Practical reasons for poor RDT performance

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The value of RDT compared with microscopy: thick blood films see also: [1] Selection and procurement of MDTs 1. Quality of test design/production. There is significant variation of performance between tests, users are advised to check test performance (see WHO testing information available from [2]. 2. Lot to lot variability. Variability between batches has been observed and should be considered if RDT performance is less than expected. 2. Heat stability of the RDT test storage potential for degradation due to heat and humidity. Extended exposure to these conditions can drastically reduce the shelf life of RDTsRDT manufacturers recommend continuous product refrigeration, from the point of manufacture all the way to rural clinics and the point of use. However, in many developing countries, cold-chain continuity cannot be guaranteed. WHO analysis of commercially available malaria RDTs found that 37 out of 50 tests claimed stability ranges of 2–30°C, and that the remaining 13 tests claimed stability up to 40°C. [18] However, in many countries, ambient temperatures regularly exceed these limits. 3. Age of test/reagents Operator-Specific Factors Operator-Inappropriate placement of reagent or blood drop Operator-Interpreting faint lineBy virtue of being targeted for use at the point-of-care, immunochromatographic RDTs must be amenable to facile and accurate interpretation by minimally trained operators. However, test lines are often subject to highly variable and user-dependent readings, which can be impacted by user biases, low signal contrast, and inadequate training materials. In the case of malarial RDTs, the WHO requires a panel detection score of 75% at a parasitaemia of 200 parasites/μl in order for the product to warrant endorsement. [16] However, the threshold for pyrogenic onset has been shown to vary from 10 to 200,000 parasites/μl, with 22% of patients developing their first acute fever at a parasite load below 200/μl. [75] Thus, even for the select assortment of commercial malaria RDTs demonstrating compliance with WHO standards, sufficiently low limits of detection have not been demonstrated for the early and accurate diagnosis of nascent infections, or of asymptomatic carriers. In order to improve test sensitivities when dealing with non-ideal biomarkers, manufacturers of RDTs often seek to use the multiplexed detection of distinct, species-specific antigens. In the case of malaria, RDTs are often multiplexed to detect HRP2 and the P. vivax variant of parasite lactate dehydrogenase (Pv-pLDH), to distinguish between infections by P. falciparum and P. vivax. Alternatively, tests can be multiplexed to select for Pf-pLDH and HRP2, which yields highly sensitive indications for falciparum malaria. [16] Multiplexed RDTs have also proven to be efficacious in discriminating between distinct diseases with non-specific symptomatic presentations. RDTs typically have a shelf-life of 18–24 months, allowing sufficient time for delivery, distribution, and use in most settings.