Performance of “VIKIA Malaria Ag Pf/Pan” (IMACCESS®), a new malaria rapid diagnostic test for detection of symptomatic malaria infections

Now that artemisinin derivatives in combination with partner drugs (artemisinin combination therapy (ACT)) are being used, parasitological confirmation has become essential before treatment in routine malaria case management, in most countries endemic for malaria[1, 2]. This ensures that anti-malarial drugs are only administered to patients who need them, thereby limiting the unnecessary use of inappropriate treatments. It also minimizes the selection and spread of drug-resistant Plasmodium falciparum parasites[3], particularly important in areas where multidrug resistance is prevalent, such as Southeast Asia[4‐7].

Microscopic examination of blood films still remains the gold standard for malaria diagnosis despite the requirement for high-quality microscopes and equipment, and for qualified personnel[8]. However, over the past two decades, rapid diagnostic tests (RDTs) for malaria have been developed for use in any situation where the only realistic alternative was clinical diagnosis of malaria, for example, at community level (e g, Village Malaria Workers’ strategy in Cambodia). These diagnostic tests are fast and easy to use, and do not require electricity or complex equipment[9]. RDTs are now available from around 60 manufacturers[10]. This profusion of suppliers and the variable quality of RDT products marketed have made it difficult for the policy makers of national malaria control programmes to determine which tests are the most suitable[11]. To address this issue, a global evaluation programme to guide RDT procurement (product testing) and assure RDT performance before and during use in the field (lot testing) was launched in 2002 by the WHO Regional Office of the Western Pacific (WPRO), in collaboration with the WHO/Global Malaria Programme, WHO/GMP), the UNICEF/UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (TDR), the Foundation for Innovative New Diagnostics (FIND), the Centers for Disease Control and Prevention (CDC, Atlanta) and numerous other partners[12].

The malaria RDTs available are all based on the same principle and like other lateral flow immune-chromatographic tests, there are various formats (dipstick, plastic cassette or card). They contain antibodies conjugated to colloidal gold particles, which bind specifically with parasite antigens. Some of them are specific for P. falciparum and detect the histidine-rich-2 protein (HRP-2). Others combine HRP-2 detection with the detection of antigens common to all species, such as lactate dehydrogenase or aldolase (combo RDTs). Combo RDTs can diagnose infections by P. falciparum or by non-P. falciparum malaria parasites (Plasmodium vivax, Plasmodium ovale and Plasmodium malariae)[13].

IMACCESS^© is a company which aims to address public health requirements by developing reliable diagnostic tools responding to the constraints associated with use in the field for populations in developing countries at affordable prices[14]. The company recently developed VIKIA Malaria Ag Pf/Pan™, a new malaria test. This new RDT, based on the same technology as other tests, allows detection of falciparum malaria (HRP-2) and non-falciparum malaria (aldolase).

In this context, the objective of the study presented here was to assess the performances of this new immunochromatographic test by following a conventional field evaluation design including microscopic examination (thick and thin blood film) and molecular detection (real-time PCR) as gold standard methods for malaria diagnosis of febrile, malaria-suspected patients seen at health centre level. In addition, the VIKIA Malaria Ag Pf/Pan™ test was also compared to another RDT widely used in Cambodia and elsewhere (CareStart Malaria™, AccessBio®).

From August to December 2011, 1,000 patients (Preah Vihear, N = 300, Veal Veng, N = 250, Ratanakiri, N = 300 and Takavit, N = 150) with an age range of one to 70 years (mean ± SD age = 22.4 ± 12.9 years; 7.0% <5 years of age, 18.0% between five and 14 years of age, and 75.0% >15 years of age) were recruited. The male/female ratio was 2.1/1. The mean ± SD axillary temperature was 38.1 ± 0.9°C (range = 37.5–41.5°C) and the mean ± SD parasitaemia density was 13,455 ± 52,694 parasites/μL (range = 20–582,500 parasites/μL).

Microscopy results showed that 389 (38.9%) of the 1,000 patients had malaria. Plasmodium falciparum was present in 39.5%, P. vivax in 58.8% and both P. falciparum and P. vivax (mixed infections) in 1.5% of the positive specimens. A total of 92 discordant results (9.2%) were found between microscopy and real-time PCR (Table2). Some 32 cases (3.2%) classified as negative by microscopy were positive by real-time PCR (sub-microscopic parasitaemia: six P. falciparum, six mixed P. falciparum/P. vivax and 21 P. vivax). In addition, 59 samples were misclassified by microscopy: 54 were classified as single infections by microscopy (43 P. falciparum, 11 P. vivax) but found to be mixed infections by real-time PCR (52 mixed P. falciparum/P. vivax and two mixed P. vivax/P. malariae); three cases classified as mixed P. falciparum/P. vivax infections were identified as single P. falciparum infections by real-time PCR; two cases classified as single P. vivax infections were found to be single P. falciparum infections by real-time PCR.

Details concerning patients positive for Plasmodium spp. by real-time PCR, microscopy, CareStart Malaria™ and VIKIA Malaria Ag Pf/Pan™ tests at several reading times (10, 15, 20, 30 and 60 min) are shown in Table3. Frequencies of false positives, false negatives and misclassified results obtained from RDTs with reference to microscopy and PCR results (including non-interpretable data; Table4) indicated that the best reading times were 20–30 min for the VIKIA Malaria Ag Pf/Pan™ test. Diagnostic performances of both RDTs are shown in Table5 and Figure3. Sensitivities for different levels of Plasmodium spp. parasitaemia are summarized in Table6, Figure4 for P. falciparum infections and Figure5 for non-P. falciparum infections.

This study reports the first evaluation of the performance of the VIKIA Malaria Ag Pf/Pan™, a new immunochromatographic test developed by IMACCESS®, using a combination of microscopy and real-time PCR as reference methods for classification of samples. This approach had the advantage of combining the high sensitivity (especially at low parasite density) and specificity (to correctly identify the parasite species) of real-time PCR, with the possibility of estimating parasite density by microscopy. As expected, more positive samples were found using real-time PCR than microscopy (+ 8.5%, 38.9% vs 42.2%), CareStart Malaria™ test (+9.7%, 38.1% vs 42.2%) or VIKIA Malaria Ag Pf/Pan™ test (from +10.4% for 60 min reading time to +27.0% for 10 min reading time, 30.8% and 37.8%, respectively vs 42.2%). A similar trend was also found for a greater sensitivity of microscopy by expert microscopists than malaria RDTs: 2.0% more positive samples by microscopy than CareStart Malaria™ test (38.8% vs 38.1%) and 2.8% (60 min reading time) to 20.8% (10 min reading time) more than with the VIKIA Malaria Ag Pf/Pan™ test (38.8% vs 37.8% and 30.8%, respectively).

The sensitivity (and the frequency of false negatives) of the VIKIA Malaria Ag Pf/Pan™ test increased with reading time, from 89.7% for P. falciparum detection and 61.5% for non-P. falciparum detection (10 min) to 94.2% for P. falciparum detection and 82.7% for non-P. falciparum detection (60 min). The specificity (and the frequency of false positives) decreased with reading time from 99.3% for P. falciparum detection and 99.5% for non-P. falciparum detection (10 min) to 98.1% for P. falciparum detection and 98.9% for non-P. falciparum detection (60 min).

The best performances of the VIKIA Malaria Ag Pf/Pan™ test for both P. falciparum and non-P. falciparum detection were observed at 20–30 min (sensitivity: 93.4% for P. falciparum and 81.1%-82.8% for non-P. falciparum; specificity: 98.6% for P. falciparum and 98.9% for non-P. falciparum). Using these reading times, the performance of the VIKIA Malaria Ag Pf/Pan™ test was similar to those of the CareStart Malaria™ test or those found in previous studies[21‐26].

Moreover, as has been previously reported, the sensitivities of both malaria RDT decreased with the level of the parasitaemia[23, 26‐30]: for P. falciparum, the sensitivity of the both tests started to decrease at levels of parasitaemia < 500 parasites/μL (100% above 500 parasites/μL) and for non-P. falciparum, the sensitivity of the CareStart Malaria™ test and the VIKIA Malaria Ag Pf/Pan™ test started to decrease below 1,000 parasites/μL or below 10,000 parasites/μL, respectively. False negative results for both malaria RDT were only found in P. falciparum samples containing <250 parasites/μL and in non-P. falciparum samples with <1,000 parasites/μL.

According to WHO recommendations for RDT performances[31], sensitivity of the VIKIA Malaria Ag Pf/Pan™ test (except when the time reading was 10 min) was greater than 95%, excluding samples with a parasitaemia <100 parasites/μL. As observed for the CareStart Malaria™ test, the VIKIA Malaria Ag Pf/Pan™ test was easy to use and interpret and simple to store with no cold chain requirement.

In conclusion, this new RDT performs similarly well as other commercially available tests (and in particular the CareStart Malaria™ test, used as comparator), and conforms to WHO recommendations for RDT performance. It appears to be a satisfactory alternative tool for the diagnosis of malaria in endemic areas.