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Should serology diagnose malaria?

A Three negative sets of blood films, taken during pyrexial episodes or rigors, would make current infection with malaria unlikely.

Biological false positives due to low-level cross-reacting antibodies and lack of antibodies in very early infections, mean serology should not be used to diagnose malaria.

An antibody-negative donor, more than six months after a visit to an endemic area, is unlikely to be carrying a potentially fatal infection. If the antibody-negative patient had hypnozoites of P. vivax or P. ovale in the liver, infection passed to the recipient could be cured with chloroquine.

Serology is occasionally useful for retrospective diagnosis and epidemiological purposes, and is the most practical and sensitive method of screening blood donors.

The P. falciparum antigen cross-reacts with the other species, and is used as a general screen. Weak titres tend to indicate false positive cross-reactions rather than chronic P. malariae malaria, which tend to give high titres.

Some infected individuals never become seronegative.

With acute or chronic malaria, usually the titres are high, and if there is any doubt one should repeat serology, recheck the exposure history, and repeat the blood films. As with all uncomfortable results, treat the patient not the result, and I recommend discussing the case with the laboratory staff.

In this case, I advise an ESR, CRP, FBC, LFTs and urinalysis. Microscopic haematuria could suggest hypernephroma or endocarditis, the latter producing a high CRP. A high ESR could indicate autoimmune disease, another cause of false-positive serology.

Marina Morgan is consultant medical microbiologist the

Royal Devon and Exeter Hospital

QWhat is the place of malaria serology or any blood tests other than thick or thin films? I have a patient who has had malaria and recently has had some night sweats (but negative films and is otherwise well) who was turned down as a blood donor as she had positive malaria serology, having previously been negative.

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