Point of care INR monitoring in cyanotic congenital heart disease-related compensatory erythrocytosis.

Introduction: People with cyanotic congenital heart disease (CHD) frequently require anticoagulation with warfarin. Point of care (POC) devices offer significant benefits such as patient-managed warfarin and rapid testing, however, there is limited experience with the use of POC in adult patients with CHD and high haematocrit. Methods: This exploratory prospective cohort study recruited people between September 2021 and December 2023. Inclusion criteria were cyanotic CHD and oxygen saturations <93% and a Hct ≥0.51. Exclusion criteria were pregnancy, antiplatelet agents, recent thromboembolic event or major medical illness. A Coaguchek XS (Roche Diagnostics) was used. Results: Five individuals were recruited to the warfarin group (Hct 0.54, 0.52-0.58, IQR 0.04) and five to the control group (Hct 0.55, 0.51-0.60, IQR 0.07). Difference in INR between methods was 0.1 (-0.3-0.2, IQR 0.2) in the warfarin and -0.1 (-0.3 - 0.1, IQR 0.2) in the cyanosis-control arms. Bland Altman analysis demonstrated no proportional bias. No eligible participants had Hct ≥0.60. One potential participant had Hct of 0.73 but POC testing was unrecordable. Correlation between POC and laboratory INR was r = 0.98, p < 0.01 (95 % CI 0.98-1.22) for the whole cohort, r = 1.0, p < 0.01 (95 % CI 0.75-1.50) in the warfarin group and r = 0.87, p = 0.06 (95 %CI -0.23-3.10) in the control group. Conclusion: POC INR testing may be an accurate alternative for individuals with compensatory erythrocytosis and Hct up to 0.60.