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Laboratory Tests and Ancillaries
Detection of Antigens
Developments in ELISA and dot blot assays directed
to the envelope/membrane (E/M) antigen and the non-structural protein (NS1)
showed that high concentrations of these antigens (immune complexes) could be
detected in patients during the early period of the disease and in patients with
primary and secondary dengue infections up to 6 days after the start of illness.
Non-structural protein 1 (NS1) ELISA has been shown
to be a useful diagnostic tool in acute dengue infections. It is produced by
all flaviviruses and is secreted from mammalian cells. Dengue NS1 antigen has
been detected in the serum of dengue virus-infected patients as early as one-day
post-symptom onset (PSO) and up to eighteen days post onset (DPO). It may also
be useful for differential diagnosis between flaviviruses because of its
specificity. NS1 antigen detection kits are commercially available and can be
used by laboratories with limited equipment and yield results within a few
hours.
Serology
The HI test is a simple, sensitive, and reproducible
test that requires paired sera, soon following hospital admission (acute) and 10-14
days after discharge (convalescent). A titer of ≥1:1280 in an acute or
convalescent phase serum sample is considered presumptive evidence of current
dengue. It is negative in an early acute blood specimen and a repeat specimen
should be tested before confirming or excluding dengue infection. It is limited
by its inability to discriminate between infections by closely related
flaviviruses.
IgM antibody capture-ELISA (MAC-ELISA) is the serological
test of choice because the ELISA IgM detected is specific for flavivirus. It is
rapid and simple, and it only requires a little sophisticated equipment. A
single, properly timed blood sample may be adequate. A high increase in the molar
fraction of IgM indicates a primary acute flavivirus infection and a low
increase in the molar fraction of IgM indicates a secondary acute flavivirus
infection. A high IgM antibody response indicates a recent primary flavivirus
infection and a low IgM antibody response indicates a recent secondary
flavivirus infection.
In MAC-ELISA, a positive dengue IgM result indicates
acute or recent past infections (up to 90 days). A single positive dengue IgM
may not be indicative of present dengue infection. MAC-ELISA may also be used
to detect CSF IgM. However, it cannot be used to identify infecting virus serotypes.
Positive results do not necessarily denote a current dengue infection. Caution
must be exercised when making decisions about patient management.
IgG ELISA is comparable
to the HI test and can be used to differentiate primary and secondary
infections.
The neutralization test
(NT) is the most specific and sensitive serologic test for dengue virus. It is
not commonly used because it is expensive, requires a long time for performance,
and is technically difficult. Serum dilution plaque reduction NT is the most
common protocol.
The complement fixation
(CF) test is the least sensitive serological assay. It appears later than IgM
or HI antibody and is usually less specific. It has greater specificity for
primary infections but is not specific in secondary infections.
Other Laboratory Tests for
Dengue Patients
CBC may show normal total WBC, but leukopenia may
develop throughout the febrile period. The mild increase in hematocrit may be
due to dehydration. Platelet counts and factors of the blood
clotting mechanism can be normal. Thrombocytopenia may occur.
Albumin level is usually low. Liver function tests such as ALT and
AST levels should be evaluated. Urinalysis may be requested to check for
hematuria.
Other diagnostic tests include virus isolation. There are also
molecular techniques including reverse transcriptase-polymerase chain reaction
(RT-PCR). Its specificity and sensitivity are better than virus isolation with
a faster turnaround time.