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.

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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).

Click for more detail on how the tests work

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 a "gold standard" test, it is important that 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..

The broad summary below applies when suitable species-specific or pan-malaria antigens tests 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 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

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