There are four criteria—all of which must be fulfilled to meet the case definition—that were first set up by a World Health Organization working group for reporting a case of DHF. The criteria, as currently stated by the World Health Organization, are:

1. Fever, or recent history of acute fever;
2. Hemorrhagic manifestations;
3. Low platelet count (100,000/mm³ or less);
4. Objective evidence of plasma leakage caused by increased vascular permeability, as reflected by one or more of the following:
   • Elevated hematocrit (defined as 20% or more over baseline, or a similar drop after volume replacement treatment);
   • Low protein; or
   • Pleural or other effusions.

Plasma leakage is the critical difference between dengue hemorrhagic fever and dengue fever and means that the patient requires fluids, sometimes large amounts of intravenous fluids.