Excerpt from UptoDate.com:
'There are no pathognomonic clinical signs or symptoms for diagnosis of malaria [12,13]. The most predictive findings of malaria are
fever without localizing symptoms,
platelet count less than 150 x 10(9)/L and
bilirubin greater than 1.3 mg/dL;
this was illustrated in a study of 1962 travelers returning with fever .
The presence of skin rash, skin ulcer, and eosinophilia was predictive of non-malarial disease.
Blood counts are often abnormal but are not specific. In a series of Canadian travelers, patients with malaria presented with anemia in 41 percent of cases . White blood cell counts were elevated above 9.8 x 10(9)/L in 3 percent of patients but were less than 5.0 x 10(9)/L in 48 percent . Platelets were low in 83 percent of Plasmodium vivax-infected patients (mean 102 x 10(9)/L), and in 62 percent of P. falciparum-infected patients (mean 137 x 10(9)/L).
Detection of parasites on Giemsa-stained blood smears by light microscopy is the gold standard for diagnosis of malaria.
Given the cyclic nature of malaria parasitemia, smears should be evaluated every 6 to 12 hours for 48 hours before the diagnosis of malaria is ruled out . The first smear is positive in 95 percent of cases.'
Local Microbiology advice suggests in low risk cases a single negative malarial film and negative malarial antigen is adequate to exclude malaria.