The diagnosis of dengue is usually made clinically. The classic picture is high fever with no localising source of infection, a petechial rash with thrombocytopenia and relative leukopenia (low platelet and white blood cell count). Care has to be taken as diagnosis of Dengue Haemorrhagic Fever (DHF) can mask end stage liver disease and vice versa. If one has persistent fever for more that 2 days then one should go for a complete blood checkup (CBC). If the platelet count and WBC count are below than their usual range one should go for Dengue Antigen test.

Dengue is always a diagnosis of exclusion, and other diseases with the same initial clinical presentation must be suspected. In order to help the clinician in the detection of severe forms of dengue (DHF/DSS), even when the definitive diagnosis has not been made yet, the following three essential laboratory tests may help in the evaluation of the real clinical conditions of the patient and its early supportive management:

	Total White Blood Cells Count: In case of dengue, this test will reveal leukopenia. The presence of leukocytosis and neutrophilia excludes the possibility of dengue and bacterial infections (leptospirosis, meningoencephalitis, septicemy, pielonephritis etc.) must be considered.
	Thrombocytopenia (less than 100.000 per mm3): Total platelets count must be obtained in every patient with symptoms suggestive of dengue for three or more days of presentation. Leptospirosis, measles, rubella, meningococcemia and septicemy may also course with thrombocytopenia
	Hematocrit (micro-hematocrit): According to the definition of DHF, it’s necessary the presence of hemoconcentration (hematocrit elevated by more than 20%); when it’s not possible to know the previous value of hematocrit, we must regard as significantly elevated the results more than 45%.

Figure 1. Case definitions of Dengue Haemorrhagic Fever (DHF). For a diagnosis of DHF Grade I, each of the four criteria must be met. Fever that is not specific and with constitutional symptoms and the only haemorrhagic proof being a positive tourniquet test are being classified under Grade I. Grade II is similar to Grade I, but Grade II has specific haemorrhagic manifestations. Signs of circulatory failure or hypertension are graded under Grade III, and lastly, profound shock with pulse and blood pressure that is undetectable is under Grade IV.

A dependable immediate information of the Dengue diagnostics in the rural areas can be performed by the introduction of Rapid Diagnostic Test kits which also differentiates between primary and secondary dengue infections. Serology and polymerase chain reaction (PCR) studies are available to confirm the diagnosis of dengue if clinically indicated. Dengue can be a life threatening fever.

Etiologic confirmation can be obtained by isolating infectious virus, demonstrating viral antigen by immunoassay, or viral genome by PCR in serum or blood. Serologic diagnosis is achieved by IgM antibody-capture by enzyme-linked immunosorbent assay (MAC - ELISA) in two blood specimen taken in a period of 14 days from each other. The first specimen, taken till the seventh day of the disease, can also be useful for virus isolation by inoculation of A. albopictus cells or adults mosquitoes, with specific indentification of virus by immunofluorescence tests employing monoclonal antibody reagents.

Postmortem diagnosis is made by virus isolation or by demonstration of viral antigen (direct immunofluorescence) from two-specimen visceral fragments (liver, spleen, linfonodes, thymus).

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Dengue and Chikungunya: Making the Diagnosis
Dengue fever is somewhat similar to the fever of chikungunya. Chikungunya and dengue are both acute febrile illnesses characterized by fever, myalgia, and lethargy. Some patients may also have maculopapular rash, nausea, vomiting, and headache. Distinguishing features of chikungunya include potentially debilitating bilateral polyarthralgia and, in some cases, arthritis. Although these signs and symptoms may assist in differentiating dengue and chikungunya, clinicians should include both illnesses in their differential diagnosis of patients with acute febrile illness and recent travel to the tropics. The CDC has a comprehensive expert opinion on differentiating chikungunya from dengue: a clinical challenge (Medscape, 14 September 2014).