Diagnosing malaria

Rapid Diagnostic Tests (RDTs)⁴

Malaria rapid diagnostic tests, sometimes called ‘dipsticks’ or malaria rapid diagnostic devices (MRDDs), assist in the diagnosis of malaria by providing evidence of the presence of malaria parasites in human blood. They are available as a simple dipstick, a cassette (a dipstick in a plastic holder), or in a card format.

Some RDTs can only detect one species (P. falciparum), and some can detect other species of the parasite (P. vivax, P. malariae and P. ovale).

Important considerations involved in choosing an RDT include:

• Sensitivity (and specificity)
• Stability
• Ease of use (impacts on sensitivity and stability)
• Cost.

Cassette format RDTs are usually 10-20% higher priced than dipstick RDTs but when used by health workers are probably more reliable than dipstick RDTs due to simplicity of preparation, and so may provide savings through improved diagnosis.

The three main groups of antigens detected by commercially available RDTs are Histidine-rich protein 2 (HRP2), specific to P. falciparum; plasmodium lactate dehydrogenase (pLDH), currently used in products that include P. falciparum-specific, pan-specific and P. vivax-specific pLDH antibodies; and aldolase (pan-specific).

Advantages

• HRP2-detecting tests are likely to have greater sensitivity than pLDH detecting tests for detection of P. falciparum infection in most environments
• pLDH and adolase-detecting RDTs are likely to be less temperature stable than HRP2, and will therefore lose sensitivity more rapidly in uncontrolled storage
• pLDH detection has advantages over HRP2 detection for monitoring of effectiveness of treatment. Microscopy is generally the preferred tool for this purpose.

Table comparing Microscopy and Rapid Diagnostic Tests¹⁵