Malaria surveys using rapid diagnostic tests and validation of results using post hoc quantification of Plasmodium falciparum histidine-rich protein 2

Deciphering immunodiffusion: In silico optimization for faster protein diagnostics

Immunodiffusion tests offer a simple yet powerful method for detecting protein antigens, but their long assay times hinder clinical utility. We unveil the complex interplay of parameters governing this process using finite element simulations. By meticulously validating our model against real-world data, we elucidate how initial concentrations and diffusivities of antigen and antibody shape the intensity, size, and formation time of the precipitin ring. Our key innovation lies in employing phase diagram analysis to map the combined effects of these parameters on assay performance. This framework enables rapid in silico parameter estimation, paving the way for the design of novel immunodiffusion assays with drastically reduced assay times. The presented approach holds immense potential for optimizing protein diagnostics for fast and reliable diagnostics.

Receipt of antimalarials among children aged 6-59 months in Nigeria from 2010 to 2021

BACKGROUND

Nigeria has the highest malaria burden globally, and anti-malarials have been commonly used to treat malaria without parasitological confirmation. In 2012, Nigeria implemented rapid diagnostic tests (RDTs) to reduce the use of anti-malarials for those without malaria and to increase the use of artemisinin-based combination therapy (ACT) for malaria treatment. This study examined changes in anti-malarial receipt among children aged 6-59 months during a 12-year period of increasing RDT availability.

METHODS

A cross-sectional analysis was conducted using the Nigeria Malaria Indicator Survey (NMIS) data from 2010 (before RDT implementation in 2012), 2015, and 2021. The analysis assessed trends in prevalence of malaria by survey RDT result, and fever and anti-malarial/ACT receipt in the 2 weeks prior to the survey. A multivariable logistic regression was used to account for the complex survey design and to examine factors associated with anti-malarial receipt, stratified by survey RDT result, a proxy for recent/current malaria infection.

RESULTS

In a nationally-representative, weighted sample of 22,802 children aged 6-59 months, fever prevalence remained stable over time, while confirmed malaria prevalence decreased from 51.2% in 2010 to 44.3% in 2015 and 38.5% in 2021 (trend test p < 0.0001). Anti-malarial use among these children decreased from 19% in 2010 to 10% in 2021 (trend test p < 0.0001), accompanied by an increase in ACT use (2% in 2010 to 8% in 2021; trend test p < 0.0001). Overall, among children who had experienced fever, 30.6% of survey RDT-positive and 36.1% of survey RDT-negative children had received anti-malarials. The proportion of anti-malarials obtained from the private sector increased from 61.8% in 2010 to 80.1% in 2021 for RDT-positive children; most of the anti-malarials received in 2021 were artemisinin-based combinations. Factors associated with anti-malarial receipt for both RDT-positive and RDT-negative children included geographic region, greater household wealth, higher maternal education, and older children.

CONCLUSION

From 2010 to 2021 in Nigeria, both malaria prevalence and anti-malarial treatments among children aged 6-59 months decreased, as RDT availability increased. Among children who had fever in the prior 2 weeks, anti-malarial receipt was similar between children with either positive or negative survey RDT results, indicative of persistent challenges in reducing inappropriate anti-malarials uptake.

A Reagent and Virus Benchmarking Panel for a Uniform Analytical Performance Assessment of N Antigen-Based Diagnostic Tests for COVID-19

Rapid diagnostic tests (RDTs) that detect antigen indicative of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection can help in making quick health care decisions and regularly monitoring groups at risk of infection. With many RDT products entering the market, it is important to rapidly evaluate their relative performance. Comparison of clinical evaluation study results is challenged by protocol design variations and study populations. Laboratory assays were developed to quantify nucleocapsid (N) and spike (S) SARS-CoV-2 antigens. Quantification of the two antigens in nasal eluates confirmed higher abundance of N than S antigen. The median concentration of N antigen was 10 times greater than S per genome equivalent. The N antigen assay was used in combination with quantitative reverse transcription (RT)-PCR to qualify a panel composed of recombinant antigens, inactivated virus, and clinical specimen pools. This benchmarking panel was applied to evaluate the analytical performance of the SD Biosensor Standard Q COVID-19 antigen (Ag) test, Abbott Panbio COVID-19 Ag rapid test, Abbott BinaxNOW COVID-19 Ag test, and the LumiraDx SARS-CoV-2 Ag test. The four tests displayed different sensitivities toward the different panel members, but all performed best with the clinical specimen pool. The concentration for a 90% probability of detection across the four tests ranged from 21 to 102 pg/mL of N antigen in the extracted sample. Benchmarking panels provide a quick way to verify the baseline performance of a diagnostic and enable direct comparisons between diagnostic tests. IMPORTANCE This study reports the results for severe acute respiratory syndrome coronavirus-2 (SARS-COV-2) nucleocapsid (N) and spike (S) antigen quantification assays and their performance against clinical reverse transcription (RT)-PCR results, thus describing an open-access quantification method for two important SARS-CoV-2 protein analytes. Characterized N antigen panels were used to evaluate the limits of detection of four different rapid tests for SARS-CoV-2 against multiple sources of nucleocapsid antigen, demonstrating proof-of-concept materials and methodology to evaluate SARS-CoV-2 rapid antigen detection tests. Quantification of N antigen was used to characterize the relationship between viral count and antigen concentration among clinical samples and panel members of both clinical sample and viral culture origin. This contributes to a deeper understanding of protein antigen and molecular analytes and presents analytical methods complementary to clinical evaluation for characterizing the performance of both laboratory-based and point-of-care rapid diagnostics for SARS-CoV-2.

Plasmodium falciparum infection prevalence among children aged 6-59 months from independent DHS and HIV surveys: Nigeria, 2018

Prevalence estimates are critical for malaria programming efforts but generating these from non-malaria surveys is not standard practice. Malaria prevalence estimates for 6-59-month-old Nigerian children were compared between two national household surveys performed simultaneously in 2018: a Demographic and Health Survey (DHS) and the Nigeria HIV/AIDS Indicator and Impact Survey (NAIIS). DHS tested via microscopy (n = 8298) and HRP2-based rapid diagnostic test (RDT, n = 11,351), and NAIIS collected dried blood spots (DBS) which were later tested for histidine-rich protein 2 (HRP2) antigen (n = 8029). National Plasmodium falciparum prevalence was 22.6% (95% CI 21.2- 24.1%) via microscopy and 36.2% (34.6- 37.8%) via RDT according to DHS, and HRP2 antigenemia was 38.3% (36.7-39.9%) by NAIIS DBS. Between the two surveys, significant rank-order correlation occurred for state-level malaria prevalence for RDT (Rho = 0.80, p < 0.001) and microscopy (Rho = 0.75, p < 0.001) versus HRP2. RDT versus HRP2 positivity showed 24 states (64.9%) with overlapping 95% confidence intervals from the two independent surveys. P. falciparum prevalence estimates among 6-59-month-olds in Nigeria were highly concordant from two simultaneous, independently conducted household surveys, regardless of malaria test utilized. This provides evidence for the value of post-hoc laboratory HRP2 detection to leverage non-malaria surveys with similar sampling designs to obtain accurate P. falciparum estimates.

Performance of antigen detection for HRP2-based malaria rapid diagnostic tests in community surveys: Tanzania, July-November 2017

BACKGROUND

Malaria rapid diagnostic tests (RDTs) based on the detection of the Plasmodium falciparum histidine-rich protein 2 (HRP2) antigen are widely used for detection of active infection with this parasite and are the only practical malaria diagnostic test in some endemic settings. External validation of RDT results from field surveys can confirm appropriate RDT performance.

METHODS

A community-based cross-sectional survey was conducted between July and November 2017 enrolling participants of all ages in households from 15 villages in four border regions of Tanzania: Geita, Kigoma, Mtwara and Ruvuma. All participants had an RDT performed in the field and provided a blood sample for later laboratory multiplex antigen detection of HRP2. In assessing the continuous HRP2 levels in participant blood versus RDT result, dose-response logistic regression provided quantitative estimates for HRP2 limit of detection (LOD).

RESULTS

From the 15 study villages, 6941 persons were enrolled that had a RDT at time of enrollment and provided a DBS for later laboratory antigen detection. RDT positive prevalence for the HRP2 band by village ranged from 20.0 to 43.6%, but the magnitude of this prevalence did not have an effect on the estimated LOD of RDTs utilized in different villages. Overall, HRP2 single-target tests had a lower LOD at the 95% probability of positive RDT (4.3 ng/mL; 95% CI 3.4-5.4) when compared to pLDH/HRP2 dual target tests (5.4 ng/mL; 4.5-6.3), though this difference was not significant. With the exception of one village, all other 14 villages (93.3%) showed RDT LOD estimates at 90% probability of positive RDT between 0.5 and 12.0 ng/mL.

CONCLUSIONS

Both HRP2-only and pLDH/HRP2 combo RDTs utilized in a 2017 Tanzania cross-sectional survey of border regions generally performed well, and reliably detected HRP2 antigen in the low ng/mL range. Though single target tests had lower levels of HRP2 detection, both tests were within similar ranges among the 15 villages. Comparison of quantitative HRP2 detection limits among study sites can help interpret RDT testing results when generating population prevalence estimates for malaria infection.

Screening strategies and laboratory assays to support Plasmodium falciparum histidine-rich protein deletion surveillance: where we are and what is needed

Rapid diagnostic tests (RDTs) detecting Plasmodium falciparum histidine-rich protein 2 (HRP2) have been an important tool for malaria diagnosis, especially in resource-limited settings lacking quality microscopy. Plasmodium falciparum parasites with deletion of the pfhrp2 gene encoding this antigen have now been identified in dozens of countries across Asia, Africa, and South America, with new reports revealing a high prevalence of deletions in some selected regions. To determine whether HRP2-based RDTs are appropriate for continued use in a locality, focused surveys and/or surveillance activities of the endemic P. falciparum population are needed. Various survey and laboratory methods have been used to determine parasite HRP2 phenotype and pfhrp2 genotype, and the data collected by these different methods need to be interpreted in the appropriate context of survey and assay utilized. Expression of the HRP2 antigen can be evaluated using point-of-care RDTs or laboratory-based immunoassays, but confirmation of a deletion (or mutation) of pfhrp2 requires more intensive laboratory molecular assays, and new tools and strategies for rigorous but practical data collection are particularly needed for large surveys. Because malaria diagnostic strategies are typically developed at the national level, nationally representative surveys and/or surveillance that encompass broad geographical areas and large populations may be required. Here is discussed contemporary assays for the phenotypic and genotypic evaluation of P. falciparum HRP2 status, consider their strengths and weaknesses, and highlight key concepts relevant to timely and resource-conscious workflows required for efficient diagnostic policy decision making.

Investigation of Plasmodium falciparum pfhrp2 and pfhrp3 gene deletions and performance of a rapid diagnostic test for identifying asymptomatic malaria infection in northern Ethiopia, 2015

BACKGROUND

Rapid diagnostic tests (RDTs) are widely used for malaria diagnosis of both symptomatic and asymptomatic infections. Although RDTs are a reliable and practical diagnostic tool, the sensitivity of histidine-rich protein 2 (HRP2)-based RDTs can be reduced if pfhrp2 or pfhrp3 (pfhrp2/3) gene deletions exist in the Plasmodium falciparum parasite population. This study evaluated dried blood spot (DBS) samples collected from a national household survey to investigate the presence of pfhrp2/3 deletions and the performance of the RDT used in the cross-sectional survey in a low transmission setting.

METHODS

The 2015 Ethiopia Malaria Indicator Survey tested household members by RDT and collected DBS samples. DBS (n = 2648) from three regions in northern Ethiopia were tested by multiplex bead-based antigen detection assay after completion of the survey. The multiplex assay detected pan-Plasmodium lactate dehydrogenase (LDH), pAldolase, and HRP2 antigens in samples. Samples suspected for pfhrp2/3 gene deletions (pLDH and/or pAldolase positive but low or absent HRP2) were further investigated by molecular assays for gene deletions. Antigen results were also compared to each individual's RDT results. Dose-response logistic regression models were fit to estimate RDT level of detection (LOD) antigen concentrations at which 50, 75, 90, and 95% of the RDTs returned a positive result during this survey.

RESULTS

Out of 2,648 samples assayed, 29 were positive for pLDH or pAldolase antigens but low or absent for HRP2 signal, and 15 of these samples (51.7%) were successfully genotyped for pfhrp2/3. Of these 15 P. falciparum infections, eight showed single deletions in pfhrp3, one showed a single pfhrp2 deletion, and six were pfhrp2/3 double-deletions. Six pfhrp2 deletions were observed in Tigray and one in Amhara. Twenty-five were positive for HRP2 by the survey RDT while the more sensitive bead assay detected 30 HRP2-positive samples. A lower concentration of HRP2 antigen generated a positive test result by RDT compared to pLDH (95% LOD: 16.9 ng/mL vs. 319.2 ng/mL, respectively).

CONCLUSIONS

There is evidence of dual pfhrp2/3 gene deletions in the Tigray and Amhara regions of Ethiopia in 2015. As the prevalence of malaria was very low (< 2%), it is difficult to make strong conclusions on RDT performance, but these results challenge the utility of biomarkers in household surveys in very low transmission settings.

Plasmodium falciparum pfhrp2 and pfhrp3 Gene Deletions from Persons with Symptomatic Malaria Infection in Ethiopia, Kenya, Madagascar, and Rwanda

Histidine-rich protein 2 (HRP2)–based rapid diagnostic tests detect Plasmodium falciparum malaria and are used throughout sub-Saharan Africa. However, deletions in the pfhrp2 and related pfhrp3 (pfhrp2/3) genes threaten use of these tests. Therapeutic efficacy studies (TESs) enroll persons with symptomatic P. falciparum infection. We screened TES samples collected during 2016–2018 in Ethiopia, Kenya, Rwanda, and Madagascar for HRP2/3, pan-Plasmodium lactate dehydrogenase, and pan-Plasmodium aldolase antigen levels and selected samples with low levels of HRP2/3 for pfhrp2/3 genotyping. We observed deletion of pfhrp3 in samples from all countries except Kenya. Single-gene deletions in pfhrp2 were observed in 1.4% (95% CI 0.2%–4.8%) of Ethiopia samples and in 0.6% (95% CI 0.2%–1.6%) of Madagascar samples, and dual pfhrp2/3 deletions were noted in 2.0% (95% CI 0.4%–5.9%) of Ethiopia samples. Although this study was not powered for precise prevalence estimates, evaluating TES samples revealed a low prevalence of pfhrp2/3 deletions in most sites.

Assessing the Performance of CareStart™ Malaria Rapid Diagnostic Tests in Northwest Ethiopia: A Cross-Sectional Study

Background

While rapid diagnostic tests are an alternative diagnostic tests for microscopy in the diagnosis of malaria in rural settings, their performance has been inconsistent. Performance of rapid diagnostic tests might be affected by manufacturing process, transportation and storage, parasitemia level, and skill of personnel who perform the tests. Therefore, periodic evaluation of the local field performance of rapid diagnostic tests is mandatory in order to make early corrections in case of decreased performance.

Methods

A facility-based cross-sectional study was conducted from January to May 2020 among 257 malaria-suspected patients attending selected health centers in Bahir Dar Zuria district. Capillary blood was collected from each participant and tested for Plasmodium infection by CareStart™ rapid diagnostic test kit and thin and thick blood film microscopy. Data were analyzed using statistical software for social sciences version 20 and MedCalc software version 19.3. Sensitivity, specificity, positive and negative predictive values, and kappa value were calculated to evaluate the performance of rapid diagnostic tests against microscopy.

Results

Among 257 study participants, 47 (18.3%) were tested positive for Plasmodium infection by at least one of the diagnostic methods. Rapid diagnostic tests revealed 3 false positive and 3 false negative results. The sensitivity and specificity of CareStart Malaria Pf/Pv Combo test were 93.2% and 98.6%, respectively (kappa = 0.918).

Conclusion

CareStart™ rapid diagnostic test has comparable performance with microscopy for malaria diagnosis. We recommend continued use of CareStart Malaria Pf/Pv Combo test at health posts in Ethiopia where microscopy is not available.

Considerations for quality assurance of multiplex malaria antigen detection assays with large sample sets

Multiplex assays for malaria antigen detection can gather data from large sample sets, but considerations for the consistency and quality assurance (QA) of mass testing lack evaluation. We present a QA framework for a study occurring November 2019 to March 2020 involving 504 assay plates detecting four Plasmodium antigens: pan-Plasmodium aldolase and lactate dehydrogenase (LDH), histidine-rich protein 2 (HRP2), P. vivax LDH (PvLDH). Controls on each plate included buffer blank, antigen negative blood, and 4-point positive dilution curve. The blank and negative blood provided consistently low signal for all targets except for pAldolase, which showed variability. Positive curve signals decreased throughout the 5-month study duration but retained a coefficient of variation (CV) of < 5%, with the exception of HRP2 in month 5 (CV of 11%). Regression fittings for inter-plate control signals provided mean and standard deviations (SDs), and of 504 assay plates, 6 (1.2%) violated the acceptable deviation limits and were repeated. For the 40,272 human blood samples assayed in this study, of 161,088 potential data points (each sample × 4 antigens), 160,641 (99.7%) successfully passed quality checks. The QA framework presented here can be utilized to ensure quality of laboratory antigen detection for large sample sets.

Laboratory Detection of Malaria Antigens: a Strong Tool for Malaria Research, Diagnosis, and Epidemiology

The identification and characterization of proteins produced during human infection with Plasmodium spp. have guided the malaria community in research, diagnosis, epidemiology, and other efforts. Recently developed methods for the detection of these proteins (antigens) in the laboratory have provided new types of data that can inform the evaluation of malaria diagnostics, epidemiological investigations, and overall malaria control strategies. Here, the focus is primarily on antigens that are currently known to be detectable in human specimens and on their impact on the understanding of malaria in human populations. We highlight historical and contemporary laboratory assays for malaria antigen detection, the concept of an antigen profile for a biospecimen, and ways in which binary results for a panel of antigens could be interpreted and utilized for different analyses. Particular emphasis is given to the direct comparison of field-level malaria diagnostics and laboratory antigen detection for the development of an external evaluation scheme. The current limitations of laboratory antigen detection are considered, and the future of this developing field is discussed.

Malaria Rapid Diagnostic Tests: Literary Review and Recommendation for a Quality Assurance, Quality Control Algorithm

Malaria rapid diagnostic tests (RDTs) have had an enormous global impact which contributed to the World Health Organization paradigm shift from empiric treatment to obtaining a parasitological diagnosis prior to treatment. Microscopy, the classic standard, requires significant expertise, equipment, electricity, and reagents. Alternatively, RDT’s lower complexity allows utilization in austere environments while achieving similar sensitivities and specificities. Worldwide, there are over 200 different RDT brands that utilize three antigens: Plasmodium histidine-rich protein 2 (PfHRP-2), Plasmodium lactate dehydrogenase (pLDH), and Plasmodium aldolase (pALDO). pfHRP-2 is produced exclusively by Plasmodium falciparum and is very Pf sensitive, but an alternative antigen or antigen combination is required for regions like Asia with significant Plasmodium vivax prevalence. RDT sensitivity also decreases with low parasitemia (<100 parasites/uL), genetic variability, and prozone effect. Thus, proper RDT selection and understanding of test limitations are essential. The Center for Disease Control recommends confirming RDT results by microscopy, but this is challenging, due to the utilization of clinical laboratory standards, like the College of American Pathologists (CAP) and the Clinical Lab Improvement Act (CLIA), and limited recourses. Our focus is to provide quality assurance and quality control strategies for resource-constrained environments and provide education on RDT limitations.

Social perceptions of malaria and diagnostic-driven malaria treatment in Burkina Faso

Malaria is a parasitic disease, endemic in many tropical and sub-tropical countries. Malaria is a well-known disease, familiar to almost all people in endemic regions, as they or their family are regularly confronted with it; everyone in these regions has probably experienced the disease, at least once in their life. To investigate the social perceptions of malaria in Burkina Faso, including its diagnosis-driven treatment, we have conducted a survey in both urban (Saint Camille Hospital, Ouagadougou HOSCO) and rural (Boussé Hospital) areas. Fifty-six individuals, mostly representatives of the society variability, were surveyed by questionnaires and 2 focus groups were organized with traditional healers. In general, populations seem to have grasped the causes, symptoms and means of preventing the disease. However, the majority of interviewees make a marked confusion between malaria and dengue; dengue fever is considered like a severe form of malaria. The care modalities (modern and/or traditional medicine) are plural and the choice of therapeutic practice depends on both the socio-economic conditions and education level of the patient. Whereas some patients mark preferences for one type of medicine, others simultaneously recourse to both; for these, a medicine does not outperform the other and their combination multiplies the chances of a quick recovery. Whether for modern or traditional medicine, the diagnosis is considered very important for effective disease management. Modern medicine uses diagnostic tools based on light microscopy and immunochromatography (rapid diagnostic tests; RDT); traditional medicine has its own diagnostic logic but nevertheless recognizes modern medicine diagnosis to guide its therapy.

90 % of those interviewed first use modern medicine to seek an accurate diagnosis of their disease and thus to receive adequate treatment. Presumptive treatments are still widely prescribed and accepted by most patients who trust the judgment of their caregiver, not perceiving any benefit to an objective diagnosis. In front of a negative diagnosis, patient reactions are diverse, some accepting investigations for other diseases (45 %), others opting for self-medication (15 %), others resorting to traditional medicine (20 %).

All are unanimous in the importance of diagnosis and are in favor of in-development diagnostic technologies, provided these obviously meet the features of reliability, ease of use, availability and, of course, economical accessibility.

Conventional and High-Sensitivity Malaria Rapid Diagnostic Test Performance in 2 Transmission Settings: Haiti 2017

Accurate malaria diagnosis is foundational for control and elimination, and Haiti relies on histidine-rich protein 2 (HRP2)-based rapid diagnostic tests (RDTs) identifying Plasmodium falciparum in clinical and community settings. In 2017, 1 household and 2 easy-access group surveys tested all participants (N = 32 506) by conventional and high-sensitivity RDTs. A subset of blood samples (n = 1154) was laboratory tested for HRP2 by bead-based immunoassay and for P. falciparum 18S rDNA by photo-induced electron transfer polymerase chain reaction. Both RDT types detected low concentrations of HRP2 with sensitivity estimates between 2.6 ng/mL and 14.6 ng/mL. Compared to the predicate HRP2 laboratory assay, RDT sensitivity ranged from 86.3% to 96.0% between tests and settings, and specificity from 90.0% to 99.6%. In the household survey, the high-sensitivity RDT provided a significantly higher number of positive tests, but this represented a very small proportion (<0.2%) of all participants. These data show that a high-sensitivity RDT may have limited utility in a malaria elimination setting like Haiti.

Community-based surveys for Plasmodium falciparum pfhrp2 and pfhrp3 gene deletions in selected regions of mainland Tanzania

BACKGROUND

Histidine-rich protein 2 (HRP2)-based malaria rapid diagnostic tests (RDTs) are effective and widely used for the detection of wild-type Plasmodium falciparum infections. Although recent studies have reported false negative HRP2 RDT results due to pfhrp2 and pfhrp3 gene deletions in different countries, there is a paucity of data on the deletions of these genes in Tanzania.

METHODS

A community-based cross-sectional survey was conducted between July and November 2017 in four regions: Geita, Kigoma, Mtwara and Ruvuma. All participants had microscopy and RDT performed in the field and provided a blood sample for laboratory multiplex antigen detection (for Plasmodium lactate dehydrogenase, aldolase, and P. falciparum HRP2). Samples showing RDT false negativity or aberrant relationship of HRP2 to pan-Plasmodium antigens were genotyped to detect the presence/absence of pfhrp2/3 genes.

RESULTS

Of all samples screened by the multiplex antigen assay (n = 7543), 2417 (32.0%) were positive for any Plasmodium antigens while 5126 (68.0%) were negative for all antigens. The vast majority of the antigen positive samples contained HRP2 (2411, 99.8%), but 6 (0.2%) had only pLDH and/or aldolase without HRP2. Overall, 13 samples had an atypical relationship between a pan-Plasmodium antigen and HRP2, but were positive by PCR. An additional 16 samples with negative HRP2 RDT results but P. falciparum positive by microscopy were also chosen for pfhrp2/3 genotyping. The summation of false negative RDT results and laboratory antigen results provided 35 total samples with confirmed P. falciparum DNA for pfhrp2/3 genotyping. Of the 35 samples, 4 (11.4%) failed to consistently amplify positive control genes; pfmsp1 and pfmsp2 and were excluded from the analysis. The pfhrp2 and pfhrp3 genes were successfully amplified in the remaining 31 (88.6%) samples, confirming an absence of deletions in these genes.

CONCLUSIONS

This study provides evidence that P. falciparum parasites in the study area have no deletions of both pfhrp2 and pfhrp3 genes. Although single gene deletions could have been missed by the multiplex antigen assay, the findings support the continued use of HRP2-based RDTs in Tanzania for routine malaria diagnosis. There is a need for the surveillance to monitor the status of pfhrp2 and/or pfhrp3 deletions in the future.

Ultra-sensitive RDT performance and antigen dynamics in a high-transmission Plasmodium falciparum setting in Mali

BACKGROUND

The recent expansion of tools designed to accurately quantify malaria parasite-produced antigens has enabled us to evaluate the performance of rapid diagnostic tests (RDTs) as a function of the antigens they detect-typically histidine rich protein 2 (HRP2) or lactate dehydrogenase (LDH).

METHODS

For this analysis, whole blood specimens from a longitudinal study in Bancoumana, Mali were used to evaluate the performance of the ultra-sensitive HRP2-based Alere™ Malaria Ag P.f RDT (uRDT). The samples were collected as part of a transmission-blocking vaccine trial in a high transmission region for Plasmodium falciparum malaria. Furthermore, antigen dynamics after successful anti-malarial drug treatment were evaluated in these samples using the Q-Plex Human Malaria Array (4-Plex) to quantify antigen concentrations.

RESULTS

The uRDT had a 50% probability of a positive result at 207 pg/mL HRP2 [95% credible interval (CrI) 160-268]. Individuals with symptomatic infection remained positive by uRDT for a median of 33 days [95% confidence interval (CI) 28-47] post anti-malarial drug treatment. Biphasic exponential decay models accurately captured the population level post-treatment dynamics of both HRP2 and Plasmodium LDH (pLDH), with the latter decaying more rapidly. Motivated by these differences in rates of decay, a novel algorithm that used HRP2:pLDH ratios to predict if an individual had active versus recently cleared P. falciparum infection was developed. The algorithm had 77.5% accuracy in correctly classifying antigen-positive individuals as those with and without active infection.

CONCLUSIONS

These results characterize the performance of the ultra-sensitive RDT and demonstrate the potential for emerging antigen-quantifying technologies in the field of malaria diagnostics to be helpful tools in distinguishing between active versus recently cleared malaria infections.

One-step PCR: A novel protocol for determination of pfhrp2 deletion status in Plasmodium falciparum

Histidine-rich protein 2 (HRP2) detecting rapid diagnostic tests (RDTs) have played an important role in enabling prompt malaria diagnosis in remote locations. However, emergence of pfhrp2 deleted parasites is threatening the efficacy of RDTs, and the World Health Organization (WHO) has highlighted surveillance of these deletions as a priority. Nested PCR is used to confirm pfhrp2 deletion but is costly and laborious. Due to spurious amplification of paralogue pfhrp3, the identity of nested exon 1 PCR product must be confirmed by sequencing. Here we describe a new one-step PCR method for detection of pfhrp2. To determine sensitivity and specificity, all PCRs were performed in triplicate. Using photo-induced electron transfer (PET) PCR detecting 18srRNA as true positive, one-step had comparable sensitivity of 95.0% (88.7–98.4%) to nested exon 1, 99.0% (94.6–99.9%) and nested exon 2, 98.0% (93.0–99.8%), and comparable specificity 93.8% (69.8–99.8%) to nested exon 1 100.0% (79.4–100.0%) and nested exon 2, 100.0% (74.4–100.0%). Sequencing revealed that one step PCR does not amplify pfhrp3. Logistic regression models applied to measure the 95% level of detection of the one-step PCR in clinical isolates provided estimates of 133p/μL (95% confidence interval (CI): 3-793p/μL) for whole blood (WB) samples and 385p/μL (95% CI: 31–2133 p/μL) for dried blood spots (DBSs). When considering protocol attributes, the one-step PCR is less expensive, faster and more suitable for high throughput. In summary, we have developed a more accurate PCR method that may be ideal for the application of the WHO protocol for investigating pfhrp2 deletions in symptomatic individuals presenting to health care facilities.

Capture and Detection of Plasmodium vivax Lactate Dehydrogenase in a Bead-Based Multiplex Immunoassay

Laboratory detection of malaria antigens has proved valuable for research and epidemiological purposes. We recently developed a bead-based multiplex antigen assay for pan-Plasmodium and Plasmodium falciparum targets. Here, we report integration of a Plasmodium vivax-specific target to this multiplex panel: P. vivax lactate dehydrogenase (PvLDH). Within the multiplex panel, assay signal for purified PvLDH antigen titrated into the single-digit picogram range. Against a panel of polymerase chain reaction (PCR)-confirmed samples from acute P. vivax infections (n = 36), sensitivity was 91.7% in using PvLDH detection for identifying the presence of parasites. Specificity against a panel of persons with no Plasmodium infection (n = 44) was 100%, and specificity against a panel of PCR-confirmed P. falciparum, Plasmodium malariae, or Plasmodium ovale infections (n = 164) was 90.2%. Addition of this PvLDH capture and detection system into the multiplex antigen panel will now allow for sensitive screening for species identification of both P. falciparum and P. vivax in the laboratory.

Combination of Serological, Antigen Detection, and DNA Data for Plasmodium falciparum Provides Robust Geospatial Estimates for Malaria Transmission in Haiti

Microscopy is the gold standard for malaria epidemiology, but laboratory and point-of-care (POC) tests detecting parasite antigen, DNA, and human antibodies against malaria have expanded this capacity. The island nation of Haiti is endemic for Plasmodium falciparum (Pf) malaria, though at a low national prevalence and heterogenous geospatial distribution. In 2015 and 2016, serosurveys were performed of children (ages 6–7 years) sampled in schools in Saut d’Eau commune (n = 1,230) and Grand Anse department (n = 1,664) of Haiti. Children received malaria antigen rapid diagnostic test and provided a filter paper blood sample for further laboratory analysis of the Pf histidine-rich protein 2 (HRP2) antigen, Pf DNA, and anti-Pf IgG antibodies. Prevalence of Pf infection ranged from 0.0–16.7% in 53 Saut d’Eau schools, and 0.0–23.8% in 56 Grand Anse schools. Anti-Pf antibody carriage exceeded 80% of students in some schools from both study sites. Geospatial prediction ellipses were created to indicate clustering of positive tests within the survey areas and overlay of all prediction ellipses for the different types of data revealed regions with high likelihood of active and ongoing Pf malaria transmission. The geospatial utilization of different types of Pf data can provide high confidence for spatial epidemiology of the parasite.

Multiplex malaria antigen detection by bead-based assay and molecular confirmation by PCR shows no evidence of Pfhrp2 and Pfhrp3 deletion in Haiti

BACKGROUND

The Plasmodium falciparum parasite is the only human malaria that produces the histidine-rich protein 2 and 3 (HRP2/3) antigens. Currently, HRP2/3 are widely used in malaria rapid diagnostic tests (RDTs), but several global reports have recently emerged showing genetic deletion of one or both of these antigens in parasites. Deletion of these antigens could pose a major concern for P. falciparum diagnosis in Haiti which currently uses RDTs based solely on the detection of the HRP2/3 antigens.

METHODS

From September 2012 through February 2014, dried blood spots (DBS) were collected in Haiti from 9317 febrile patients presenting to 17 health facilities in 5 departments throughout the country as part of a bed net intervention study. All DBS from RDT positive persons and a random sampling of DBS from RDT negative persons were assayed for P. falciparum DNA by nested and PET-PCR (n = 2695 total). All PCR positive samples (n = 331) and a subset of PCR negative samples (n = 95) were assayed for three malaria antigens by a multiplex bead assay: pan-Plasmodium aldolase (pAldo), pan-Plasmodium lactate dehydrogenase (pLDH), and HRP2/3. Any samples positive for P. falciparum DNA, but negative for HRP2/3 antigens were tested by nested PCR for Pfhrp2 and Pfhrp3 gene deletions.

RESULTS

Of 2695 DBS tested for Plasmodium DNA, 345 (12.8%) were originally found to be positive for P. falciparum DNA; 331 of these had DBS available for antigen detection. Of these, 266 (80.4%) were positive for pAldo, 221 (66.8%) positive for pLDH, and 324 (97.9%) were positive for HRP2/3 antigens. Seven samples (2.1%) positive for P. falciparum DNA were not positive for any of the three antigens by the bead assay, and were investigated for potential Pfhrp2/3 gene deletion by PCR. These samples either successfully amplified Pfhrp2/3 genes or were at an estimated parasite density too low for sufficient DNA to perform successful genotyping.

CONCLUSIONS

Malaria positive samples in multiple Haitian sites were found to contain the HRP2/3 antigens, and no evidence was found of Pfhrp2/3 deletions. Malaria RDTs based on the detection of the HRP2/3 antigens remain a reliable P. falciparum diagnostic tool as Haiti works towards malaria elimination.

Assessing Performance of HRP2 Antigen Detection for Malaria Diagnosis in Mozambique

Rapid diagnostic tests (RDTs) that detect the Plasmodium falciparum-specific histidine-rich protein 2 (PfHRP2) antigen are the primary methods for malaria diagnosis in Mozambique. However, these tests do not detect infections with non-falciparum malaria or Pfhrp2- and Pfhrp3-deleted P. falciparum parasites. To assess the appropriateness of conventional PfHRP2-only RDTs for malaria diagnosis in Mozambique, samples collected during a health facility survey conducted in three provinces of Mozambique were screened using antigen detection methods and further characterized by molecular techniques. Samples from 1,861 outpatients of all ages and symptoms attending 117 randomly selected public health facilities in 2018 were analyzed with an ultrasensitive bead-based immunoassay for the presence of PfHRP2, pan-Plasmodium aldolase (pAldo), and pan-Plasmodium lactate dehydrogenase (pLDH). The presence of PfHRP2 in patient blood detected using the bead-based assay was compared to the results of PfHRP2-based RDTs performed during the routine health facility consult and during the survey reexamination at the exit interview. Samples with discordant antigen profiles (negative for PfHRP2 but positive for pAldo and/or pLDH) were further characterized by photoinduced electron transfer PCR (PET-PCR). Using the bead-based laboratory assay as the gold standard, the sensitivities of the conventional RDTs administered during the routine health facility consult and the exit interview were 90% and 83%, respectively, and the specificities were 91% and 97%, respectively. Of 710 samples positive for at least one antigen, 704 (99.2%) were positive for PfHRP2. Six (0.8% of total) discordant samples lacked PfHRP2 but were positive for pAldo and/or pLDH; 3 of these (0.4% of total) were Plasmodium ovale monoinfections or coinfections where P. ovale was the dominant species. The remaining 3 discordant samples were negative by PET-PCR. The sensitivity and specificity of the conventional RDTs performed in the routine health facility consults and survey exit interviews were acceptable, and there was no evidence of Pfhrp2- and Pfhrp3-deleted parasites. Monoinfections with non-falciparum malaria species comprised <1% of the total malaria infections. Nearly all malaria antigen-positive patients had detectable PfHRP2, confirming that this antigen remains an appropriate malaria diagnostic target in the surveyed provinces.

False-negative malaria rapid diagnostic test results and their impact on community-based malaria surveys in sub-Saharan Africa

Surveillance and diagnosis of Plasmodium falciparum malaria relies predominantly on rapid diagnostic tests (RDT). However, false-negative (FN) RDT results are known to occur for a variety of reasons, including operator error, poor storage conditions, pfhrp2/3 gene deletions, poor performance of specific RDT brands and lots, and low-parasite density infections. We used RDT and microscopy results from 85 000 children enrolled in Demographic Health Surveys and Malaria Indicator Surveys from 2009 to 2015 across 19 countries to explore the distribution of and risk factors for FN-RDTs in sub-Saharan Africa, where malaria’s impact is greatest. We sought to (1) identify spatial and demographic patterns of FN-RDT results, defined as a negative RDT but positive gold standard microscopy test, and (2) estimate the percentage of infections missed within community-based malaria surveys due to FN-RDT results. Across all studies, 19.9% (95% CI 19.0% to 20.9%) of microscopy-positive subjects were negative by RDT. The distribution of FN-RDT results was spatially heterogeneous. The variance in FN-RDT results was best explained by the prevalence of malaria, with an increase in FN-RDT results observed at lower transmission intensities, among younger subjects, and in urban areas. The observed proportion of FN-RDT results was not predicted by differences in RDT brand or lot performance alone. These findings characterise how the probability of detection by RDTs varies in different transmission settings and emphasise the need for careful interpretation of prevalence estimates based on surveys employing RDTs alone. Further studies are needed to characterise the cost-effectiveness of improved malaria diagnostics (eg, PCR or highly sensitive RDTs) in community-based surveys, especially in regions of low transmission intensity or high urbanicity.

Performance of Antigen Concentration Thresholds for Attributing Fever to Malaria among Outpatients in Angola

The density of malaria parasites is a key determinant of whether an infected individual develops fever. While the pyrogenic threshold for malaria parasite density has been well studied, there are no analogous data on the antigen levels associated with fever during infection. Samples from 797 afebrile and 457 febrile outpatients from two provinces in Angola with known concentrations of histidine-rich protein 2 (HRP2), aldolase, and lactate dehydrogenase (LDH) antigens were analyzed by Bayesian latent class modeling to attribute malarial etiology to the fevers and to estimate the sensitivity and specificity of different antigen thresholds for detection of malaria fevers. Among patients with aldolase or LDH levels detectable with a bead-based assay, the concentrations of these two antigens did not differ between afebrile and febrile patients. In contrast, the concentrations of HRP2 were substantially higher in febrile HRP2-positive patients than in afebrile HRP2-positive patients. When HRP2 concentrations were considered, the malaria-attributable fractions of fever cases were 0.092 in Huambo Province and 0.39 in Uíge Province. Diagnostic tests detecting HRP2 with limits of detection (LODs) in the range of 3,000 to 10,000 pg/µl would provide ideal sensitivity and specificity for determination of malarial etiology among febrile persons.

Screening for Pfhrp2/3-Deleted Plasmodium falciparum, Non-falciparum, and Low-Density Malaria Infections by a Multiplex Antigen Assay

Background

Detection of Plasmodium antigens provides evidence of malaria infection status and is the basis for most malaria diagnosis.

Methods

We developed a sensitive bead-based multiplex assay for laboratory use, which simultaneously detects pan-Plasmodium aldolase (pAldo), pan-Plasmodium lactate dehydrogenase (pLDH), and P. falciparum histidine-rich protein 2 (PfHRP2) antigens. The assay was validated against purified recombinant antigens, monospecies malaria infections, and noninfected blood samples. To test against samples collected in an endemic setting, Angolan outpatient samples (n = 1267) were assayed.

Results

Of 466 Angolan samples positive for at least 1 antigen, the most common antigen profiles were PfHRP2+/pAldo+/pLDH+ (167, 36%), PfHRP2+/pAldo-/pLDH- (163, 35%), and PfHRP2+/pAldo+/pLDH- (129, 28%). Antigen profile was predictive of polymerase chain reaction (PCR) positivity and parasite density. Eight Angolan samples (1.7%) had no or very low PfHRP2 but were positive for 1 or both of the other antigens. PCR analysis confirmed 3 (0.6%) were P. ovale infections and 2 (0.4%) represented P. falciparum parasites lacking Pfhrp2 and/or Pfhrp3.

Conclusions

These are the first reports of Pfhrp2/3 deletion mutants in Angola. High-throughput multiplex antigen detection can inexpensively screen for low-density P. falciparum, non-falciparum, and Pfhrp2/3-deleted parasites to provide population-level antigen estimates and identify specimens requiring further molecular characterization.

Simultaneous Quantification of Plasmodium Antigens and Host Factor C-Reactive Protein in Asymptomatic Individuals with Confirmed Malaria by Use of a Novel Multiplex Immunoassay

Malaria rapid diagnostic tests (RDTs) primarily detect Plasmodium falciparum antigen histidine-rich protein 2 (HRP2) and the malaria-conserved antigen lactate dehydrogenase (LDH) for P. vivax and other malaria species. The performance of RDTs and their utility is dependent on circulating antigen concentration distributions in infected individuals in a population in which malaria is endemic and on the limit of detection of the RDT for the antigens.

Malaria rapid diagnostic tests (RDTs) primarily detect Plasmodium falciparum antigen histidine-rich protein 2 (HRP2) and the malaria-conserved antigen lactate dehydrogenase (LDH) for P. vivax and other malaria species. The performance of RDTs and their utility is dependent on circulating antigen concentration distributions in infected individuals in a population in which malaria is endemic and on the limit of detection of the RDT for the antigens. A multiplexed immunoassay for the quantification of HRP2, P. vivax LDH, and all-malaria LDH (pan LDH) was developed to accurately measure circulating antigen concentration and antigen distribution in a population with endemic malaria. The assay also measures C-reactive protein (CRP) levels as an indicator of inflammation. Validation was conducted with clinical specimens from 397 asymptomatic donors from Myanmar and Uganda, confirmed by PCR for infection, and from participants in induced blood-stage malaria challenge studies. The assay lower limits of detection for HRP2, pan LDH, P. vivax LDH, and CRP were 0.2 pg/ml, 9.3 pg/ml, 1.5 pg/ml, and 26.6 ng/ml, respectively. At thresholds for HRP2, pan LDH, and P. vivax LDH of 2.3 pg/ml, 47.8 pg/ml, and 75.1 pg/ml, respectively, and a specificity ≥98.5%, the sensitivities for ultrasensitive PCR-confirmed infections were 93.4%, 84.9%, and 48.9%, respectively. Plasmodium LDH (pLDH) concentration, in contrast to that of HRP2, correlated closely with parasite density. CRP levels were moderately higher in P. falciparum infections with confirmed antigenemia versus those in clinical specimens with no antigen. The 4-plex array is a sensitive tool for quantifying diagnostic antigens in malaria infections and supporting the evaluation of new ultrasensitive RDTs.

Using Reference Quantitative Polymerase Chain Reaction to Assess the Clinical Performance of the Paracheck-Pf® Rapid Diagnostic Test in a Field Setting in Uganda

Malaria rapid diagnostic tests (RDTs) are widely used in clinical and surveillance settings. However, the performance of most RDTs has not been characterized at parasite densities below detection by microscopy. We present findings from Uganda, where RDT results from 491 participants with suspected malaria were correlated with quantitative polymerase chain reaction (qPCR)-defined parasitemia. Compared with qPCR, the sensitivity and specificity of the RDT for Plasmodium falciparum mono-infections were 76% (95% confidence interval [CI]: 68-83%) and 95% (95% CI: 92-97%), respectively. The sensitivity of the RDT at parasite densities between 0.2 and 200 parasites/μL was surprisingly high (87%, 95% CI: 74-94%). The high sensitivity of the RDT is likely because of histidine-rich protein 2 from submicroscopic infections, gametocytes, or sequestered parasites. These findings underscore the importance of evaluating different RDTs in field studies against qPCR reference testing to better define the sensitivity and specificity, particularly at low parasite densities.