Identifying forensic psychiatric populations in national health registers: a Danish validation study

The identification of patients in forensic psychiatric care within national health registries has historically posed significant challenges, limiting research to small-scale studies and restricting its scope. This study aims to evaluate the utility of the Danish National Patient Register (DNPR) for research purposes by assessing the criterion concurrent validity of administrative trajectory markers for identifying incident patients receiving forensic psychiatric care.

We employed a population-wide design, analyzing data from the entire Danish population aged 15 years and older (almost 5 million individuals). Incident forensic psychiatric patients between January and December, 2022, were identified through a trajectory start marker in the DNPR. Validity was assessed by comparing these cases to confirmed cases from the Central Criminal Register (CCR). Test statistics, including sensitivity, specificity, positive predictive value (PPV), and negative predictive value, were calculated.

A total of 323 forensic psychiatric patients were identified in the DNPR, of whom 289 (89.5%) were confirmed as incident cases in the CCR. The sensitivity of any trajectory start marker was .755 (.708-.797), and the PPV was .895 (.856, .926). PPV varied across the five Danish regions and between individual trajectory markers, with the highest PPV (.950) observed among patients sentenced to outpatient psychiatric treatment with the option of hospitalization.

The findings suggest that trajectory markers are a promising approach for reliably identifying incident forensic psychiatric cases within Danish national health registers. Further studies are needed to confirm their validity across different contexts, informing clinical practice and policy decisions in forensic psychiatry.