Diagnosis

The main method of malaria diagnosis has been the microscopic examination of blood. Besides blood, both saliva and urine have been investigated as alternative, less invasive specimens.

a) Blood films

The most economic, preferred, and reliable diagnosis of malaria is microscopic examination of blood films because each of the four major parasite species has distinguishing characteristics. Two sorts of blood film are traditionally used:

1. Thin film

• allow species identification because the parasite's appearance is best preserved in this preparation. 

     2.  Thick film

• allow the microscopist to screen a larger volume of blood and are about eleven times more sensitive than the thin film, so picking up low levels of infection is easier on the thick film, but the appearance of the parasite is much more distorted and therefore distinguishing between the different species can be much more difficult.  
• Diagnosis of species can be difficult because the early trophozoites ("ring form") of all four species look identical which is a single ring form.  

Thick and thin blood blood films

 

b) Antigen tests

•  Immunochromatographic tests (also called Malaria Rapid Diagnostic Tests, Antigen-Capture Assay or "Dipsticks") use finger-stick or venous blood and the completed test takes a total of 15–20 minutes
• results are read visually as the presence or absence of colored stripes on the dipstick, so they are suitable for use in the field. 
• The threshold of detection  is in the range of 100 parasites/µl of blood  
• One disadvantage is that dipstick tests are qualitative but not quantitative - they can determine if parasites are present in the blood, but not how many.
• The first rapid diagnostic tests were using P. falciparum glutamate dehydrogenase as antigen.
• PGluDH was soon replaced by P.falciparum lactate dehydrogenase, a 33 kDa oxidoreductase [EC 1.1.1.27].
• It is the last enzyme of the glycolytic pathway, essential for ATP generation and one of the most abundant enzymes expressed by P.falciparum. 
• PLDH does not persist in the blood but clears about the same time as the parasites following successful treatment. 
• The lack of antigen persistence after treatment makes the pLDH test useful in predicting treatment failure. In this respect, pLDH is similar to pGluDH. 

c) Molecular methods

• PCR (and other molecular methods) is more accurate than microscopy. 
• However, it is expensive, and requires a specialized laboratory.
• Levels of parasitemia are not necessarily correlative with the progression of disease, particularly when the parasite is able to adhere to blood vessel walls.