BACKGROUND The rapidly evolving pandemic of novel 2009 swine-origin influenza A (H1N1) virus (S-OIV) demands that accurate and practical diagnostics be urgently evaluated for their potential clinical utility. OBJECTIVE To determine the diagnostic accuracy of a rapid influenza diagnostic test (RIDT) and direct fluorescent antibody (DFA) assay for S-OIV by using reverse-transcription polymerase chain reaction (RT-PCR) as the reference standard. METHODS We prospectively recruited children (aged 0-17 years) assessed in the emergency department of a pediatric referral hospital and a community pediatric clinic for influenza-like illness between May 22 and July 25, 2009. RIDT (performed on-site) and DFA were compared with RT-PCR to determine their sensitivity and specificity for S-OIV. We also compared the sensitivity of RIDT for S-OIV to that for seasonal influenza over 2 preceding seasons. RESULTS Of 820 children enrolled, 651 were from the emergency department and 169 were from the clinic. RIDT sensitivity was 62% (95% confidence interval [CI]: 52%-70%) for S-OIV, with a specificity of 99% (95% CI: 92%-100%). DFA sensitivity was 83% (95% CI: 75%-89%) and was superior to that of RIDT (P < .001). RIDT sensitivity for S-OIV was comparable to that for seasonal influenza when using DFA supplemented with culture as the reference standard. RIDT sensitivity for influenza viruses was significantly higher in children 5 years of age or younger (P = .003) and in patients presenting < or =2 days after symptom onset (P < .001). CONCLUSIONS The sensitivity of RIDT for detection of S-OIV is higher than recently reported in mixed adult-pediatric populations but remains suboptimal.