What Is HRP2 and Why Is It Key for Malaria Tests?

HRP2, short for histidine-rich protein 2, is a protein produced by the deadliest malaria parasite, Plasmodium falciparum, and it serves as the main target for most malaria rapid diagnostic tests (RDTs) used worldwide. The protein is abundant in the blood of infected people, which makes it relatively easy to detect on a simple test strip without laboratory equipment. Over 200 RDT brands currently rely on one or more of three parasite antigens, with HRP2 being the most sensitive for P. falciparum detection.¹ But HRP2’s dominance in diagnostics has created problems that public health officials are now racing to solve.

What HRP2 Does Inside the Parasite

Plasmodium falciparum survives inside red blood cells by digesting hemoglobin for nutrients. That process releases heme, a toxic iron-containing molecule that would kill the parasite if left unchecked. HRP2 helps the parasite deal with this waste. Biochemical studies have shown that recombinant HRP2 binds heme and can promote its conversion into hemozoin, a crystalline form the parasite can safely store.² Each HRP2 molecule can bind roughly 15 heme molecules, giving it substantial detoxification capacity.³

HRP2 may also protect the parasite from heme-driven damage in other ways. Research using a synthetic peptide based on HRP2’s structure found it could pull heme away from cell membranes and reduce heme-induced destruction of red blood cells.⁴ So the protein appears to play a dual role: converting toxic heme into inert crystals and shielding surrounding cells from damage. Despite its importance for the parasite’s survival machinery, HRP2 became medically significant mainly because of a fortunate quirk: the parasite secretes it in large quantities into the bloodstream, making infected people walking reservoirs of a detectable biomarker.

How RDTs Use HRP2 to Detect Malaria

Malaria rapid diagnostic tests work on the same basic principle as a home pregnancy test. A drop of blood is placed on a test strip, where it flows along a membrane. If HRP2 is present, it binds to antibodies attached to colored particles, then this complex is captured at a visible test line by a second set of antibodies. The result appears as a colored band within about 15 minutes. The first commercial HRP2-based RDT, called ParaSight-F, used monoclonal antibodies targeting a repeating amino acid sequence in HRP2 and a colorimetric detection system with liposomes containing pink dye.⁵

The reason HRP2 became the preferred target over other parasite proteins is straightforward: it is produced in much higher concentrations in the blood during infection, which means the test picks it up more reliably at lower parasite densities. HRP2 is found across all blood-stage forms of the parasite, including the sexual stages responsible for transmission.⁶ This abundance is what made HRP2-based RDTs a game-changer for malaria control in remote areas where microscopy is not available. However, this dependence on a single protein from a single parasite species has created vulnerabilities that are becoming increasingly hard to ignore.

Why HRP2 Tests Stay Positive After Treatment

One of the most frustrating features of HRP2-based RDTs is that they keep reading positive long after the malaria parasites have been cleared from the blood. HRP2 is a soluble protein that circulates freely in the bloodstream, and the body takes weeks to break it down completely. In a study of children treated with antimalarials, all parasites were gone from blood smears by day 3, but the RDT remained positive in about 95% of them. By day 28, the test was still positive in roughly 81% of the children.⁷

Some studies have found even longer clearance times. Research in Kenyan children from a high-transmission area reported a median antigen clearance time of about 51 days, with heavier initial infections taking longer to clear.⁸ This persistence is driven by the initial parasite load, not by ongoing infection. A separate study confirmed that lingering HRP2 after treatment was associated with higher baseline parasite densities and was not connected to the presence of gametocytes, the transmission stages of the parasite.⁹

For clinicians in the field, this creates a real headache. If someone was treated for malaria three weeks ago and develops a new fever, a positive HRP2-based RDT cannot distinguish between leftover antigen from the old infection and a genuine new episode. In high-transmission settings where reinfection is common, this ambiguity can lead to either unnecessary retreatment or missed new infections.

The Gene Deletion Problem

Perhaps the most serious threat to HRP2-based diagnostics is that some P. falciparum parasites have lost the gene that produces HRP2 altogether. Without the gene, the parasite produces no HRP2 protein, and the RDT returns a false-negative result. A person with malaria walks away from the test thinking they are malaria-free.

This phenomenon was first identified in South America, where deletions of the pfhrp2 gene (and its related gene pfhrp3) have been most dramatic. Research in Peru showed that these deletions did not arise from a single genetic event but occurred independently multiple times across different parasite lineages.¹⁰ A systematic review and meta-analysis found the highest deletion rates in South and Central America, where about 18% of P. falciparum parasites carried pfhrp2 deletions, compared to about 4% in Africa and 3% in Asia.¹¹

The concern is that widespread use of HRP2-based RDTs may itself be driving the spread of these deletions. The logic is straightforward: if a parasite lacks HRP2, it avoids detection, the patient does not get treated, and the parasite survives to reproduce and spread. Over time, parasites carrying the deletion gain an evolutionary edge in areas where HRP2-based testing is the primary diagnostic tool.¹² This is essentially natural selection shaped by human diagnostic strategy.

How Widespread Are Deletions in Africa

Africa bears the overwhelming majority of the global malaria burden, so gene deletions there are an especially acute concern. The picture across the continent is uneven. A systematic review of studies published between 2010 and 2019 found deletion rates ranging from as low as 0.4% to as high as 62% across 12 African countries.¹³ That enormous range partly reflects real geographic variation, but it also reflects inconsistent study methods, with different labs using different techniques and thresholds for calling a deletion.

A more controlled multi-country study of samples from Ethiopia, Kenya, Madagascar, and Rwanda found a much lower prevalence. Single-gene pfhrp2 deletions were seen in about 1.4% of Ethiopian samples and 0.6% of Madagascar samples. Dual deletions of both pfhrp2 and pfhrp3 were found in about 2% of Ethiopian samples.¹⁴ In most African settings, the deletions are still uncommon enough that HRP2-based RDTs remain useful. But with billions of tests deployed annually, even a small percentage of false negatives translates into large absolute numbers of missed cases, and the worry is that the percentage will grow over time under continued selective pressure.

The HRP3 Backup and Cross-Reactivity

P. falciparum carries a second gene, pfhrp3, that produces a structurally similar protein called HRP3. Because HRP2 and HRP3 share amino acid sequences, many HRP2-targeting RDTs will cross-react with HRP3. This turns out to be partially protective against the deletion problem. Lab testing showed that a parasite strain lacking the pfhrp2 gene but retaining pfhrp3 could still trigger a positive result on HRP2-targeting RDTs, especially at parasite densities above 1,000 parasites per microliter.¹⁵ Even at densities as low as 100 parasites per microliter, all three RDT products tested still showed faint positive bands for this strain.

This cross-reactivity is a mixed blessing. It means that parasites with single pfhrp2 deletions (but intact pfhrp3) may still be caught by standard tests, especially when the infection is not too light. But parasites that have lost both genes produce neither protein, and these double deletions are the ones that truly evade HRP2-based testing. The distinction between single and double deletions matters enormously for policy. Variable detection of certain target sequences, low parasite loads, and gene deletions can all result in false-negative RDTs, leading to missed diagnoses and delayed treatment.¹⁶

Alternative Biomarkers and Combination Tests

Given HRP2’s limitations, the other two biomarkers used in malaria RDTs are getting more attention. Plasmodium lactate dehydrogenase (pLDH) is an enzyme the parasite needs for its energy metabolism, and it comes in species-specific and pan-species forms, allowing tests to distinguish between P. falciparum and other malaria species.¹⁷ The third target, Plasmodium aldolase, is another metabolic enzyme conserved across all human malaria species.¹

The advantage of pLDH and aldolase is that they are unaffected by pfhrp2 gene deletions. An early immunochromatographic test targeting pLDH could detect infections at roughly 200 parasites per microliter of blood.¹⁸ These enzymes also clear from the bloodstream quickly once parasites are eliminated, so pLDH-based tests do not suffer from the weeks-long false-positive problem that plagues HRP2 tests. The trade-off is sensitivity. HRP2 is simply more abundant in the blood during most P. falciparum infections, and pLDH-based tests are less reliable at detecting low-density infections.

The practical solution increasingly adopted is a combination RDT that includes both HRP2 and pLDH detection lines on the same test strip. In a field evaluation, RDTs with LDH-based detection lines successfully caught P. falciparum samples carrying double pfhrp2/pfhrp3 deletions at densities above 1,000 parasites per microliter, catching infections that an HRP2-only test would have missed entirely.¹⁹ A broader laboratory study reinforced this: pan-LDH-only RDTs detected over 94% of parasites with pfhrp2 deletions, while HRP2-only RDTs detected just about 70% of single-deletion strains and dropped further against double deletions.²⁰

Ultra-Sensitive RDTs

Standard HRP2-based RDTs have a detection threshold of roughly 800 to 1,000 picograms per milliliter of HRP2, which corresponds to around 100 to 200 parasites per microliter of blood.²¹ That threshold catches most symptomatic infections but misses a large reservoir of people who carry the parasite at very low levels without feeling sick. These asymptomatic carriers still transmit malaria through mosquitoes, making them invisible fuel for ongoing transmission.

Ultra-sensitive RDTs (uRDTs) have been developed with a detection limit more than ten-fold lower than standard tests. In field evaluations, the uRDT picked up asymptomatic infections that standard RDTs missed entirely. In Uganda, its sensitivity against molecular testing was 84%, compared to 62% for the standard RDT. In Myanmar, the standard test detected none of the asymptomatic infections tested, while the uRDT caught 44%.²² The uRDT also detected new infections about a day and a half earlier than the standard test in controlled human infection studies.

A survey in southern Ghana found the uRDT correctly identified P. falciparum in an additional 9.3% of participants who had been missed by the standard RDT.²³ That extra sensitivity matters most for elimination campaigns, where finding and treating every carrier, not just symptomatic patients, is the goal. The downside is that ultra-sensitive tests amplify the persistence problem: because they detect much lower concentrations of HRP2, they stay positive even longer after successful treatment.

False Positives, False Negatives, and the Prozone Effect

Beyond gene deletions and post-treatment persistence, HRP2-based RDTs have a few more quirks worth knowing about. One is the prozone effect, a paradox where extremely high concentrations of HRP2 can actually make the test line appear weaker rather than stronger. This happens because an overwhelming amount of antigen saturates the test antibodies and prevents the normal sandwich complex from forming at the capture line. Lab testing showed this effect kicked in at HRP2 concentrations above 15,000 nanograms per milliliter, corresponding to parasite densities above about 312,000 per microliter.²⁴ These are extreme infections, typically seen in severe malaria. While the prozone effect has been confirmed as a cause of false-negative results in cases of very high parasite loads, the reduced line intensity did not always produce outright negative results.²⁵

On the false-positive side, certain autoimmune conditions can interfere with HRP2-based tests. Rheumatoid factor, an antibody found at elevated levels in people with rheumatoid arthritis and some chronic infections, can cause the test to show a positive line even with no malaria present. Testing of 92 patients with high rheumatoid factor levels showed false-positive rates between 2% and 13%, depending on the RDT brand.²⁶ In malaria-endemic regions where chronic infections are common, this overlap adds another layer of diagnostic noise.⁵

Tracking Gene Deletions at Scale

Knowing where gene deletions are spreading requires large-scale surveillance, and that surveillance depends on molecular tools that can detect missing genes in thousands of blood samples. Two newer approaches have been developed for high-throughput screening. A quantitative PCR assay called qHRP2/3-del correctly identified pfhrp2 and pfhrp3 deletion status in about 93% of samples with parasite densities above 5 parasites per microliter, making it suitable for processing large numbers of field-collected samples.²⁷

A separate approach using digital PCR achieved reliable gene deletion typing across a wide range of parasite densities, from fewer than 2 to nearly 50,000 parasites per microliter.²⁸ These methods are designed for reference laboratories and national malaria programs, not for bedside use. But they are essential for answering the question that determines diagnostic policy: at what point have deletions become common enough in a given area that HRP2-only RDTs should be replaced with combination tests or pLDH-based alternatives? The World Health Organization has recommended that countries switch away from HRP2-only tests when deletion prevalence exceeds 5% in symptomatic patients, and these molecular surveillance tools are what make that threshold measurable.

Heat, Humidity, and Real-World Performance

An RDT can be perfectly designed on paper and still fail in the field if the test strips degrade before they reach a patient. HRP2-based RDTs are lateral-flow immunoassays that depend on antibodies maintaining their structure and function, and antibodies are proteins that can denature in heat. In tropical regions where malaria is most prevalent, supply chains often involve long stretches of transport and storage without climate control. Laboratory heat-stability testing showed that most HRP2-based RDTs maintained good sensitivity for P. falciparum (above 90%) when stored at 35°C and 45°C for up to 90 days. But by day 100 at those temperatures, most showed a sharp decline in performance.²⁹ Interestingly, a brief 48-hour exposure to 60°C did not cause the same damage, suggesting that prolonged moderate heat is more destructive than a short spike.

This matters practically because a health worker in a remote clinic may be using RDTs that sat in a warehouse or the back of a truck for months under tropical conditions. Procurement programs and the WHO’s product testing scheme, which has been evaluating and comparing commercial RDTs since 2008, account for heat stability in their quality ratings. But once a test leaves the controlled supply chain, its performance depends on how it was actually stored, and that is difficult to monitor in the last mile of delivery.