Background: Early and accurate diagnosis of dengue fever (DF) is indispensable for patient management and outbreak control in endemic areas such as Bangladesh. S ns1 as an early diagnostic marker is a challenge, and there are many formats of “rapid test” (RDT) today, the sensitivity and specificity of all RDT in relation to molecular gold standard has yet not been well defined for use among more vulnerable population with highest dengue burden. The objective of this study was to assess the diagnostic performance of a commercial NS1 RDT and an NS1 ELISA with reference to real-time reverse transcription-polymerase chain reaction (RT-PCR) for early dengue diagnosis. Methods: A prospective diagnostic accuracy study was conducted from July 2024 to June 2025 at two tertiary hospitals in Dhaka. We enrolled 200 consecutive patients with acute febrile illness (≤5 days) meeting the WHO suspected dengue case definition. Serum was simultaneously tested with the SD BIOLINE Dengue Duo NS1 rapid diagnostic test (RDT), the PANBIO™ Dengue Early enzyme linked immunosorbent assay (ELISA) and a multiplex real time reverse transcriptase PCR. Sensitivity, specificity, predictive values and concordance (Cohen’s kappa) were estimated with 95% confidence intervals (CI). Performance was analyzed by day of illness and association with RT-PCR cycle threshold (Ct) values. Results: Out of the total 200 patients, confirmed RICT dengue-positive was found in 124 (62.0%) by RT-PCR. The NS1 ELISA was significantly more sensitive compared with the RDT (91.9% [85.6-96.1] versus 74.2% [65.5-81.6]; p<0.001). Specificities were 96.1% (88.9-99.2) with ELISA and 92.1% (83.6-97.0) with RDT. ELISA testing had good concordance with RT-PCR (κ=0.87), whereas the RDT administered only a moderate concordance (κ=0.66). Sensitivity of both tests waned with delay in presentation, but this was more marked for the RDT which registered a sensitivity of 59.1% by day 4-5. The most influential factor for both tests false negative was high RT-PCR Ct values (low viral load). Conclusion: The NS1 ELISA is far superior to the NS 1 RDT for early diagnosis of dengue in Bangladesh hospital. Though the RDT serves as a rapid triage tool, it comes with a high false-negative rate after the early febrile stage, and should be used cautiously. A reflex testing algorithm with RDT as initial screening, followed by ELISA confirmation of negative RDT cases, should be considered in order to maximize early case detection and patient management.