According to WHO 1997 classification, Dengue fever is classified into three major categories
1. Dengue Fever (DF)
2. Dengue hemorrhagic fever (DHF)
3. Dengue Shock Syndrome (DSS)
Dengue hemorrhagic fever is characterized by plasma leakage and manifests as hemoconcentration, pleural effusion, and/or ascites - along with fever and thrombocytopenia. According to WHO guidelines, DHF requires the documentation of hemorrhagic tendency.
One easy method is via tourniquet test. The blood pressure cuff should be inflated on the upper arm to a point midway between the systolic and diastolic pressures for 5 minutes. A positive test is described as 10 or more petechiae per 1 inch. Said that test may give false-negative results if a patient is in profound shock, or false-positive when patient is in a recovering state.
Link to WHO document is given in the reference below.
1. World Health Organization. Dengue haemorrhagic fever: Diagnosis, treatment, prevention and control, 2nd edition. WHO, Geneva 1997. http://apps.who.int/iris/bitstream/10665/41988/1/9241545003_eng.pdf (Accessed on May 18, 2020).