The performance of RDTs in diagnosing PAM was evaluated against microscopy and nested Plasmodium species diagnostic PCR in a cohort of pregnant women in north-eastern Tanzania. The use of RDTs might act as an appropriate complementary diagnostic tool for malaria instead of only relying on presumptive treatment based on clinical grounds in areas with limited expert microscopy and laboratory infrastructure. Prescription of any drug during pregnancy is a challenging task due to potential risks of harming the foetus , over-prescription and subsequent risk of drug resistance development . Simple, cheap, reliable, accurate, easy to use, sensitive and specific diagnostic tests that can identify genuine malarial cases are the only means of allowing accurate malaria detection and rational treatment. With the escalating anti-malarial drug resistance which necessitates the deployment of expensive artemisinin-based combination therapy, there is a need to prescribe anti-malarial drugs only to patients with true malarial illness . The present study demonstrates that RDTs can act as a diagnostic tool to manage malaria during pregnancy in resource poor settings with limited access to expert microscopy as they are easy to use and perform better than microscopy. Based on the PCR results, the different types of RDTs used in this study were able to capture over 40% of sub-microscopic infections missed by microscopy.
According to the current Ministry of Health and Social Welfare’s policy in Tanzania on malaria diagnosis, the use of RDTs is not yet implemented as a routine practice for pregnant women at antenatal clinics. Therefore, there is a need to provide evidence-based data on the best diagnostic supplement/alternative for malaria diagnosis during pregnancy in areas with limited laboratory infrastructure. This will assist the National Malaria Control Programme in Tanzania and beyond when considering RDTs as a possible routine diagnostic tool in malaria diagnosis during pregnancy. According to the recent WHO report  on the performance of different RDTs, it has been shown that they are easy to use, are heat stable and have the ability to detect parasitaemia as low as 200 asexual stages/μl. This makes RDTs an ideal diagnostic supplement to malaria diagnosis in resource constrained settings.
In many of the malaria endemic regions including Tanzania, healthcare delivery in peripheral settings is compromised by the lack of well equipped laboratories and personnel with sufficient expertise in malaria microscopy . Microscopy could be as sensitive as RDTs or even more sensitive when done well. However, adequate infrastructure, maintenance of good quality microscopy and proper training on expert microscopy are not always present in many malaria endemic settings. Mismanagement of sub-microscopic infections could result in low but persistent parasitaemia that may culminate in adverse pregnancy outcomes [42, 43]. Under-diagnosis and/or wrong diagnosis of true malarial infections may lead to infections going untreated or being wrongly treated as non-malarial illnesses, with subsequent adverse pregnancy outcomes and/or acting as potential reservoirs of transmission. However, in all malaria endemic settings, children with febrile illnesses are treated by anti-malarials and/or other antimicrobials following the World Health Organization guidelines’ on integrated management of childhood illnesses.
Singer et al. have shown that microscopy underestimates the real malarial burden during pregnancy. Nonetheless in their study, contrary to this study, PCR detected more positive cases as compared to RDTs, whilst assessing only placental blood samples. The present study might differ from that of Singer and colleagues in malaria transmission intensities and also in the current study only a small proportion of RDT and microscopy negative samples were checked by PCR due to limited resources and time. However, when taken as a proportion of placental blood the 351 samples checked by PCR accounts for 54% of the available placental blood samples with complete data. The message portrayed here is that microscopy was shown to have underestimated the true malarial prevalence after PCR correction.
The persistence of HRP-2 circulation in the blood more than two weeks even after successful clearance of IEs in the bloodstream is one of the concerns on the usefulness of HRP-2 based RDTs in malaria diagnosis, as has been reported by many studies [35, 36, 38, 39, 60]. However, in most of these studies microscopy was used as gold standard without PCR correction and this might have categorized sub-microscopic infections as false positives due to the limited sensitivity of microscopy. In the present study, all the RDT positive but microscopy negative samples were checked by PCR and the analyses showed that a large proportion of the RDT positive but microscopy negative samples were in fact sub-microscopic infections. Treatment of these few false positive women with anti-malarials might provide some prophylactic effect against subsequent infections outweighing the risk of not treating genuine sub-microscopic infections missed by microscopy that could have a profound effect on the pregnancy outcome. The relatively poor performance of microscopy compared to RDT cannot be explained by suboptimal conditions for microscopy as the present study was conducted in parallel with a large clinical trial . Therefore, laboratory conditions were excellent and all the laboratory technologists had ample experience in malaria diagnosis and were undergoing proficiency microscopy tests on a regular basis.
The performance of RDTs in malaria diagnosis in the present study is in agreement with studies by Tjitra, Batwala and Tham et al.[26, 27, 62] showing that RDTs were performing better than microscopy in malaria diagnosis under field conditions. Likewise, Bell et al. conducted a study in the Philippines in an area of low endemicity and reported that sub-microscopic infections missed by microscopy but captured by RDTs were actually true infections after PCR correction. On the other hand, the current study is not in agreement with a study by Schachterle et al. that showed that RDTs had high rates of false positives and negatives in a region of hypoendemicity. However, the results of that study were purely based on microscopy data without PCR correction.
RDTs missed some few infections, which were positive in repeated microscopic investigations. This could be due to assay degradation as a result of humid conditions or batch variability of the RDTs , delay in HRP-2 surge after increased parasite density  or due to deletion of hrp-2 genes in some parasites . Some studies have also reported reduced sensitivity of RDTs as a result of low parasitaemia [33, 65] and this could explain the reason for the few cases with low parasite densities missed by RDTs [49, 50].
This study indicates that RDTs outperform expert microscopy in detecting asymptomatic P. falciparum in pregnant women. Given the difficulties in establishing reliable microscopy based diagnostic services, RDTs are good alternative for the detection and in the management of P. falciparum infections in pregnant women. RDTs can both be used to detect infections not cleared by IPTp or to detect infections where the malaria endemicity is too low to warrant IPTp.