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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 (lysis); this "lysed" sample then diffuses along the strip where it mixes with labelled-antibodies that detect the presence of parasites, ultimately these will 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

How good are they?

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. Click for merits of RDTs and microscopy

In brief:

1. Practicalities are vital: test selection, storage, use and reading are major causes of test failure world-wide.Click for more on practicalities
2. Performance varies between species and design: be aware of the limitations of your selected test.Click for more on limitations

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

Click for less common antibody strategies

TEST FORMATS

Single test-band RDT tests

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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. However, this is at the cost missing less prevelant species, and the lack of a pan-malaria antibody means that cases with HRP2-gene delection may be missed.

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Multi-band RDT tests

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These use two (or occasionally more) test bands together with a control band.

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 antibdy.

This format can be very effective providing its' limitations are understood by users. In particular the poor sensitivity of pan-malarial antibodies for some species.

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. iF your test differs then you may need to be adapt this advice.

See Guides below for common test patterns:

Interpretation guide: SINGLE BAND tests

Interpretation guide: MULTI BAND tests

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