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Investigations

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

investigations

A full blood count should be done at the first visit

should be carried out in all patients presenting with symptoms
should be repeated daily until the critical phase is over
leucopenia and thrombocytopenia (which may occur as early as the second day of fever) make the diagnosis of dengue very likely
- leucopenia usually precedes the onset of the critical phase and has been associated with severe disease
- leucopenia +positive tourniquet test in dengue endemic areas has a positive predictive value of 70-80%
- thrombocytopenia is usually mild in classic dengue (may be severe in some patients)
increased haematocrit may occur in about 10% of patients due to dehydration
- the haematocrit in the early febrile phase could be used as the patient’s own baseline
- rapid thrombocytopenia + rising haematocrit suggests progress to the plasma leakage/critical phase

In addition the following tests should also be carried out in patients with dengue infection:

liver function tests – usually increased
clotting studies – useful in patients with haemorrhagic signs

A definitive diagnosis can be made by dengue specific laboratory investigations. However it is not necessary for the acute management of patients (except with unusual manifestations) (1,2).

confirmatory laboratory tests for dengue fever

The main aims of dengue laboratory investigations are:

to confirm the clinical diagnosis
to provide information for epidemiological surveillance

Laboratory diagnosis of dengue can be made by:

detecting virus or any of its components
- virus isolation
  - can be isolated from serum, plasma, leucocytes or necropsy tissue
  - methods include – mosquito & mosquito cell culture inoculation
- nucleic acid detection
  - reverse transcriptase-polymerase chain reaction (RT-PCR) is used
  - can be carried out in the first 5 days of fever onset
  - tissue, whole blood, serum or plasma specimen can be used
- antigen detection
  - detection of non structural protein 1 (NS1)
  - enzyme linked immunosorbent assay (ELISA) or rapid kits can be used
  - can be done from day 1-5 of illness
  - a serum specimen should be used
investigating serological response after infection
- detection of IgM and IgG
  - IgM ELISA and IgG ELISA are tests of choice
    - carried out after first 5 days of illness
    - detection of IgG during the first few days strongly indicates secondary infection
    - presence of IgG & IgM in single serum sample is highly suggestive of dengue infection
    - IgM or IgG seroconversion in paired serum samples or fourfold IgG titre increase in paired serum samples confirms the diagnosis
  - IgM rapid tests
    - easy to use
    - less accurate due to cross reaction with other infectious agents and in autoimmune disorders
  - haemagglutination inhibition test
    - useful for diagnosing secondary dengue infection

Reference:

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