Underestimation of unfractionated heparin therapy assessment due to platelet activation when using partial-draw (pediatric) citrate collection tubes

The “so-called” pediatric tubes are often used when collecting smaller blood volume is necessary, particularly in pediatric patients or in case of difficult/recurrent sampling. The aim of this multicenter study was to compare coagulation test results evaluated in evacuated polymer tubes containing 0.109 M citrate (1 vol./9 vol.) specifically designed to allow either a partial (2.0 mL,“pediatric”) or a total (3.5 mL) filling. No significantly relevant discrepancy was found between routine coagulation test results in both tubes collected from untreated patients and from patients on vitamin K antagonist or low molecular weight heparin. In contrast, aPTT was significantly shorter and anti-FXa activity was significantly lower in partial-draw than in full-draw tubes collected from 46 patients receiving unfractionated heparin (UFH). This discrepancy was likely related to increased platelet activation in partial-draw tubes, as suggested by higher platelet factor 4 plasma concentrations and platelet P-Selectin expression in partial-draw than in full-draw citrate tubes. To confirm this hypothesis, we then evaluated partial-draw tubes containing CTAD, a mixture of anticoagulant and antiplatelet agents. In 25 patients on UFH, aPTT and anti-FXa activity were not significantly different in partial-draw CTAD tubes and in full-draw citrate tubes. In conclusion, despite increased platelet activation, samples collected into partial-draw citrate tubes allow accurate routine coagulation testing in all patients but those requiring UFH assessment, in which their use could lead to significant underestimation of anticoagulation. In such cases, partial-draw tubes containing CTAD could be validly used to monitor heparin therapy as well as to perform routine coagulation testing.