A stable prevalence hides case turnover: reclassifying depression from ICD-10 to ICD-11 in a Danish general-population sample.

BACKGROUND: ICD-11 revised the diagnostic criteria for a depressive episode by adding hopelessness as a symptom, moving low energy from core to accompanying, merging low self-confidence with guilt, and raising the symptom threshold. The Major Depression Inventory (MDI) was built to screen for depression using ICD-10 and has no hopelessness item. We asked how the switch to ICD-11 would change who counts as a case. METHODS: We scored ICD-10, ICD-11 and DSM diagnoses in a normative Danish population survey conducted in 2000, in which MDI and SCL-92 were administered to adults aged 18-80 (53% female, using established MDI algorithms and a co-administered hopelessness item (SCL-92 item 54) for ICD-11. RESULTS: Among the 1130 respondents, the overall prevalence was identical: 43 cases, i.e., 3.81% [95% CI 2.84; 5.09] under both ICD-10 and ICD-11. Beneath that identical rate, cases churned: only 33 of the 43 cases overlapped between criteria; 10 dropped out, and 10 were new. Dropped cases fell out because ICD-11 raised the symptom threshold, while gained cases entered through the relaxed core-symptom rule and the new hopelessness symptom. Hopelessness drove 7 of 10 gains and 0 of 10 drops. DSM prevalence was lower (3.27% [2.38; 4.48]) but agreed almost perfectly with ICD-11 (kappa = 0.92), the gap reflects hopelessness counting independently only in ICD-11. CONCLUSIONS: An identical headline rate from ICD-10 to ICD-11 can conceal a substantial change in case membership. Studies that compare depression prevalence across ICD versions should examine case-level agreement, not just the aggregate rate.