Rapid influenza diagnostic tests (RIDTs) are commonly used by clinicians to guide patient management. Data on sensitivities among hospitalized patients are limited. Here, we evaluated the clinical and virologic factors affecting the sensitivities of 2 commercially available RIDTs (BinaxNOW Influenza A&B and QuickVue Influenza A+B) on nasopharyngeal aspirate (NPA) specimens collected from elderly patients and young children hospitalized for influenza. Influenza cases and age-matched negative controls were prospectively enrolled during the 2011-2012 influenza season in Hong Kong. NPA specimens were collected at presentation before antiviral treatment. Real-time reverse transcription-PCR (RT-PCR) results were used as references for the sensitivity analyses. One hundred patients (57 influenza cases and 43 controls) were studied. Both RIDTs had 100% specificities. The sensitivities of the BinaxNOW Influenza A&B and QuickVue Influenza A+B tests were 70% and 82%, respectively. For both tests, the sensitivities were lower in cases with presentation times beyond 2 days of illness onset than for those within this time (50 to 71% versus 85 to 91%, respectively). There were trends toward lower sensitivities for influenza B than for influenza A (66 to 81% versus 76 to 84%, respectively), among young children than among the elderly patients (63 to 78% versus 80 to 88%, respectively), and among cases with pneumonia than those without pneumonia (75% versus 82 to 94%, respectively). The sensitivities of the RIDTs decreased with reduced NPA viral RNA levels (5.6 to 15.0% reduction per 1-log decrease), which declined progressively after illness onset (Spearman's rho, -0.47 [P < 0.05] and -0.66 [P < 0.001] for influenza A and B, respectively). Collectively, late presentation, a low NPA viral load, and probably lower respiratory manifestation are factors associated with reduced sensitivities of RIDTs for diagnosing influenza in hospitalized patients. A negative RIDT result should be interpreted with caution.