Internal deterministic record linkage using indirect identifiers for matching of same-patient hospital transfers and early readmissions after acute coronary syndrome in a nationwide hospital discharge database: A retrospective observational validation study

Objectives: To assess validity of record linkage using multiple indirect personal identifiers to identify same-patient hospitalisations and definition of episode of care (EC) due to acute coronary syndrome (ACS). Methods: Using national hospital discharge data to identify all admissions due to ACS, we used six different linkage rules using indirect identifiers with increasing level of detail and compared validity against a pseudonymised unique identifier used as gold standard (GS). Contiguous hospitalisations within each matched group of hospitalizations occurring within 28 days of each other were considered one EC. We classified hospitalisations according to time between the first pair of hospitalisations as hospital transfer (HT: ≤1 day), early readmission (ER: 2-28 days) or recurrent cases (>28 days). Results: There were 146 671 hospitalisations (unlinked), 121 987 ACS 28-day EC (linked GS), with 18 398 HTs (≤1 day), and 6286 ERs (≤28 days). Linkage rules using demographic and residence code variables produced linkage rates with highest validity for rule using sex, date of birth and four-digit residence code with sensitivity of 98.4 (95% CI: 98.4 to 98.5); specificity of 97.8 (95% CI: 97.6 to 98.0) and Cohen's κ of 0.9 to detect ACS-EC, compared with GS linkage rule. Similarly, validity for HT and ER was high and of similar magnitude, with sensitivity ranging between 97.2% and 98.1%, and specificity between 98.8% and 99.9%, respectively. Conclusions: Our internal linkage validation study using indirect patient identifiers will allow calibration of incidence rates and performance indicators, accounting for the effect of HT and readmissions.