Evaluating the Concordance between ICD-10 and Stroke Severity as Measured by the NIHSS

Background: The National Institutes of Health Stroke Scale (NIHSS) scores have been used to evaluate Acute Ischemic Stroke (AIS) severity in clinical settings. Through the International Classification of Diseases, Tenth Revision Code (ICD-10), documentation of NIHSS scores has been made possible for administrative purposes and has since been increasingly adopted in insurance claims. Per CMS guidelines, the stroke ICD-10 diagnosis code must be documented by the treating physician, but ICD-10 NIHSS scores can be documented by any healthcare provider involved in the patient's care. Accuracy of the administratively collected NIHSS compared to expert clinical evaluation as documented in the Paul Coverdell registry is however still uncertain. Methods: Leveraging a linked dataset comprised of the Paul Coverdell National Acute Stroke Program (PCNASP) clinical registry and probabilistically matched individuals on Medicare Claims data, we sampled patients aged 65 and above admitted for AIS across nine states, from 2016 to 2019. We excluded those lacking documentation for either clinical or ICD-10 based NIHSS scores. We then examined score concordance from both databases and measured discordance as the absolute difference between the PCNASP and ICD-10-based NIHSS scores. Results: Among 66,837 matched patients, mean NIHSS scores for PCNASP and Medicare ICD-10 were 7.26 (95% CI: 7.20 - 7.32) and 7.40 (95% CI: 7.34 - 7.46), respectively. Concordance between the two scores was high as indicated by an intraclass correlation coefficient of 0.93. Conclusion: The high concordance between clinical and ICD-10 NIHSS scores highlights the latter's potential as measure of stroke severity derived from structured claims data.