Decoding the Poison Mystery - Linking TESS and ICD-9-CM Codes

Poisonings are a growing public health problem, and are difficult to relate in separate data sources. To code cases of poisoning, Poison Centers use Toxic Exposure Surveillance System (TESS) codes, whereas public health professionals use International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9-CM) codes. Currently, there is no standard translation between these two coding systems. Previous work on a one-way poison crosswalk between TESS substance and ICD-9-CM condition codes has been done, but never validated.

Data from the Northern New England Poison Center (NNEPC) for July 2004-December 2004 were linked to New Hampshire Inpatient Hospital (IP) and Emergency Department Hospital (ED) data using probabilistic linkage techniques. For each related poisoning case, NNEPC assigned TESS codes were compared to hospital assigned ICD-9-CM codes.

The crosswalk was 50% effective. Of the 93 NNEPC poisoning cases managed at one hospital, 84 (90%) were linked to IP or ED cases. Of these 84 linked cases, 42 matched the one-way crosswalk used. Typically hospital data for linked cases that did not match included the condition (e.g., foreign object) not the poison code (e.g., paint).

This method brings us closer to the Institute of Medicine recommendation of establishing a common poison definition across data sources. However, this one-way crosswalk missed 50% of poison cases with linked data. Therefore, it is necessary to do further analysis to understand why cases are missed, which will lead to the development of a standard crosswalk for better poisoning surveillance.