The Incidence of Anaphylaxis in a Large Health Maintenance Organization: A Review of International Classification of Diseases Coding and Epinephrine Auto-Injector Prescribing

Rationale: Accurate estimates of the incidence of anaphylaxis are limited. Current ICD-9 codes make it difficult to accurately diagnose anaphylaxis and to assess appropriate epinephrine prescribing. The purpose of this study is to quantify the incidence and demographic character of patients with anaphylaxis-related ICD-9 codes in a large health maintenance organization, and analyze epinephrine prescribing and dispensing rates. 

Methods: All patients included had at least 12 months of continuous membership over a 4-year period from January 1st, 2008 to December 31st, 2012 and were selected based on anaphylaxis-related ICD-9 codes (n=159,172). This algorithm was extrapolated from a previous study that used expanded ICD-9 codes to identify more cases of anaphylaxis. Individual chart reviews found that many expanded ICD-9 codes represented unconfirmed cases of anaphylaxis, and therefore were excluded, resulting in analysis of 52,405 patients. 

Results:  Incidence of anaphylaxis over 4 years was 2.07%, with female predominance (56.5%) over males (43.5%). Epinephrine was prescribed in 16.2% of total cases. Highest rates of epinephrine prescription were for traditional ICD-9 codes 995.0 (other anaphylactic shock) and 995.60-995.69 (anaphylactic shock caused by food) at 49.3% and 58.6%, respectively. Of the cases where an epinephrine auto-injector was prescribed, it was dispensed 95.9% of the time, independent of copayment amount.

Conclusions: Low epinephrine auto-injector prescribing rates in cases of anaphylaxis suggests the continued difficulty in the diagnosis of anaphylaxis and may result in suboptimal treatment of potential future episodes. Additionally, these data suggest that the expanded algorithm of anaphylaxis-related ICD-9 codes, as reported previously, may overestimate true cases of anaphylaxis.