The impact of the ICD coding transition on neurological diagnoses

The transition from the International Classification of Diseases (ICD) -9 coding system to the ICD-10 coding system has important implications for clinical research as it can significantly affect the prevalence of more common neurological diagnoses. There is evidence of a dramatic decrease in the prevalence of status epilepticus, while the prevalence of idiopathic intracranial hypertension, critical disease myopathy, intracerebral hemorrhage, ischemic stroke and Parkinson’s disease increased significantly due to the study result published in Neurology due to the coding transition.

The United States switched its coding system from the ICD-9 to the ICD-10 system on October 1, 2015. The aim of the current study was to examine the impact of the coding transition from ICD-9 to ICD-10 on the prevalence of 16 common neurological diagnoses by comparing diagnoses before (using the ICD-9 coding system) and after (using the ICD-10 coding system) the ICD coding transition.

Using data from the National Inpatient Sample, study researchers identified 16 common neurological diagnoses from January 2014 to September 2015 and October 2015 to December 2017. They then compared the prevalence of each diagnosis before and after the ICD coding transition.

With the exception of a small increase in the monthly rate of change associated with the coding transition for subarachnoid hemorrhage (from 4.32 to 24.32 diagnoses per month), the monthly rate of change in other neurological diagnosis frequencies remained unchanged over time between ICD-9 and ICD-9. 10 coding systems.

Significant changes in cross-sectional prevalence were seen in 6 of 16 neurological diagnoses, including a dramatic decrease in diagnoses of status epilepticus (odds ratio) [OR]0.30; 99.7% CI, 0.26-0.34) between ICD-9 and ICD-10. On the other hand, the coding transition was with an increase in the diagnoses of idiopathic intracranial hypertension (OR 1.2; 99.7% CI 1.05-1.37), myopathy in critical illness (OR 1.14; 99.7% CI 1 , 01-1,3). intracerebral hemorrhage (OR 1.08; 99.7% CI 1.03-1.14), ischemic stroke (OR 1.04; 99.7% CI 1.02-1.06) and Parkinson’s disease (OR 1 , 18; 99.7% CI 1.14-1.22).

The study had several limitations, including data limited to 2 years of full ICD-10 coding data, potential mundane trends unrelated to coding transition, no patient-level clinical data available, and the inability to improve the accuracy and diagnostic validity of diagnostic codes to rate .

“[O]Our data suggest that changes in administrative coding can systematically affect estimates of the prevalence of neurological disease, which has important implications for epidemiology, outcome research, and recruitment efforts for clinical trials based on health databases, ”concluded the study’s researchers.


Hamedani AG, Blank L, DP Thibault, Willis AW. Influence of the ICD-9 to ICD-10 coding transition on prevalence trends in neurology. Neurology. Published online January 29, 2021. doi: 10.1212 / CPJ.0000000000001046

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