Assessment of ICD-9-based case definitions for influenza-like illness surveillance.

Population-based surveillance of influenza routinely relies on administrative medical encounter databases and ICD-9 codes. However, an assessment of the ICD-9 codes used for the Department of Defense (DoD) influenza-like illness (ILI) case definition has not been conducted since 2007. As coding practices may have changed over time, this analysis was done to determine the sensitivity, specificity, and positive predictive value (PPV) of the current ILI case definition and three alternative case definitions for the 2014-2015 influenza season. Influenza laboratory tests conducted on specimens from DoD beneficiaries during the 2014-2015 season were matched to ambulatory and inpatient medical encounters. The current DoD ILI case definition had high sensitivity (92%) but low specificity (30%) and moderate PPV (63%). A more specific ILI case definition utilizing only codes with greater than 75% influenza positivity for the matched laboratory test had high specificity (96%) and PPV (96%) and moderate sensitivity (62%). The current ILI case definition is sufficient for broad, sensitive population-based surveillance; however, an alternative case definition may be more appropriate when there is a need to maximize specificity.