OBJECTIVES : Acute hepatic porphyrias (AHPs) are rare genetic diseases consisting of acute and chronic manifestations caused by mutations in heme synthesis enzymes. Prior to implementation of ICD-10-CM, identifying AHP patients was limited due to ICD-9-CM having only one non-specific porphyria code. This study developed an algorithm to identify AHP patients in a claims database. METHODS : Truven Health MarketScan Research Databases were used to identify a definitive AHP population using ICD-10-CM E80.21 (acute intermittent [hepatic] porphyria) between 10/1/2015 – 12/31/2015 and possible AHP patients between 1/1/2010 – 09/30/2015. A four-pronged approach to develop an algorithm combined non-specific AHP claims, common symptoms, diagnostic testing, and hemin (only approved treatment) to identify patients prior to 10/1/2015. The algorithm was tested by comparing the 6-month emergency room (ER) visit frequency prior to diagnosis among the definitive AHP population with the non-specific population given the high frequency of ER visits in this population due to acute attacks. RESULTS : CONCLUSIONS : Identifying rare diseases using claims is challenging with non-specific diagnosis codes. This is the first claims-based algorithm to identify AHP patients using ICD-9-CM in combination with symptoms and urinalysis.