Background: When malaria is suspected in case of fever after travel in endemic areas, a first test with a negative result does not exclude the diagnosis. The current recommendation is to repeat the malaria test at 24-hour intervals, with up to two additional tests, as long as the result is negative. This recommendation is precautionary rather than evidence-based. A retrospective analysis was conducted to investigate the appropriateness of this recommendation by estimating the proportion of tests with negative result at first and subsequently with a positive one at second or third attempt, and to identify potential risk factors for delayed diagnosis.
Methods: A retrospective study was conducted at the Policlinique Médicale Universitaire (PMU) [at present Centre for Primary Care and Public Health (Unisanté)] and in the Centre Hospitalier et Universitaire Vaudois (CHUV) covering a period of 15 years (2005-2020). All patients tested once for malaria were included. The main outcome measure was the proportion of patients with a first negative test result, subsequently positive on second or third test over the total patients with suspected malaria assessed. Demographic, travel, clinical, and laboratory variables were collected from patients’ records to identify potential predictors of an initially negative and then positive test result.
Results: 4,972 patients were included retrospectively. Of those, 4,557 (91.7%) had definitive negative test results. 415 (8.3%) had a positive result on the first test (332/415 (80%) with Plasmodium falciparum, 40/415 (9.6%) with Plasmodium vivax, 21/415 (5.1%) with Plasmodium ovale, 12/415 (2.9%) with Plasmodium vivax/ovale, 9/415 (2.2%) with Plasmodium malariae, and 1/415 (0.2%) with Plasmodium knowlesi). Three (0.06%) had a positive result on the second test (all with Plasmodium falciparum) after a first negative result, 1 (0.02%) had a positive test result (with Plasmodium falciparum) after 2 negative results. One of the four patients positive after initial negative test was 5-week pregnant and another one simultaneously had pneumonia associated with pleural effusion and acute hepatitis E. Due to the low number of secondary positive test results, no analysis on potential predictors could be performed.
Conclusions: The current recommendation of serial malaria testing is not supported by the present study, a fortiori for those who do not present with a strong clinical or laboratory predictor of malaria, or those who have a possible alternative diagnosis.