Malaria is one of the most well known tropical diseases worldwide. In the UK, malaria is characterised by fever in travellers returning from endemic areas. Malaria is caused by the Plasmodium protozoa, which are transmitted by Anopheles mosquitoes.
Within the UK, malaria is the most common imported tropical disease from patients travelling to endemic areas. The condition can be life-threatening, especially with P. falciparum (most common cause of malaria). Therefore, knowing how we test for malaria is essential for anyone working in healthcare, especially with the increase in global travel (albeit not currently!).
The diagnosis of malaria is based on malarial films (thick and thin slides) and rapid diagnostic tests.
This refers to microscopic examination of a blood film. It remains the ‘gold-standard’ diagnostic tool for malaria.
A blood specimen is taken from a patient and prepared on a slide using a Giemsa stain. Light microscopy is then used to identify the parasite within erythrocytes. Films are useful in both quantification of the parasitaemia burden and identification of the Plasmodium species. Highest yield of detection is around the time of fever.
Traditionally, a total of three thick and thin blood films should be completed within a 36 hour period to exclude malaria.
Rapid diagnostic tests
These are newer diagnostic tests that can be completed within 15-20 minutes. They involve detection of malarial antigens (e.g. histidine-rich protein 2 or Plasmodium lactate dehydrogenase) in an infected patients blood. It utilises antibodies mounted on a testing strip.
Some rapid diagnostic tests can only detect P. falciparum, whereas others can differentiate between species. They must be combined with films as there are rare cases of false negatives (albeit sensitivity and speciality > 90%) and they cannot quantify the parasite burden.