Comparison of Heparin Anti-Xa Activity From Central Venous Catheter Samples Using a 5 mL Syringe Flush or a Vacuum Tube Flush Versus Peripheral Vein Samples in ICU Patients

Unfractionated heparin (UFH) is widely used in intensive care units (ICUs) and requires close monitoring, most commonly through the measurement of anti-Xa activity. The reference method for anti-Xa monitoring involves blood sampling by peripheral vascular puncture. However, in ICU patients, peripheral access may be challenging or painful, and central venous catheters are often available and already used for UFH infusion. Sampling from these central lines could be a convenient alternative, but residual heparin in the catheter may contaminate the sample, leading to falsely elevated anti-Xa results. The CASSANDRA study (Central catheter Anti-Xa Sampling Study for Accurate aNalysis and Reliable Dosage Assessment) is a prospective, monocentric, comparative study designed to assess whether anti-Xa activity levels obtained from central venous catheter samples are equivalent to those from peripheral vein samples. To be eligible for inclusion, patients must already have a central venous catheter in place and require continuous intravenous unfractionated heparin administration through the distal lumen of the central venous catheter. A three-way stopcock will be placed upstream of the infusion tubing to allow temporary interruption of UFH infusion during sampling. Two catheter flushing techniques will be compared: * Method A: A 5 mL flush performed with a syringe over 5 seconds. * Method B: A 5 mL flush using a vacuum tube (standard discard tube). For each patient, four pairs of simultaneous blood samples will be collected, according to a randomized sequence alternating between methods A and B. In each pair, one sample will be drawn from the central venous catheter and the other from a fresh peripheral vein puncture (reference). All anti-Xa assays will be performed, but only the results from peripheral vein samples (reference method) will be made available to clinicians. Anti-Xa results from central venous catheter samples will remain blinded to the clinical team to avoid influencing patient management. The primary outcome is the absolute difference in anti-Xa activity between central venous catheter samples (either method) and peripheral samples. Equivalence is defined as a mean difference not exceeding 0.05 IU/mL. Secondary outcomes include Bland-Altman agreement limits between each central method and the peripheral reference. Results of this study may support the safe use of central venous sampling for routine anti-Xa monitoring in ICU patients, provided an appropriate flushing method is used.