Initial Diagnostic Test for Dengue in Acute Febrile Illness
In a patient with acute febrile illness in a dengue-endemic area presenting within 7 days of symptom onset, order dengue PCR/NAAT or NS1 antigen detection on serum as the initial diagnostic test. 1
Diagnostic Algorithm Based on Symptom Duration
Early Presentation (≤7 days from symptom onset)
Dengue PCR/NAAT on serum is the preferred initial test because viral RNA is detectable for 4–6 days after symptom onset, providing the highest sensitivity during the acute viremic phase. 1
NS1 antigen detection serves as an excellent alternative to NAAT, as NS1 is secreted from infected cells as early as 1 day after symptom onset and remains detectable for up to 10 days. 1, 2
Both serum and plasma are acceptable specimen types and may be transported at room temperature if processed within 2 hours of collection. 1
Peak NS1 sensitivity (75–90%) occurs during the first 3–5 days of illness, with sensitivity beginning to decline after day 5 but remaining useful up to day 10. 2, 3, 4
Late Presentation (>7 days from symptom onset)
IgM capture ELISA (MAC-ELISA) becomes the primary diagnostic test after the first week of illness, as IgM antibodies appear during the first week and can persist for 2–3 months. 1, 2
If initial NS1 or PCR/NAAT is negative in patients ≤7 days, perform IgM antibody testing on the same specimen because a negative molecular result does not rule out dengue—viremia may have declined or the onset date may be inaccurate. 1
Critical Considerations for Test Selection
Primary vs. Secondary Infection Impact
NS1 sensitivity is significantly higher in primary infections (98.8%) compared to secondary infections (83.5%), which is particularly important in endemic areas where secondary infections predominate. 3, 4
In secondary dengue infections, IgM antibodies may not be detectable, making NS1 testing particularly valuable despite its reduced sensitivity. 2
Serotype and Timing Effects
NS1 sensitivity varies by dengue serotype and decreases with time after symptom onset—showing 100% sensitivity early during illness but dropping below 50% by day 7. 3, 4
The positive rate of NS1 antigen is 100% on day 2,92.3% on day 3,76.9% on day 4,56.5% on day 5, declining to 43.1% on day 6 and 29.8% on day 7. 4
Confirmatory Testing When Needed
Plaque reduction neutralization test (PRNT) is the reference standard when cross-reactivity with other flaviviruses (Zika, West Nile, yellow fever, Japanese encephalitis) is suspected. 1, 2
PRNT titer ≥10 defines a positive result and provides improved specificity over commercial serologic assays. 1
Document complete vaccination history for yellow fever, Japanese encephalitis, and other flavivirus vaccines, as these can cause false-positive IgM results due to cross-reactivity. 1, 2
Common Pitfalls to Avoid
Do not delay dengue testing based on assumptions about incubation—if a patient has been in an endemic area within the past 14 days and presents with compatible symptoms, testing should be performed immediately. 1
Never assume acute infection based on positive IgG antibodies alone, as IgG can persist for months to years after the initial infection. 2, 5
A negative IgM test during the first few days of illness does not rule out dengue infection, as antibodies may not have developed yet—repeat testing after 5–7 days is indicated if clinical suspicion remains high. 2
In endemic areas with high secondary infection rates, interpret NS1 results with caution due to reduced sensitivity (63.2–83.5% overall), though specificity remains very high (98.4–100%). 6, 3, 7
Special Population Considerations
For pregnant women, perform simultaneous NAAT for both dengue and Zika viruses regardless of outbreak patterns, due to risks of maternal death, hemorrhage, preeclampsia, and vertical transmission. 1, 2
Before dengue testing, first perform a rapid malaria test to exclude malaria in any febrile traveler from endemic regions. 1