential to appreciate that RDT tests meet a specific task:
RDTs provide a rapid means to indicate the possible presence or absence of malaria infection
While in some circumstances they are the only choice, RDTs should never be considered to a "gold standard" test, and the significant issues with particular speces and with low parasite counts must be recognised. Briefly:
- Tests made vary in quality or sensitivity, either by manufacturer or due to batch variation. The World Health Organisation has a rolling programme that evaluates performance of different tests.
- Practical aspects of use are vital: tests must be stored, performed and read correctly - this is one of the mast important causes of test failure world-wide.
- Sensitivity and false positive rates vary between species, and also according to the detection system used. Most tests have a significantly reduced sensitivity at lower parasite concentrations. You should be aware of the strengths and limitations of the test you use.
The sensitivities of RDT tests accross species are broadly summarised below, but note these are for general guidance as performance may vary between test types and conditions:
| 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 may detect this species well when parasitaemia is high. |
Notes:
1 HRP2 mutation mean that in some geographical areas the diagnostic sensitivity may be significantlu 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 differert manufacurers recognise different malarial antigens. The antigens recognised offer give different characteristics and confer different benefits or drawbacks. You should consider this when interpreting results:
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
*Plasmodium lactate dehydrogenase (PLDH): 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 |
The following formats are available:
Single band tests (comprising a single test band and a control) (image)
- simplicity and lower cost
- suitable only where there is a sigle dominant malaria species.
The single secies P.falciparum diagnostic test has been successfully applied in Africa areas where infections with this species account for more than 95% of infections.
Multi-band tests: the most frequent form identifies the dominant or most important species with a species-specific band for the selected region, then a second band idetifies antigens expressed by all malaria species. Two formats are used:
PF/general PV/general
Test selection
| INTERPRETATION and PROBLEMS |