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Malaria Diagnosis
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Malaria
Diagnosis

Diagnostic methods

Parasitological diagnosis
Microscopy (stained thick and thin blood films) has been the “gold standard” for malaria diagnosis for ages. However, the recent development of highly sensitive and specific molecular methods has changed this situation! PCR-based methods are now considered as the new “gold standard”. The detection limit using rt-PCR is 50 parasites/ml of whole blood. This is approx. 200 times lower than the detection limit of well-trained and experienced microscopists.
Blood smears should be prepared on admission of the patient even if there is no pattern of intermittent fever.
Both thick and thin blood smears should be prepared.
At least 200–300 oil immersion fields should be examined.
Giemsa is the stain of choice for blood parasites.
Note: malarial parasites may be missed using automated differential instruments.

Molecular diagnosis

Different formats of PCR assays (conventional or real-time) have been developed allowing the confirmation of uncertain parasitological species determinations, the efficient detection of mixed infections or the detection of drug resistance markers.
In addition, molecular methods allow the genotyping of Plasmodium strains.
Recently a quantitative real-time Nucleic Acid Sequence-based Amplification (qt-NASBA) technique has been developed. This technique detects parasite RNA which is reversely transcribed to cDNA. The sensitivity of qt-NASBA is comparable to qt-PCR (detection limit at 20 P. falciparum parasites per ml of blood). However, NASBA is much faster (results in 4 instead of 16 hours). Compared to microscopy, the sensitivity is approximately 1000 times higher!

Antigen detection
Many commercially available rapid diagnostic tests (RDTs) detect histidine-rich protein-2 (HRP2) or parasite-specific lactate dehydrogenase (pLDH) by immunochromatographical procedures in whole-blood samples.

Antibody detection
The detection of anti-malarial serum antibodies is not adequate to diagnose an early acute infection. During the first febrile episode, no antibodies are detectable in most cases. Indications for serological investigations are restricted:
• to diagnose relapses of P. vivax and P. ovale
• to diagnose persistent malaria in persons returning with uncertain diagnosis from endemic areas
The most widely used serological test is IFA with P. falciparum parasites from culture. Home-made ELISA tests with crude or recombinant antigens have also been developed.

 

Diagnostic strategies

  1. To diagnose a case of unknown fever in individuals from an endemic area
    In holoendemic areas where most people have blood parasites and are asymptomatic carriers, it is difficult to prove the aetiology of a febrile episode. Higher densities of parasitaemia are correlated with a higher probability that the fever is due to malaria. Very often parasitological examination is not possible in the field.
     
  2. To diagnose malaria in a returning traveller
    This represents an urgent diagnosis and results should be available in a few hours. This can be achieved by either an experienced microscopist applying classical parasitological methods (thin and thick blood films) or a fast rt-PCR method to detect parasite DNA. Rapid diagnostic tests detecting parasite antigens have their limitations in sensitivity and species determination. They are an alternative if other methods are not available.
     
  3. To monitor control trials in an endemic area
    For this purpose, genotyping of malaria parasites has become an important method. It allows the differentiation of persisting strains from newly acquired ones in drug trials or allows one to detect a potential effect of vaccines on strain predominance.

 

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