Rapid Diagnostic Tests for the Field: Malaria, HIV & Dengue
A point-of-care RDT can confirm a case in fifteen minutes without a power supply, a centrifuge, or a trained microscopist. It can also miss the very pathogen it was bought to detect. This guide separates what an RDT is genuinely good for from what it quietly cannot tell you — and how to build that judgement into a survey kit that holds up at 40°C.
What an RDT can — and cannot — tell you
A lateral-flow rapid diagnostic test answers one question: is a specific antigen or antibody present above the assay's detection threshold in this sample, right now. That is a narrower claim than clinicians often read into a coloured line. An RDT does not quantify parasite density, distinguish current from recent infection in most antibody formats, identify drug resistance, or replace a confirmatory method when the pre-test probability and the result disagree.
Its strengths are exactly the ones a laboratory cannot offer in an outbreak: a result at the bedside or the tent flap, no cold centrifugation, no mains electricity, and a workflow a trained community health worker can run reliably. The discipline is to treat the line as a screening signal interpreted against clinical context and local epidemiology — never as a standalone verdict. A negative malaria RDT in a febrile child in a high-transmission zone is a prompt to look harder, not a discharge.
Malaria RDTs and HRP2/3 deletion
The majority of P. falciparum RDTs detect histidine-rich protein 2 (HRP2). Parasite strains carrying deletions of the pfhrp2 and pfhrp3 genes produce no HRP2 antigen, so an HRP2-only test reads negative in a genuinely infected, sometimes severely ill, patient. These deletions have been documented at programme-relevant frequencies across parts of the Horn of Africa, the Amazon basin, and South Asia, and the WHO has flagged them as a threat to RDT-based case management where prevalence crosses the surveillance threshold.
The field mitigation is deliberate test selection: in regions with known or suspected deletions, choose a combination RDT that also detects pan-Plasmodium lactate dehydrogenase (pLDH), which is gene-independent of HRP2. Pair this with a standing rule that clinical malaria with a negative HRP2 RDT escalates to microscopy or molecular confirmation. Document the discordance — those records are surveillance data, not paperwork.
HIV and dengue: test-selection criteria
HIV. Antibody-only RDTs miss the acute window before seroconversion; fourth-generation tests that also detect p24 antigen narrow that gap but do not close it. Field programmes should follow a validated national testing algorithm — typically two or three sequential assays from different manufacturers to confirm a reactive result — rather than acting on a single reactive line. Self-test and confirmatory roles are distinct; do not collapse them.
Dengue. Timing drives test choice. NS1 antigen is detectable from roughly day one to day seven of fever and is the workhorse for early presentation; IgM/IgG antibody formats become useful later and help distinguish primary from secondary infection. A combined NS1/IgM/IgG cassette covers more of the febrile curve but still demands that you record the day of illness — a day-six NS1-negative result is uninformative, not reassuring. Cross-reactivity with other flaviviruses is a real limit in co-endemic zones.
Heat stability, shelf life, and field storage
RDTs are biological reagents on nitrocellulose, not hardware. The single most common cause of a false result in the field is a degraded test, and degradation rarely announces itself.
- 01Step
Respect the labelled storage range — and the real one
Most RDTs tolerate 2–30°C, but transient excursions inside a closed cool box on a tarmac airstrip can pass 40°C. Use a temperature logger in the diagnostics box, not just the vaccine cold chain, and treat the manufacturer's range as a ceiling, not a target.
- 02Step
Track shelf life against arrival, not manufacture
A test with eighteen months of stated shelf life may reach a remote site with a third already spent in transit and warehousing. Record received-stock expiry on intake and rotate first-expiry-first-out. Short-dated lots go to high-throughput sites, not the standby kit at the back of the truck.
- 03Step
Open pouches just before use
The foil pouch and its desiccant are the humidity barrier. Once opened, the cassette has minutes, not hours. In high-humidity settings a cassette laid out 'ready' degrades visibly — open one test per patient, at the point of testing.
- 04Step
Quarantine heat-exposed stock
If a logger shows an out-of-range excursion, do not assume the tests survived it. Segregate the affected lot, run a known-positive and known-negative control, and only return it to service if both read correctly.
Reading, recording, and QA without a lab
Quality assurance does not require a laboratory — it requires a routine that survives staff turnover and a bad night.
- 01Step
Read at the stated time, under adequate light
A faint test line is still a positive line; a line that appears after the read window is not. Standardise on a timer and a torch. Inter-reader variation on weak lines is a known error source — where stakes are high, have a second reader confirm faint results.
- 02Step
Validate every result against the control line
No control line means an invalid test, regardless of what the test line shows. Discard and repeat with a fresh cassette. An absent control line is the most reliable signal that a lot or a storage failure has compromised the stock.
- 03Step
Run lot-level positive and negative controls
On opening a new lot, and after any cold-chain alarm, run external quality-control samples. Log lot number, expiry, and control outcome in a register that travels with the kit.
- 04Step
Record the result as surveillance data
Capture test type, lot, day of illness where relevant, and discordances with clinical judgement. These records are how HRP2 deletions, assay drift, and false-negative clusters get detected at programme level — not by any single test.
Queries to the desk
Q1.When should we escalate from an RDT to microscopy?
Escalate whenever clinical suspicion and RDT result conflict: a febrile patient in an endemic zone with a negative malaria RDT, a severely ill patient regardless of RDT result, suspected mixed or non-falciparum species, and any setting with known HRP2/3 deletions. Microscopy also gives parasite density for monitoring treatment response, which no qualitative RDT can. Maintain at least one referral pathway to a microscopist or molecular platform for these cases.
Q2.Can a negative RDT rule out infection?
Not on its own. A negative result lowers probability but is bounded by the assay's detection threshold, the timing of the sample relative to infection (the HIV window period, the dengue NS1 curve), and antigen-deletion strains in malaria. Interpret negatives against pre-test probability; a high-probability case with a negative test warrants confirmation, not closure.
Q3.How do we handle co-endemic febrile illness where malaria, dengue, and others overlap?
Build a tiered survey kit rather than relying on one test. Lead with the highest-probability pathogen by season and locale, but carry combination malaria RDTs (HRP2 + pLDH), a timed dengue panel (NS1 plus IgM/IgG), and the nationally validated HIV algorithm. Record day of illness on every dengue test and treat flavivirus cross-reactivity as a stated limit, escalating ambiguous cases for confirmation.
Q4.What belongs in a multi-disease survey kit?
Combination malaria RDTs sized to expected caseload; a timed dengue cassette; HIV tests in the sequence your algorithm requires from at least two manufacturers; external quality-control samples per assay; a temperature logger dedicated to the diagnostics box; lancets, buffer, and sharps disposal; and a lot/expiry register. Procure to WHO prequalification status where available, and verify each lot's labelled storage range against the conditions you actually expect to meet.
Equipment graded for the field, not the catalog
This guide sits within our Field Diagnostics pillar alongside the portable-microscopy, point-of-care analyzer, and HIV field-testing reviews. If you are assembling a survey kit for a specific transmission setting, our editorial team can point you to the relevant evidence and review notes.