Avoiding misdiagnosis of imported malaria: screening of emergency department samples with thrombocytopenia detects clinically unsuspected cases.

BACKGROUND: Misdiagnosis of imported malaria is not uncommon and even abnormal routine laboratory tests may not trigger malaria smears. However, blind screening of all thrombocytopenic samples might be a possible way to detect clinically unsuspected malaria cases in the accident and emergency department (AED).
METHODS: The frequency and degree of thrombocytopenia was determined in two cohorts of malaria patients (Lisbon, Portugal and Berlin, Germany). The frequency of thrombocytopenia in full blood count (FBC) samples from patients presenting at the ED at a large teaching hospital in Lisbon, Portugal, was determined and compared with urgent samples from in-patients, both determined at a dedicated emergency laboratory. A cut-off value was established for screening of all FBC samples from the ED with Giemsa-stained thick-blood films.
RESULTS: In 4,362 unselected samples from the ED in Lisbon a thrombocytopenia of <150,000/microL was found in 7% and of <100,000/microL in 2.2% of cases (2.5 samples/day with <100.000/microL). In patients with malaria, a thrombocytopenia was found in 75% and 77% (<150,000/microL) or 53% and 45% (<100,000/microL) in Lisbon, (n=60) and Berlin (n=170), respectively. Blind screening of all samples with <100,000 thrombocytes/microL by thick-blood film microscopy led to the diagnosis of 5 unsuspected malaria cases (3 Plasmodium falciparum, 1 Plasmodium vivax and 1 Plasmodium ovale), during the study. The diagnosis of each unsuspected malaria case would have cost 21 hours of dedicated technician's time.
CONCLUSIONS: The problem of clinically unsuspected malaria seems to be more common than generally expected and is dependent on the local incidence of malaria as well as clinical and laboratory expertise. The blind screening of all thrombocytopenic samples with <100,000/microL may be a cost-effective way to reduce the misdiagnosis of imported malaria.