The clinical laboratory findings of dengue fever include low levels of white blood cells (leukopenia) and platelets (thrombocytopenia) and, often, an elevated level of the enzyme serum aminotransferase. The diagnosis of dengue fever is usually confirmed in the laboratory by serologic tests on blood samples from the patient. (In very technical terms, this is done by antigen-detection ELISA or PR-PCR during the acute phase of the disease or by IgM ELISA or paired serology during the recovery phase of dengue fever). During the acute phase of the illness, the dengue virus that causes the disease can also be readily isolated from the patient's bloodstream. A drop in the platelet count below 1,00,000 per cmm is usually seen between the 3rd and 7th days. The WBC counts may be variable but usually shows leucopenia with relative lymphocytosis. Hemoconcentration is almost always present with the hematocrit raising more than 20% of the average value for the age and population. Other wise an absolute value of more than 40% is diagnostic of DHF. This is direct evidence of plasma leakage, unless it is interfered by administration of fluids or massive internal hemorrhage.