A5.1 Symptom-based (clinical) diagnosis
The signs and symptoms of malaria, such as fever, chills, headache and anorexia, are non-specific and are common to many diseases and conditions. Malaria is a common cause of fever and illness in endemic areas (1, 2), but it is not possible to apply any one set of clinical criteria to the diagnosis of all types of malaria in all patient populations. The appropriateness of particular clinical diagnostic criteria varies from area to area according to the intensity of transmission, the species of malaria parasite, other prevailing causes of fever, and the health service infrastructure (3). One of the factors leading to a change in the clinical epidemiology of malaria in some areas is the prevalence of HIV/AIDS. This disease can increase the risk of acquiring malaria or the progression to severe malaria, depending on malaria transmission in the area and the age of the patient. The prevalence of HIV/AIDS can also lead to an increase in the incidence of febrile disease that is not malaria, and can therefore cause further difficulties in the symptom-based diagnosis of malaria (4).
Two different studies in The Gambia have shown that a sensitivity of 70-88% and a specificity of 63-82% for malaria diagnosis could be achieved using a weighting and scoring system for clinical signs and symptoms. These methods may be too complicated to implement and supervise under operational conditions in the field, and many of the key symptoms and signs of malaria in one area may not be applicable elsewhere. For instance, reduced feeding in a child is more likely to indicate malaria in The Gambia than in Ethiopia (5, 6).
Fever alone is as effective a criterion for diagnosis as clinical algorithms; a review of 10 studies indicated that use of the more restrictive criteria of clinical algorithms resulted in only trivial savings in drug costs compared with use of a fever-based diagnosis, even in areas of low malaria prevalence. In areas of high prevalence it greatly increases the probability of missing malaria infections (7).
A5.2 Light microscopy
In addition to providing a diagnosis with a high degree of sensitivity and specificity when performed well, microscopy allows quantification of malaria parasites and identification of the infecting species. It is inexpensive, the cost varying from US$ 0.40-0.70 per slide and is considered to be the "gold standard" against which the sensitivity and specificity of other methods must be assessed. A skilled microscopist is able to detect asexual parasites at densities of fewer than 10 per µl of blood but under typical field conditions the limit of sensitivity is approximately 100 parasites per µl (8). Light microscopy has important advantages:
• low direct costs if the infrastructure to maintain the service is available,
• high sensitivity if the quality of microscopy is high,
• differentiation between plasmodia species,
• determination of parasite densities,
• can be used to diagnose many other conditions.
It can be difficult to maintain good quality of microscopy, for various reasons: the need for adequate training and supervision of laboratory staff; the need to rely on electricity at night time; delays in providing results to patients; and the need for maintaining quality assurance and control of laboratory services.
Numerous attempts have been made to improve malaria microscopy, but none has proven superior to the classical method of Giemsa-staining and oil-immersion microscopy for performance in typical health-care settings (9).
A5.3 Rapid diagnostic tests
Rapid diagnostic tests (RDTs) are immunochromatographic tests that detect parasite-specific antigens in a finger-prick blood sample. Some tests detect only one species (Plasmodium falciparum), others detect one or more of the other three species of human malaria parasites (P. vivax, P. malariae and P. ovale) (10-12). RDTs are available commercially in different formats, as dipsticks, cassettes or cards. Cassettes and cards are easier to use in difficult conditions outside health facilities.
RDTs are simple to perform and interpret, and do not require electricity or special equipment. WHO recommends that such tests should have a sensitivity of > 95% in detecting plasmodia at densities of more than 100 parasites per µl of blood. Programme and project managers should make their own choice among the many products available, using the criteria recommended by WHO (www.wpro. who.int/rdt) as there is as yet no international mechanism for pre-qualification of RDTs.
Current tests are based on the detection of histidine-rich protein 2 (HRP2), which is specific for P. falciparum, pan-specific or species-specific parasite lactate dehydrogenase (pLDH), or other pan-specific antigens such as aldolase. These antigens have different characteristics, which may affect suitability for use in different situations, and this should be taken into account when developing RDT policy. These tests have many potential advantages, including:
• the ability to provide rapid results,
• fewer requirements for training and skilled personnel (a general health worker can be trained in one day),
• reinforcement of patient confidence in the diagnosis and in the health service in general.
There are also potential disadvantages, including:
• the likelihood of misinterpreting a positive result as indicating malaria in patients with parasitaemia incidental to another illness, in particular when host immunity is high; the inability in the case of some RDTs, to distinguish new infections from a recently and effectively treated infection; this is due to the persistence of certain target antigens (e.g. HRP2) in the blood for 1-3 weeks after effective treatment. The persistence of PfHRP2 in blood for at least one week after treatment can be used in the diagnosis of severe malaria in low transmission areas where artemisinin derivatives are widely available. Patients may have cleared peripheral parasitaemia because of inadequate self treatment, but the PfHRP2 test will be strongly positive.
• unpredictable sensitivity in the field (13-20), mainly because test performance is greatly affected by adverse environmental conditions such as high temperature and humidity.
Published sensitivities of RDTs for P. falciparum range from comparable to those of good field microscopy (>90% at 100-500 parasites/µl of blood) to very poor (40-50%) for some widely used products. Sensitivities are generally lower for other species. The reasons for poor sensitivity are not clear. They may include: poor test manufacture, damage due to exposure to high temperature or humidity, incorrect handling by end-users, possible geographical variation in the test antigen, and poor comparative microscopy (12). Several studies have shown that health workers, volunteers and private sector providers can, with some support and follow-up, learn to use RDTs correctly with relative ease.
The use of a confirmatory diagnosis with either microscopy or RDTs is expected to reduce the overuse of antimalarials by ensuring that treatment is targeted on patients with confirmed malaria infections as opposed to treating all patients with fever. There is, however, little documented evidence that this is so. The main problem is that providers of care, although they may be willing to perform diagnostic tests, do not always comply with the results, especially when they are negative. Being aware that delay in providing effective treatment can be fatal for a malaria patient, they are often reluctant to withhold treatment on the basis of a negative result. WHO is currently supporting operational research projects designed to address these issues.
A5.4 Immunodiagnosis and PCR-based molecular detection methods
Detection of antibodies to parasites, which may be useful for epidemiological studies, is neither sensitive, specific, nor rapid enough to be of use in the management of patients suspected of having malaria (21).
Techniques to detect parasite DNA based on the polymerase chain reaction (PCR) are highly sensitive and very useful for detecting mixed infections, in particular at low parasite densities. They are also useful for studies on drug resistance and other specialized epidemiological investigations (22), but are not generally available in malaria endemic areas.
What are the effects of treatment based on clinical diagnosis compared with treatment based on diagnosis with the use of malaria microscopy or RDTs?a
Interventions: use of RDTs or microscopy for diagnosis
Summary of RCTs: despite the existence of a number of studies on the sensitivity and specificity of various methods for diagnosing malaria, there are no RCTs on the impact of a confirmatory diagnosis as an intervention.
Expert comment: treatment of all people with fever leads to overuse of antimalarials in most settings. With the introduction of more expensive antimalarials, there is a need to target treatment more effectively on people with malaria infections by using confirmatory testing. This will have the additional benefits of improving patient care and providing better epidemio-logical surveillance data. However, in young children in areas of intense transmission, it is believed that the risk associated with relying on a parasito-logical diagnosis (death owing to a false negative result) may outweigh the benefits.
Recommendation: except for children in areas of intense transmission, confirmatory (parasitological) testing should be introduced and used to supplement clinical criteria in diagnosis.
Controlled trials and operational research in various settings are urgently needed.
a See also references (7) and (23).
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