Validity of ICD-10-based algorithms to identify patients with influenza in inpatient and outpatient settings

Purpose: To evaluate the validity of ICD-10-CM code-based algorithms as proxies for influenza in inpatient and outpatient settings in the USA. Methods: Administrative claims data (2015–2018) from the largest commercial insurer in New Jersey (NJ), USA, were probabilistically linked to outpatient and inpatient electronic health record (EHR) data containing influenza test results from a large NJ health system. The primary claims-based algorithms defined influenza as presence of an ICD-10-CM code for influenza, stratified by setting (inpatient/outpatient) and code position for inpatient encounters. Test characteristics and 95% confidence intervals (CIs) were calculated using test-positive influenza as a reference standard. Test characteristics of alternative outpatient algorithms incorporating CPT/HCPCS testing codes and anti-influenza medication pharmacy claims were also calculated. Results: There were 430 documented influenza test results within the study period (295 inpatient, 135 outpatient). The claims-based influenza definition had a sensitivity of 84.9% (95% CI 72.9%–92.1%), specificity of 96.3% (95% CI 93.1%–98.0%), and PPV of 83.3% (95% CI 71.3%–91.0%) in the inpatient setting, and a sensitivity of 76.7% (95% CI 59.1%–88.2%), specificity of 96.2% (95% CI 90.6%–98.5%), PPV of 85.2% (95% CI 67.5%–94.1%) in the outpatient setting. Primary inpatient discharge diagnoses had a sensitivity of 54.7% (95% CI 41.5%–67.3%), specificity of 99.6% (95% CI 97.7%–99.9%), and PPV of 96.7% (95% CI 83.3%–99.4%). CPT/HCPCS codes and anti-influenza medication claims were present for few outpatient encounters (sensitivity 3%–10%). Conclusions: In a large US healthcare system, inpatient ICD-10-CM codes for influenza, particularly primary inpatient diagnoses, had high predictive value for test-positive influenza. Outpatient ICD-10-CM codes were moderately predictive of test-positive influenza.