A negative blood smear makes the diagnosis of malaria unlikely. However, because non-immune individuals may be symptomatic at very low parasite densities which may be initially undetectable, blood smears should be repeated every 12 – 24 hours for a total of three sets before the diagnosis of malaria can be ruled out.
Laboratorians may lack experience with malaria and fail to detect parasites when examining blood smears under the microscope. In some malaria-endemic areas, malaria transmission is so intense that a large proportion of the population is infected but not made ill by the parasites.
Such carriers have developed just enough immunity to protect them from malarial illness but not from malarial infection. In that situation, finding malaria parasites in an ill person does not necessarily mean that the illness is caused by the parasites. In the US, symptomatic and asymptomatic infections should be treated.
As a national reference center for malaria diagnosis, CDC provides diagnostic and technical assistance on malaria diagnosis. CDC provides reference microscopic diagnosis and other specialized tests such as PCR. Additionally, telediagnosis and training are provided for malaria and other parasitic diseases on CDC's DPDx website. Public health laboratories may also conduct malaria diagnostic testing, healthcare providers should check with their health departments for assistance with testing and/or specimen submission to CDC.
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