Testing for malaria: Rapid diagnostic tests and PCR

We’ve seen that diagnosing malaria subtypes can be achieved by looking at the blood film—in fact, you should still review a smear for each patient suspected to have malaria. But in developing countries, where malaria is common, and access to microscopy may be limited, rapid diagnostic tests (RDTs) are needed.

Rapid diagnostic tests allow us to screen for malaria. These tests are conducted using filter paper strips that contain three types of antibodies—anti-falciparum malaria antibodies, general anti-malaria antibodies, and anti-human antibodies.

To test for malaria place serum from a patient on one of these filter paper strips, and allow it to soak in. After about 15 minutes, look at the strip. If only the control line is visible, the patient doesn't likely have malaria. If they have something other than falciparum malaria, the anti-malaria strip will be visible, but not the anti-falciparum strip. And if the patient is infected with falciparum malaria—severe malaria—all the lines will be visible. These tests have a sensitivity and specificity of greater than 90%.

Polymerase chain reaction (PCR) is the most sensitive test for malaria at more than 90% sensitivity. While microscopy can detect infected red blood cells with a parasite load of 50 parasites / µL, PCR is able to detect as few as 1 parasite / µL. But of course, it’s also the most labor-intensive, the most expensive, and takes the longest to get results. PCR is available, to a limited extent, in developing countries. But in the developed world, it’s a common way to make the diagnosis, especially in returning travelers.

PCR uses a polymerase enzyme to copy the deoxyribonucleic acid (DNA) of the malaria parasite. This process increases or amplifies, the number of DNA nucleic acids so they can be more easily picked up by laboratory tests such as protein assays. So PCR can be used to diagnose malaria from a very small blood sample volume, or a blood sample that has been dried on filter paper.

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