Imported malaria investigation — DTM&H MCQ

Correct answer: B — Urgent thick and thin blood films repeated if initially negative

The correct answer is B, urgent thick and thin blood films repeated if initially negative. In a febrile traveller returning from a malaria-endemic area with thrombocytopenia and no chemoprophylaxis, malaria must be actively excluded, and microscopy remains the reference standard for diagnosis, species identification and parasitaemia quantification. A negative rapid diagnostic test does not rule out infection, particularly early in illness when parasitaemia may be low or if a non-falciparum species with poor RDT antigen expression is present. UK guidance is explicit that a single negative film or RDT does not exclude malaria, and films should be repeated over the following 24 to 48 hours (a total of three sets 12 to 24 hours apart) before the diagnosis is safely discounted. Why the other options are wrong: C. Dengue NS1 antigen as the sole first-line test: dengue is a relevant differential but cannot be assumed as the sole diagnosis while malaria, a rapidly progressive and potentially fatal illness, remains unexcluded; NS1 alone also misses the thrombocytopenia-causing possibility of malaria. A. Malaria serology with convalescent sample: antibody serology reflects past exposure, is too slow for acute diagnosis, and has no role in guiding urgent management of a currently unwell patient. D. Stool microscopy for ova and parasites: this investigates intestinal parasites, not the cause of acute febrile illness with thrombocytopenia after only 36 hours of symptoms. E. Blood cultures after discharge: bacteraemia is a differential worth considering, but discharging a patient with unexplained fever and thrombocytopenia before excluding malaria is unsafe, and cultures should be taken before, not after, discharge. Key point: a negative malaria RDT or single blood film never excludes malaria in a returned traveller; repeat thick and thin films are mandatory until malaria is confidently ruled out.

Reference: UK Health Security Agency / GOV.UK, Malaria prevention guidelines for travellers from the UK, section on Diagnosis: 'One negative blood film or RDT does not exclude a diagnosis of malaria.' https://www.gov.uk/government/publications/malaria-prevention-guidelines-for-travellers-from-the-uk-2026/diagnosis