Dr Jyotirmoy Pal writes on the presentation of dengue, tell-tale clinical signs and confirmatory tests that can help establish a diagnosis.

Diagnosis of dengue is primarily based on evaluation of meticulous history, clinical examination, haematocrit and/or full blood count (FBC). These parameters also help to determine the phase in which the patient is in, the presence or absence of any danger signs, the haemodynamic state and hydration status of the patient and whether the patient needs admission.

Laboratory diagnosis of dengue infection is essential for confirmation of clinical diagnosis and for providing necessary information for epidemiological surveillance. For adequate dengue diagnosis, three aspects should be considered:

• Serological and virological markers in respect to the time of dengue virus infection
• Characteristics of clinical samples
• The type of diagnostic method used with respect to clinical illness

Laboratory diagnostic tests used to detect dengue virus infection

1. NS1 antigen tests (ELISA based): Dengue NS1 antigen is an essential tool for the diagnosis of dengue infections during the early stages. Though it is a simple test, it has high sensitivity and specificity. Since NS1 assay can detect dengue infection early in the viremic stage, it has major epidemiological significance in preventing transmission. This test is also useful for differential diagnostics between different flaviviruses because of its high specificity.
2. IgM capture enzyme-linked immunosorbent assay (MAC- ELISA): It is a very simple test and has been used widely in the last few years. It is based on the detection of the dengue specific IgM antibodies which appears a little earlier than IgG antibodies and becomes detectable by day 5 of illness. This test is an essential tool for surveillance of dengue fever/dengue haemorrhagic fever. In non-endemic areas, this test can also be used for surveillance of viral illness and for population-based random serosurveys.
3. Isolation of dengue virus: Many strains of dengue virus can be isolated from clinical specimens such as plasma, acute-phase serum, washed buffy coat and autopsy tissues of spleen, liver, thymus, lymph nodes and mosquitoes captured from nature. The sample has to be collected within the first five days of illness and processed immediately. Since it takes 7-10 days to isolate the virus from a clinical specimen, it is not considered a very useful laboratory test for beginning the treatment of patients with dengue fever.
4. Polymerase chain reaction (PCR): Reverse transcription-polymerase chain reaction (RT-PCR) and nucleic acid sequence-based amplification (NASBA) is the new standard method for detection of dengue virus from acute-phase serum samples.
5. IgG-ELISA: It is a simple test that is done to differentiate between primary and secondary dengue infections. It indicates only the past infections and henceforth is not considered a good diagnostic test.
6. Serological tests: Complement fixation (CF), hemagglutination-inhibition (HI) and neutralization test (NT) are not widely used due to different technical problems.
7. RDTs: A good number of commercial RDT kits for NS1 antigen and anti-dengue IgM/IgG antibodies are available which can give results within 15 to 25 minutes. Due to lack of validation and low sensitivity and specificity, the use of RDTs is not recommended.
8. Additional tests like serum electrolytes, blood sugar level, urea, creatinine, liver function test, lactate or bicarbonate, electrocardiogram (ECG) and cardiac enzymes assay can be considered in patients with severe disease and comorbidities. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) becomes significantly elevated in dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) as compared to dengue fever (DF).
9. A decreasing leucocyte and platelet count makes the diagnosis of dengue virus infection very likely.
10. Immature platelet fraction (IPF): IPF may be used as a predictive marker of platelet recovery. In dengue patients, the platelet count usually gets recovered within 24 to 48 hours if the value of IPF is greater than 8%.

Dengue diagnostics and characteristics of sample

Probable and confirmed dengue diagnosis, interpretation or results and characteristics of sample

Collection of samples

Proper methods of collection, processing, storage and transport of specimens are very essential for the laboratory diagnosis of dengue. Collection of blood samples should be done maintaining all universal precautions. Proper information (such as day of onset of fever, day of collection of samples, the type of test that is needed to be performed, etc) should be clearly furnished while sending samples for laboratory confirmation.

National Vector Borne Disease Control Programme (NVBDCP) recommended tests for laboratory diagnosis

For laboratory confirmation of dengue virus infection, the Government of India (GOI) recommends an ELISA-based test for detection of NS1 antigen from the first day onwards and a MAC-ELISA test after the fifth day of onset of disease. Apex Referral Laboratories (ARLs) equipped with advanced diagnostic facilities and sentinel surveillance hospitals are being identified across the country for surveillance of dengue infections.

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

Dr Jyotirmoy Pal is a Professor of Medicine from Kolkata.