Accuracy of International Classification of Diseases Codes for Identifying Acute Optic Neuritis

The accuracy of International Classification of Diseases (ICD) codes for identifying cases of acute optic neuritis (aON) is not known. A prior study reported 61% accuracy for ICD code plus MRI consistent with aON within 2 months. This study determined accuracy for ICD code plus MRI within 2 months regardless of results. Retrospective chart review was conducted using a medical record research repository of a tertiary care institution from 1998 to 2019. Subjects with ICD-9/10 codes for ON and an MRI brain and/or orbits within 2 months of earliest (initial) ICD code were included. MRI was classified as positive or negative for aON based on report noting gadolinium-contrast enhancement. Clinical diagnosis at the time of initial code was classified as aON, prior ON, considered ON, alternative diagnosis, or unknown based on review of physician authored clinical notes within 7 days of the initial code. Accuracy of ICD code for aON, acute or prior ON, and acute, prior, or considered ON were calculated for all subjects and stratified based on MRI result. Two hundred fifty-one subjects had MRI results within 2 months of their initial ON ICD code (49 positive MRI [previously reported]; 202 negative MRI). Among those with negative MRI, 32 (16%) had aON, 40 (20%) had prior ON, 19 (9%) considered ON as a diagnosis, 92 (46%) had other confirmed diagnoses, and 19 (9%) had unknown diagnosis at time of code. Considering all subjects, accuracy for ICD code was 25% for acute ON, 41% for acute or prior ON, and 48% for acute, prior, or considered ON. Positive MRI, increased number of ON ICD codes, a code given by an ophthalmologist or neurologist within 2 months, and the presence of a neurology encounter within 2 months were associated with an increased accuracy for clinical aON diagnosis. In the setting of an MRI within 2 months, ICD codes for ON have low accuracy for acute ON and only slightly better accuracy for acute or prior ON. Accuracy is higher for cases with a positive MRI than those with a negative MRI, suggesting positive MRI in conjunction with ICD codes may help more accurately identify cases. Reliance on ICD and Current Procedural Terminology codes alone to identify aON cases may introduce substantial misclassification bias in claims-based research.