Bead-based immunoassay allows sub-picogram detection of histidine-rich protein 2 from Plasmodium falciparum and estimates reliability of malaria rapid diagnostic tests

Characterization of Plasmodium Lactate Dehydrogenase and Histidine-Rich Protein 2 Clearance Patterns via Rapid On-Bead Detection from a Single Dried Blood Spot

A rapid, on-bead enzyme-linked immunosorbent assay for Plasmodium lactate dehydrogenase (pLDH) and Plasmodium falciparum histidine-rich protein 2 (HRP2) was adapted for use with dried blood spot (DBS) samples. This assay detected both biomarkers from a single DBS sample with only 45 minutes of total incubation time and detection limits of 600 ± 500 pM (pLDH) and 69 ± 30 pM (HRP2), corresponding to 150 and 24 parasites/μL, respectively. This sensitive and reproducible on-bead detection method was used to quantify pLDH and HRP2 in patient DBS samples from rural Zambia collected at multiple time points after treatment. Biomarker clearance patterns relative to parasite clearance were determined; pLDH clearance followed closely with parasite clearance, whereas most patients maintained detectable levels of HRP2 for 35–52 days after treatment. Furthermore, weak-to-moderate correlations between biomarker concentration and parasite densities were found for both biomarkers. This work demonstrates the utility of the developed assay for epidemiological study and surveillance of malaria.

Posttreatment HRP2 Clearance in Patients with Uncomplicated Plasmodium falciparum Malaria

Background

The response to antimalarial treatment is assessed using serial microscopy. New techniques for accurate measurement of the Plasmodium falciparum histidine-rich protein 2 (HRP2) antigen have allowed for monitoring of the antigen concentration over time, offering a potential alternative for assessing treatment response.

Methods

Posttreatment HRP2 concentrations were measured in samples obtained longitudinally from 537 individuals with P. falciparum malaria who were participating in efficacy trials in Angola, Tanzania, and Senegal. The HRP2 half-life was estimated using a first-order kinetics clearance model. The association between the HRP2 concentration 3 days after treatment and recrudescence of infection was assessed.

Results

Despite substantial variation in HRP2 concentrations among participants at baseline, concentrations consistently showed a first-order exponential decline. The median half-life of HRP2 was estimated to be 4.5 days (interquartile range [IQR], 3.3-6.6 days) in Angola, 4.7 days (IQR, 4.0-5.9 days) in Tanzania, and 3.0 days (IQR, 2.1-4.5 days) in Senegal. The day 3 HRP2 concentration was predictive of eventual recrudescence, with an area under the receiver operating characteristic curve of 0.86 (95% confidence interval, .73-.99).

Conclusions

Consistent HRP2 clearance dynamics following successful antimalarial treatment imply a common underlying mechanism of biological clearance. Patients who ultimately did not respond to treatment did not exhibit this same pattern of clearance, even in the absence of other indications of inadequate response to treatment.

Selecting analytical biomarkers for diagnostic applications: a first principles approach

INTRODUCTION

Biomarkers are objective indications of a medical state that can be measured accurately and reproducibly. Traditional biomarkers enable diagnosis of disease through detection of disease-specific molecules, disease-mediated molecular changes, or distinct physiological or anatomical signatures. Areas covered: This work provides a framework for selecting biomarkers that are most likely to provide useful information about a patient's disease state. Though the authors emphasize markers related to disease, this work is also applicable to biomarkers for monitoring physiological changes such as ovulation or pregnancy. Additionally, the scope was restricted to biomarkers that are amenable to analytical detection across a range of health care levels, including low resource settings. The authors describe trade-offs between biomarkers' sensitivity/specificity for a disease-causing agent, the complexity of detection, and how this knowledge can be applied to the development of diagnostic tests. This report also details additional assessment criteria for successful tests. Expert commentary: Biomarker selection should primarily be driven by an attempt to answer an explicit clinical question (preferably causative relationship of the biomarker to disease-state), and only then by test development expediency (ease of detection). This framework is useful for stakeholders from test developers to clinicians to identify the trade-offs for diagnostic biomarkers for any use case.

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

BACKGROUND

Rapid diagnostic test (RDT) positivity is supplanting microscopy as the standard measure of malaria burden at the population level. However, there is currently no standard for externally validating RDT results from field surveys.

METHODS

Individuals' blood concentration of the Plasmodium falciparum histidine rich protein 2 (HRP2) protein were compared to results of HRP2-detecting RDTs in participants from field surveys in Angola, Mozambique, Haiti, and Senegal. A logistic regression model was used to estimate the HRP2 concentrations corresponding to the 50 and 90% level of detection (LOD) specific for each survey.

RESULTS

There was a sigmoidal dose-response relationship between HRP2 concentration and RDT positivity for all surveys. Variation was noted in estimates for field RDT sensitivity, with the 50% LOD ranging between 0.076 and 6.1 ng/mL and the 90% LOD ranging between 1.1 and 53 ng/mL. Surveys conducted in two different provinces of Angola using the same brand of RDT and same study methodology showed a threefold difference in LOD.

CONCLUSIONS

Measures of malaria prevalence estimated using population RDT positivity should be interpreted in the context of potentially large variation in RDT LODs between, and even within, surveys. Surveys based on RDT positivity would benefit from external validation of field RDT results by comparing RDT positivity and antigen concentration.

High-Sensitivity Assays for Plasmodium falciparum Infection by Immuno-Polymerase Chain Reaction Detection of PfIDEh and PfLDH Antigens

Background

Rapid diagnostic tests based on Plasmodium falciparum histidine-rich protein II (PfHRP-II) and P. falciparum lactate dehydrogenase (PfLDH) antigens are widely deployed for detection of P. falciparum infection; however, these tests often miss cases of low-level parasitemia, and PfHRP-II tests can give false-negative results when P. falciparum strains do not express this antigen.

Methods

We screened proteomic data for highly expressed P. falciparum proteins and compared their features to those of PfHRP-II and PfLDH biomarkers. Search criteria included high levels of expression, conservation in all parasite strains, and good correlation of antigen levels with parasitemia and its clearance after drug treatment. Different assay methods were compared for sensitive detection of parasitemia in P. falciparum cultures.

Results

Among potential new biomarkers, a P. falciparum homolog of insulin-degrading enzyme (PfIDEh) met our search criteria. Comparative enzyme-linked immunosorbent assays with monoclonal antibodies against PfLDH or PfIDEh showed detection limits of 100-200 parasites/µL and 200-400 parasites/µL, respectively. Detection was dramatically improved by use of real-time immuno-polymerase chain reaction (PCR), to parasitemia limits of 0.02 parasite/µL and 0.78 parasite/µL in PfLDH- and PfIDEh-based assays, respectively.

Conclusions

The ability of PfLDH- or PfIDEh-based immuno-PCR assays to detect <1 parasite/µL suggests that improvements of bound antibody sensor technology may greatly increase the sensitivity of malaria rapid diagnostic tests.

Estimating the Added Utility of Highly Sensitive Histidine-Rich Protein 2 Detection in Outpatient Clinics in Sub-Saharan Africa

Most malaria testing is by rapid diagnostic tests (RDTs) that detect Plasmodium falciparum histidine-rich protein 2 (HRP2). Recently, several RDT manufacturers have developed highly sensitive RDTs (hsRDTs), promising a limit of detection (LOD) orders of magnitude lower than conventional RDTs. To model the added utility of hsRDTs, HRP2 concentration in Angolan outpatients was measured quantitatively using an ultrasensitive bead-based assay. The distribution of HRP2 concentration was bimodal in both afebrile and febrile patients. The conventional RDT was able to detect 81% of all HRP2-positive febrile patients and 52-77% of HRP2-positive afebrile patients. The added utility of hsRDTs was estimated to be greater in afebrile patients, where an hsRDT with a LOD of 200 pg/mL would detect an additional 50-60% of HRP2-positive persons compared with a conventional RDT with a LOD of 3,000 pg/mL. In febrile patients, the hsRDT would detect an additional 10-20% of cases. Conventional RDTs already capture the vast majority of symptomatic HRP2-positive individuals, and hsRDTs would have to reach a sufficiently low LOD approaching 200 pg/mL to provide added utility in identifying HRP2-positive, asymptomatic individuals.