There are a few articles this week in Malaria World discussing accuracy of tests. There are two types of tests is common use. The first is old fashioned microscopy that is considered the ‘gold standard’. A drop of blood is smeared on an microscope slide and it is inspected visually for plasmodia. This method requires a basic laboratory and a skilled trained operator.
The second method is RTD (rapid diagnostic tests). mRDTs (m for malaria) detect the presence of histidine rich proteins released from parasitized red blood cells of a drop of blood. Their use does not require a laboratory or as much training. They have been widely used for routine malaria diagnosis in many rural areas of sub-Saharan Africa.
It is a similar situation to that which applied during the COVID ‘pandemic’. Lateral flow RTDs could be used by anyone and allowed increased testing when compared to PCR (polymerase chain reaction) that was considered the ‘gold standard’. PCR is occasionally used for malaria detection but probably has many false positives (See ‘PCR Detects More Asymptomatic Malaria Cases than RDT’).
The lead article in Malaria World is ‘Researchers question reliability of Abbott’s rapid malaria tests’ by Offord in Science. WHO is concerned that 11 “affected” lots from two Abbott RDTs—Pf/Pv and Pf/Pan—that were associated with “faint lines and false negative results” in reports from “multiple research groups.” They are concerned that the tests do not detect cases especially in Asia. False negatives may be less likely in countries where malaria transmission and parasite densities are higher, as in parts of Africa, says Nick White, a malaria researcher at MORU who is co-authoring an academic paper describing SMRU’s findings.
The second paper of interest on this topic in Malaria World this week is scientific study called ‘Point-of-Care Evaluation of Malaria Rapid Diagnostic Test (mRDT) for Detection of Plasmodium falciparum Among Children Under 5 Years of Age Attending Panyadoli Health Center III in Kiryandongo Refugee Settlement, Mid-Western Uganda’ by Acan et al. The cross-sectional study was conducted among refugee children under 5 years old from February to April 2023. All eligible refugee children aged ≤5 years with suspected malaria symptoms, such as anorexia, vomiting, or abdominal discomfort, with or without diarrhoea, or with a body temperature above 37.4°C or a history of fever within the past 24–48h were included.
380 blood specimens were obtained using the finger prick method and examined for malaria parasites using mRDT (Carestart by Apacor – pictured) and microscopy. A structured questionnaire was used to collect sociodemographic characteristics of the respondents. Data were analyzed using descriptive statistics, while Kappa value was used to provide insights into the agreement between the two diagnostic methods.
The prevalence of malaria using mRDT and microscopy was 12.8% (95% CI: 8.0%–17.8%) and 12.2% (95% CI: 7.4%–17.4%), respectively. mRTD detects a few more cases on average than the ‘gold standard’. But are they mostly the same cases? Cohen’s Kappa Statistic is used to measure the level of agreement between two raters or judges who each classify items into mutually exclusive categories. The concludes that the strong agreement between mRDT and microscopy (Kappa=0.75) further confirms the effectiveness of mRDT as a diagnostic tool. This is considered substantial agreement (0.61-0.80) but not near perfect agreement (0.81-0.99). There is not perfect overlap of the two methods.