Performance of Thromboelastography 6s Difference in Reaction Time Between Heparinase-Free and Heparinase-Containing Channel for Detecting Subtherapeutic Unfractionated Heparin Anticoagulation in a Pediatric Cardiac ICU

OBJECTIVES: In cases where anti-Xa activity is not routinely measurable, estimating heparin efficacy using the activated partial thromboplastin time (APTT) is challenging, and misestimation may risk bleeding or thrombotic complications. We aimed to assess the utility of the difference in reaction time between thromboelastography 6s heparinase-free and heparinase-containing channel, termed “ΔR,” as a potential diagnostic test for monitoring the heparin effect in a pediatric cardiac ICU (PCICU), using anti-Xa activity as a reference standard, and APTT (seconds and preoperative ratio; APTT at sampling divided by the preoperative value) as comparators. DESIGN: Single-center, prospective observational diagnostic performance study. SETTING: The PCICU of a national cardiovascular center in Japan. PATIENTS: Consecutive patients younger than 18 years old who received unfractionated heparin in the PCICU between July 5, 2024, and May 31, 2025. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: The ΔR, anti-Xa activity, and APTT (seconds and preoperative ratio) were measured for each patient on concurrent samples. Among 59 patients, the ΔR correlated moderately with anti-Xa (r = 0.67; p < 0.001), whereas correlations between anti-Xa activity and APTT were weak (seconds: r = 0.06; p = 0.65 and ratio: r = 0.24; p = 0.066). For the primary target condition (anti-Xa < 0.1 international units/mL), ΔR achieved an area under the curve of 0.934 (95% CI, 0.841–0.996) with an optimal cutoff of 5.6 min (95% CI, 4.35–6.35), yielding 96% sensitivity and 83% specificity, thus outperforming APTT (seconds and ratio; p < 0.001). CONCLUSIONS: ΔR showed a moderate correlation with anti-Xa and high discriminative performance in identifying subtherapeutic anticoagulation, thus supporting its use as a surrogate marker for heparin monitoring in PCICUs. However, further external validation and outcome-based studies are needed to confirm these findings.