With automatic hemocytometer the platelet count was determined. Through World Health Organization criteria DHF was determined i.e. the platelets count must be 100 000 cells/ mm3, hematocrit elevation 20% above the average age group and hemorrhagic manifestation [7,17]. All the DHF cases were further grouped as I–IV. Dengue fever was defined as an elevation in hematocrit of < 20% and on the right lateral decubitus chest radiograph with no detectable pleural effusion [18]. Procedure for this study was approved by the Bioethics Committee of Hayat Abad Medical Complex, Peshawar Pakistan. Written informed consent was provided to parents and guardians of all patients.
Hemagglutination inhibition test was performed according to the method of Clarke and Casals [19], a paired plasma sample with an interval longer than 7 days was used. When the hemagglutination inhibition titer was 1:2560 in the plasma of