What are the diagnostic criteria, differential diagnosis, and recommended laboratory tests for a patient suspected of having dengue fever?

Dengue Diagnosis and Laboratory Testing

For suspected dengue, order dengue NS1 antigen test and/or NAAT (PCR) plus dengue IgM and IgG antibody testing on serum if the patient presents within 7 days of symptom onset; if presenting after 7 days, order dengue IgM and IgG antibody testing with consideration for confirmatory PRNT testing. 1, 2

Clinical Diagnostic Criteria

Suspect dengue in any patient with:

• Fever PLUS one or more of the following: nausea, vomiting, rash, headache, retro-orbital pain, myalgia, arthralgia, positive tourniquet test, leukopenia, or dengue warning signs 1
• Travel history or residence in dengue-endemic areas within the past 14 days (incubation period 3-14 days, typically 4-8 days) 2

The clinical presentation alone cannot reliably distinguish dengue from other febrile illnesses, making laboratory confirmation essential 3, 4.

Differential Diagnosis

Consider these alternative diagnoses in patients with suspected dengue:

• Other flaviviruses: Zika virus (especially important in pregnant women), West Nile virus 1
• Other arboviruses: Chikungunya 1
• Parasitic infections: Malaria 1
• Viral infections: Rubella, measles, hepatitis A, parvovirus, adenovirus, enterovirus 1
• Bacterial infections: Leptospirosis, rickettsiosis, group A streptococcal infections 1

Cross-reactivity with other flaviviruses is a significant confounding factor in serological testing, particularly with Zika virus 2, 5.

Laboratory Testing Algorithm

For Patients Presenting ≤7 Days After Symptom Onset:

Order the following tests on serum:

• Dengue NS1 antigen test (can be positive from day 1 of symptoms through day 7-10) 6, 2
• Dengue NAAT/PCR (most sensitive in first week when viremic) 2
• Dengue IgM antibody test 1, 2
• Dengue IgG antibody test 5

The NS1 antigen test is FDA-cleared and provides excellent sensitivity (81-94%) for early diagnosis, with similar detection frequency and duration as viral RNA 1, 6, 3. NAAT is the preferred diagnostic method during the first week according to WHO 2.

For Patients Presenting >7 Days After Symptom Onset:

Order the following tests on serum:

• Dengue IgM antibody test (primary serological test after first week) 1, 2
• Dengue IgG antibody test 5
• Consider PRNT (Plaque Reduction Neutralization Test) for confirmation if available 1

IgM antibodies typically develop during the first week and remain detectable for 2-3 months 2. Serological testing becomes more appropriate after the first week when viremia has cleared 2.

Interpretation of Laboratory Results

Acute Phase (≤7 Days):

• Positive NS1/NAAT + Negative IgM/IgG: Acute primary dengue infection 5
• Positive NS1/NAAT + Positive IgG: Acute secondary dengue infection 5
• Positive NS1/NAAT + Positive IgM: Acute dengue infection (primary or secondary) 2
• Negative NS1/NAAT + Negative IgM/IgG: Does not rule out dengue if collected very early (first 24 hours) 6

Convalescent Phase (>7 Days):

• Negative NS1/NAAT + Positive IgM + Negative IgG: Recent primary dengue infection 5
• Negative NS1/NAAT + Positive IgM + Positive IgG: Recent secondary dengue infection 5
• Negative NS1/NAAT + Negative IgM + Positive IgG: Past dengue infection 5

Confirmatory Testing with PRNT:

• Dengue PRNT ≥10 + Zika PRNT <10: Recent dengue virus infection 1
• Dengue PRNT <10 + Zika PRNT ≥10: Recent Zika virus infection 1
• Both PRNT ≥10: Recent flavivirus infection (cannot distinguish specific virus) 1
• Both PRNT <10: No evidence of dengue or Zika infection 1

Special Considerations for Pregnant Women

Pregnant women require comprehensive testing due to potential complications:

• Perform both NAAT and IgM antibody testing on serum 1
• Perform NAAT on urine specimen for Zika virus detection 1
• Collect specimens as soon as possible and within 12 weeks of symptom onset 1
• Always test for both dengue and Zika virus regardless of outbreak status 1

Dengue infection during pregnancy increases risk for maternal death, hemorrhage, preeclampsia, eclampsia, and vertical transmission 1, 2.

Common Pitfalls and Caveats

Avoid these diagnostic errors:

• NS1 sensitivity is significantly lower in secondary dengue infections compared to primary infections (may miss up to 50% of secondary cases) 7
• Negative NS1 or NAAT in the first 24 hours or after 7 days does not exclude dengue 6
• IgM testing alone has poor sensitivity (13-40%) in acute phase and should not be used as the sole diagnostic test 8
• Without confirmatory PRNT, positive IgG results should be interpreted as "presumptive" due to cross-reactivity 5
• IgM antibodies can persist for 2-3 months, so positive IgM does not necessarily indicate acute infection 2
• Cross-reactivity with other flaviviruses (Zika, West Nile, yellow fever) complicates serological interpretation 2, 5

Best practice: Use combination testing (NS1/NAAT plus serology) rather than relying on a single test method for optimal diagnostic sensitivity 9, 8.