What diagnostic tests should be requested for a patient suspected of having dengue (Dengue Virus infection) in an endemic area, particularly in individuals with underlying health conditions, the elderly, or young children?

Diagnostic Testing for Suspected Dengue in Endemic Areas

For patients presenting within 7 days of symptom onset, order dengue virus NAAT (nucleic acid amplification test/PCR) on serum as the primary diagnostic test, and if available, add NS1 antigen testing for enhanced sensitivity. 1

Testing Algorithm Based on Timing of Presentation

Early Presentation (≤7 Days After Symptom Onset)

Primary approach:

• Perform dengue virus NAAT (RT-PCR) on serum specimen 1
• Alternatively or additionally, perform NS1 antigen testing (can substitute for NAAT if molecular testing unavailable) 1, 2
• Some laboratories may choose to run both NAAT and IgM antibody testing simultaneously rather than sequentially 1

Specimen types:

• Serum is the preferred specimen for all testing 1
• Some NAATs can also be performed on plasma, whole blood, cerebrospinal fluid, or urine 1

Important consideration for endemic areas:

• In regions where both dengue and Zika virus circulate, perform NAATs for both dengue and Zika virus simultaneously to differentiate between these flaviviruses 1

Late Presentation (>7 Days After Symptom Onset)

Primary approach:

• Perform dengue IgM antibody testing (IgM capture ELISA/MAC-ELISA) on serum 1, 2
• IgM antibodies become detectable after the first week of illness 1

If NAAT was negative in early presentation:

• Follow up with IgM antibody testing on the same or subsequent specimen 1

Confirmatory Testing When Needed

When to perform confirmatory PRNT (Plaque Reduction Neutralization Test):

• Positive IgM antibody result without positive NAAT or NS1, when definitive diagnosis is needed for clinical or epidemiologic purposes 1
• Indeterminate IgM antibody results 1
• Perform PRNTs against dengue, Zika, and other flaviviruses endemic to your region 1

Interpretation of PRNT results:

• PRNT titer ≥10 against dengue with negative PRNTs (<10) against Zika and other flaviviruses confirms recent dengue infection 1
• PRNT titer ≥10 for both dengue and Zika indicates recent flavivirus infection but cannot identify the specific virus 1

Special Populations Requiring Enhanced Testing

Pregnant Women

Critical difference in approach:

• Test ALL pregnant women with both NAAT and serology for dengue AND Zika virus, regardless of local outbreak patterns 2, 3, 4
• Collect both serum and urine specimens as soon as possible within 12 weeks of symptom onset 3, 4
• Perform concurrent dengue and Zika virus NAATs and IgM antibody testing 3, 4
• This comprehensive approach is mandatory due to risks of maternal death, hemorrhage, preeclampsia, vertical transmission, and fetal complications including microcephaly 2, 3, 4

Elderly and Young Children

• Use the same diagnostic algorithm as adults based on timing of presentation 1
• These populations warrant lower threshold for hospitalization and closer monitoring, but diagnostic testing approach remains unchanged 2

Practical Considerations for Point-of-Care Testing

Rapid diagnostic tests (RDTs):

• Lateral flow immunochromatographic tests combining NS1 antigen detection with IgM/IgG antibodies are available and highly practical for endemic settings 5, 6, 7
• NS1 rapid tests show high specificity but significantly lower sensitivity in secondary dengue infections compared to primary infections 8, 6
• Rapid tests combining NS1 and IgG have very high positive likelihood ratios and can optimize management 2

Performance characteristics:

• NS1 ELISA and PCR show similar sensitivity (approximately 88-89%) 6
• Commercial lateral flow NS1 tests demonstrate sensitivity ranging from 81-94% 6
• NS1 testing is most useful within the first 5-7 days of illness 5, 9

Critical Pitfalls to Avoid

Cross-reactivity issues:

• Document vaccination history for yellow fever, Japanese encephalitis, and tick-borne encephalitis, as these can cause false-positive results due to flavivirus cross-reactivity 2
• IgM antibodies can persist for months following infection, so positive IgM alone does not confirm acute infection timing 1

Timing errors:

• A negative IgM test in specimens collected <7 days after onset may simply reflect collection before antibody development and does not rule out dengue 1
• Negative NAAT does not exclude dengue due to declining viremia over time and possible inaccurate reporting of symptom onset dates 1

Interpretation in endemic settings:

• For specimens collected 7 days to 12 weeks after symptom onset, negative IgM to both dengue and Zika rules out recent infection with either virus 1
• Without confirmatory PRNTs, positive IgM results should be interpreted as "presumptive recent dengue infection" rather than definitive diagnosis 1

Supporting Laboratory Tests for Clinical Management

Essential monitoring tests (not diagnostic but critical for management):

• Daily complete blood count to track platelet counts and hematocrit levels 2, 3, 4
• Monitor for thrombocytopenia (platelets ≤100,000/mm³) and rising hematocrit (>20% increase from baseline) as indicators of severe disease 2
• Absence of thrombocytopenia significantly reduces probability of dengue 2

Additional tests when indicated:

• Liver function tests if clinical hepatitis suspected 2
• Blood and urine cultures if fever persists to evaluate for secondary bacterial infection 2

Reporting Requirements

Mandatory notification:

• Dengue is a nationally notifiable condition and must be reported to state, territorial, or local health departments 3, 4