Target condition being diagnosed

Malaria is a life‐threatening illness caused by the parasitic protozoan Plasmodium, which is transmitted by many species of anopheline mosquitoes. In 2006, an estimated 3.3 billion people, almost half the world's population, were living in malaria endemic areas, and there were between 189 and 327 million cases of malaria (World Health Organization 2008a). Around 87% of cases were in Africa, 9% in South East Asia, 2% in the Middle East and 1% in South America (World Health Organization 2008a). In the same year, nearly a million people died from malaria; 91% were in Africa and 85% were children under the age of five years (World Health Organization 2008a). Malaria may be uncomplicated or severe. Most cases are uncomplicated, commonly presenting with fever, and sometimes other non‐specific symptoms including headache, and aches and pains elsewhere in the body (Gilles 1991; World Health Organization 2003). Severe malaria presents with confusion or drowsiness with extreme weakness, and may lead to coma and other life‐threatening complications involving many organs.

The two most common species of malaria parasites are Plasmodium falciparum and Plasmodium vivax. Falciparum malaria is the most common cause of severe malaria and malaria deaths and can also cause other complications such as anaemia and, in pregnancy, low birth weight babies. Vivax malaria is a relapsing form, which is rarely fatal but can cause serious anaemia in children. Malaria is a curable disease, and therefore malaria related morbidity and mortality can be reduced. Early, prompt and accurate diagnosis followed by appropriate treatment is the key to effective disease management (World Health Organization 2003) and is a basic tenet of current malaria control policy (World Health Organization 2005; Bell 2006).

People who are exposed repeatedly to malaria infection develop a partial and incomplete immunity. This means that in highly endemic areas those most at risk are children under the age of five, who have not yet had the chance to develop immunity. In less endemic areas, or areas of seasonal or epidemic transmission, older children and adults are also at risk due to less developed immunity. Travellers from non‐endemic to endemic countries are at highest risk because they have no immunity at all.

Index test(s)

Rapid diagnostic tests (RDTs)(World Health Organization 2003) detect parasite‐specific antigens in a drop of fresh blood through lateral flow immunochromatography (World Health Organization 2006). The World Health Organization currently lists 96 commercially available test kits meeting ISO131485 manufacturing standards (World Health Organization 2009). RDTs do not require a laboratory or any special equipment (World Health Organization 2006), are simple to use and can give results as a simple positive/ negative result, at thresholds pre‐set by the manufacturers, within 15 minutes (Talman 2007). RDTs are therefore, in general, suitable for remote areas with limited facilities and relatively untrained staff. However, they have a limited shelf life and need to be kept dry and away from extremes of temperature. They may also fail to detect malaria where there are low levels of parasites in the blood, for example in young children with low immunity, and false positives are possible due to cross reactions or gametocytaemia (Kakkilaya 2003).

These assays detect one or several antigens, the most common are histidine‐rich protein‐2 (HRP‐2), aldolase and lactate dehydrogenase (pLDH) (Talman 2007). Detection of HRP‐2 is a marker for P. falciparum while pLDH may be specific for P. falciparum, P. vivax or may detect all species (including P. ovale and P. gambiae). Aldolase is pan‐specific, detecting all types of malaria but not differentiating between them. There are seven types of commercially available test, using different antigen combinations (Bell 2006):

• Type I: HRP‐2 (P. falciparum‐specific). Possible results: no Pf, Pf, invalid

• Type II: HRP‐2 (P. falciparum specific) and aldolase (pan‐specific). Possible results: no malaria, Pf or mixed, Pv, Po and/or Pm, invalid

• Type III: HRP‐2 (P. falciparum‐specific) and pLDH (pan‐specific). Possible results: no malaria, Pf or mixed, Pv, Po and/or Pm, invalid

• Type IV: pLDH (P. falciparum‐specific) and pLDH (pan‐specific). Possible results: no malaria, Pf or mixed, invalid.

• Type V: pLDL (P. falciparum‐specific) and pLDH (P. vivax‐specific). Possible results: no malaria, Pf, Pv, Pf and Pv, invalid

• Type VI: HRP‐2 (P. falciparum‐specific), pLDH (pan‐specific) and pLDH (P. vivax‐specific). Possible results: no malaria, Pf and Pv +/‐ Po and/or Pm, Pf +/‐ Po and/or Pm, Pv +/‐ Po and/or Pm, Po and/or Pm, invalid

• Type VII: Aldolase. Possible results: no malaria, Pf, Pv, Po and/or Pm, invalid

HRP‐2 can stay in the blood for 28 days after initiating the antimalarial therapy (Kakkilaya 2003). Because of this 'persistent antigenaemia', it is not possible to use these tests in assessing parasite clearance following treatment, and false positive results may be found in patients who have recently been treated for malaria. In contrast, pLDH is rapidly cleared from the blood following parasite death; in fact it may clear more rapidly than the dead parasites (World Health Organization 2009).

Alternative test(s)

Microscopic examination of Giemsa‐stained thick and thin blood films remain the conventional laboratory method and are still regarded as the ‘gold standard’. Microscopic examination provides a good sensitivity and specificity (although very low parasitaemias may not be detected), and it allows species and stage differentiations and quantification of parasites, all of which are important in assessing the disease severity and prescribing appropriate therapy. Intensive examination is more likely to reveal parasitaemia, so the test is carried out with a fixed number of fields being examined.

Polymerase chain reaction (PCR) which is molecular method based on DNA amplification is the most accurate method of detecting parasites in the blood. This technique is currently not widely available due to logistic constraints and the need of specially trained technicians and a well equipped laboratory. It is usually used only for research purposes.

Rationale

A diagnostic test which is simple to perform, rapid and accurate is important in many situations to ensure prompt specific treatment, reduce misdiagnosis of non‐malarial illness as malaria, limit the development of drug resistance (Talman 2007) and reduce drug wastage. The World Health Organization lists some of the situations where RDTs can be particularly useful as remote areas without access to expert microscopy, complex emergencies, and severe malaria, where rapid diagnosis is essential to save lives (World Health Organization 2000).

The World Health Organization now recommends artemisinin‐based combination therapy (ACT) for the treatment of malaria (World Health Organization 2006a). This is more effective and also more expensive than previously used antimalarial drugs (Sinclair 2009; Omari 2006). Since the introduction of ACTs, World Health Organization policy for highly malaria endemic areas has changed from treating all people with fever, to recommending only treating following diagnostic confirmation by microscopy or RDTs, with the exception of children under the age of five. However, local practice is often very different, some studies indicating that, even where diagnostic tests are used, treatment for malaria is often still given to all patients with fever (Hamer 2007).

The relative costs of microscopy and RDTs vary according to context. Where there is a relatively high prevalence of malaria and an established microscopy service, microscopy would usually be less expensive than RDTs, because most of the costs associated with microscopy are fixed costs, and microscopy can also be used to diagnose other diseases. In areas where malaria is less prevalent, or very rural areas where access to good quality microscopy services is limited, RDTs may be less expensive than microscopy (World Health Organization 2008.). The costs of RDTs also depends on the type of test used, which will depend on the types of malaria parasite endemic in the area; World Health Organization describes three zones (World Health Organization 2005a):

• Zone 1‐ P. falciparum only or other species almost always as a mixed infection (most of Sub‐saharan Africa, lowland Papua New Guinea). Tests using HRP‐2 to detect P. falciparum only are appropriate.

• Zone 2 ‐ both P. falciparum and P. vivax, most commonly as a single species (Asian and the Americas, Ethiopian highlands). Combination RDTs are needed, which detect all species and distinguish between P. falciparum and P. vivax.

• Zone 3 ‐ non‐falciparum only (vivax only areas of East Asia and Central Asia, some highland areas elsewhere). Pan‐specific or vivax‐specific RDTs are appropriate.

RDTs may be used to confirm diagnosis before commencing treatment in people with symptoms of malaria where confirmation by microscopy is currently unavailable or unused, thereby increasing the specificity of diagnosis, which would otherwise be made on symptoms only. Alternatively, RDTs may be used to replace microscopy for confirmatory diagnosis, where logistic factors and relative costs indicate that this may be beneficial. The usefulness of RDTs in these roles will depend to a large extent on their accuracy, which will be explored in this review. The sensitivity and specificity thresholds that decide whether a test is useful in practice will depend upon the situation; as malaria endemicity varies enormously by geographic area, and positive and negative predictive values will vary considerably with endemicity. In addition, microscopy is not a perfect reference standard in itself, and the relative accuracy of RDTs and microscopy will depend to a large extent on the performance of the laboratory facilities and personnel available for microscopy.

Previously published systematic reviews have focused on the accuracy of RDTs for diagnosing malaria in travellers returning to non‐endemic countries from endemic countries (Marx 2005) or on one particular test only (Cruciani 2004). As far as we know this will be the first systematic review to assess the accuracy of the full range of RDTs for diagnosing malaria in people with symptoms in endemic areas.