Annually, over 2.5 billion people are at risk for infection with dengue virus (DENV), while between 50 and 100 million people contract the infection. There is an urgent need for alternative diagnostic tools that can detect DENV during acute infection. Recent studies have shown that DENV nonstructural protein 1 (NS1) is detectable in the blood as early as the onset of symptoms and persists well into the convalescent phase of the infection. We evaluated the utility of the Bio-Rad Platelia DENV NS1 antigen capture kit in combination with real-time reverse transcriptase PCR (RT-PCR) and an IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA) for refining a new algorithm for the diagnosis of acute- or convalescent-phase DENV infection with a single clinical sample. We tested the Bio-Rad kit with three panels of sera. These panels were designed to evaluate the sensitivities of the NS1 kit for (i) early-convalescent-phase samples, (ii) acute-phase samples with false-negative PCR results, and (iii) IgM-negative convalescent-phase samples from patients with confirmed secondary DENV infections. Results show that NS1 can be detected in 22% of serum samples collected more than 10 days after the onset of illness and in 22% of samples that did not elicit an IgM response. Additionally, NS1 was detected in 37% of the tested acute-phase samples with false-negative PCR results, suggesting that NS1 detection may be valuable in increasing the sensitivity of current acute-phase diagnostics. These results will improve diagnosis with a single acute-phase or early-convalescent-phase sample for disease surveillance and clinical diagnosis.