Operational Heterogeneity of Capillary Refill Time Across Two Public ICU Databases: A Methodological Analysis

Background: Capillary refill time (CRT) is an increasingly endorsed bedside perfusion marker in septic shock.Publicly available intensive care unit (ICU) databases such as MIMIC-IV and eICU-CRD enable large-scale multi-database research, but the cross-database comparability of CRT operationalization has not been systematically examined. Methods: We conducted a retrospective, descriptive, cross-database methodological analysis of adult ICU stays in MIMIC-IV v3.1 (2008-2022) and eICU-CRD v2.0 (2014-2015). CRT availability, definitional thresholds, recording patterns and time windows, bilateral measurement consistency, and selection bias in the CRT-documented population were characterized descriptively. Results: Among 94,458 MIMIC-IV and 200,234 eICU-CRD adult ICU stays, CRT availability differed markedly:79.85% of MIMIC-IV stays had at least one CRT record within the first 24 hours, versus 6.35% of eICU-CRD stays across the entire ICU stay, a 73.5-percentage-point gap. The abnormality threshold was 3 seconds in MIMIC-IV and 2 seconds in eICU-CRD. Among CRT-available MIMIC-IV stays, the mean number of CRT records in the first 24 hours was 7.0 (median 6, interquartile range (IQR) 4-10); 99.6% had bilateral records, with 99.4% left-right agreement (Cohen's kappa 0.914). The temporal distribution of CRT records was strikingly similar between databases: only 28.7% of MIMIC-IV and 29.1% of eICU-CRD records occurred within 24 hours of ICU admission (including pre-admission records), with roughly 71% in both databases recorded later in the stay. In eICU-CRD, the CRT-documented population had higher illness severity than the undocumented population (Acute Physiology and Chronic Health Evaluation (APACHE) IV 60.0 versus 54.4; standardized mean difference 0.21), consistent with selection by indication. Structured peripheral perfusion index fields were unavailable in both databases. Conclusions: CRT operationalization differs fundamentally across MIMIC-IV and eICU-CRD in availability, definitional thresholds, recording patterns, bilateral structure, and patient selection. Cross-database CRT research should explicitly declare definitional heterogeneity, avoid pooling across thresholds, and treat sparsely documented subsets as selected populations. Selection bias was evident in both databases, in opposite directions. Severity-score-based comparison was possible only in eICU-CRD. Standardized common data elements for bedside perfusion assessment are needed to make cross-database CRT research valid by design.