Should insulin billing procedure codes be considered for exposure ascertainment in pharmacoepidemiology studies?

Abstract

Objectives: The purpose of the study was to examine whether Healthcare Common Procedure Coding System (HCPCS) billing codes should be used in conjunction with National Drug Codes (NDCs) to establish insulin exposure episodes. Materials and Methods: We identified insulin claims billed by NDCs or HCPCS codes in FDA’s Sentinel System from 2013 to 2018. We created insulin exposure episodes separately based on NDCs only, HCPCS only, and a combination of both NDC and HCPCS. We considered gaps of <30 days between valid billing codes as continuous exposure. Patients were followed until the earliest of (1) episode end date, (2) death, (3) disenrollment, (4) query end date, and (5) evidence of the opposite exposure defining code type (for cohorts defined by only NDCs or only HCPCS). We examined the median duration of incident episodes, requiring no NDC or HCPCS codes in the 183 days (washout period) before the first billing code and prevalent episodes (no washout period required). For patients with more than 1 treatment episode, we calculated median gap length between episodes. Results: We identified 107,528,855 insulin claims using NDCs or HCPCS. Of these, 98.5% were billed using NDCs. HCPCS were largely billed during emergency and ambulatory visits (52.5% and 38%, respectively). We identified 6,350,872 incident and 12,922,593 prevalent NDC episodes; and 6,821,075 incident and 13,465,108 prevalent NDC-HCPCS episodes. The median length of the first incident NDC. NDC episodes was 110 days (IQR: 60; 212); 31 (IQR: 19; 31) days for HCPCS only and 90 (IQR: 19-31) for NDC-HCPCS episodes. The median gap between the first and second episodes was shorter for incident NDC episodes than HCPCS episodes (NDC: 49 [IQR: 17; 132]; HCPCS: 249 [IQR: 93; 550]). Prevalent episodes showed similar trends. Conclusion: HCPCS insulin codes appeared to indicate either 1 time or sporadic occurrences with long gaps between two codes. HCPCS codes in conjunction with NDC codes increased the number but reduced the median length of insulin episodes. Unless studies investigate the effects of insulin administered in specific settings to identify transient adverse reactions treated with insulin, we do not encourage the use of HCPCS to establish insulin exposure episodes.