Background It is not clear how well administrative data identify incident disease in complex chronic disorders like Systemic Lupus Erythematosus (SLE). We aimed to clarify accuracy of ICD-10-coding in incident SLE by comparing incidence-rates from code-based case-definitions and confirmed SLE diagnosis by expert clinical assessment in a defined population.
Methods From administrative data, we identified all individual cases registered with a SLE-specific ICD-10 code (M32) during 1999–2017 in three Southeast Norway counties(2.1 million). All cases were manually chart-reviewed to confirm SLE diagnosis. To prevent against admixture of prevalent cases, we defined incident by presence of M32 in 2004–2017, but not in 1999–2003. Incidence-rates were estimated from five case-definition; (a-c) first occurrence of one-, two- and three or more M32-codes 2004–2017, (d) SLE diagnosis confirmed by chart-review and (e) SLE classified by 1997 ACR classification criteria. To define accuracy, we applied incidence-rate ratios obtained from dividing M32-derived incidence-rates to those from SLE diagnosis.
Results Of 1975 unique cases registered with a M32-code 1999–2017, chart-review confirmed SLE diagnosis in 936 cases (45%), while 1033 (52%) had conditions other than SLE.
Of 936 cases with confirmed SLE diagnosis, 323 (34%) were incident 2004–2017 (table 1). (figure 1a-c) shows the incidence-curves by different SLE case-definitions. Overall, the incidence-rate ratio from two or more M32 code divided by SLE diagnosis was 2.1 (95% confidence interval 1.8–2.4). Accuracy of ICD-coding was low for incident SLE across all ages, except in those under 25 years were the incidence-rate ratio was 1.0 (95% confidence interval 0.8–1.4) (figure 1d).
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