Evaluation of clot formation on coronary stents by in vitro flow system: Use of d-dimer assay

Background: Thrombosis is a major complication following stent implantation into coronary vessels. The development of anticoagulant coated stents, brought up the need for compared studies using a well controlled system. We have established a flow system by which citrate anticoagulated blood is perfused through a stent inserted into siliconized tube. Methods: Platelet adhesion on the stents was determined by acid phosphatase assay and by the chromium 51 labeling method. Clot formation on the stents was evaluated by the incubation of the stent in PBS enriched with tissue plasminogen activator (tPA, 50,000 U/ml) followed by the determination of D-dimer in the supernatant. Soluble fibrin in the circulated blood was determined by D-dimer assay after incubation of samples from the perfused blood with t-PA. Results: Clot formation on stents as measured by the D-dimer levels in supernatants was correlated with the addition of low CaC12 concentrations into the perfused blood. When CaCI2 at the concentration of 2mM was added, D-dimer level in the perfused blood was not increased. However, increased clot formation on the stems was observed which could be inhibited by the addition of 0.25 U/ml heparin. Platelet adhesion to the stent was not affected by the addition of low concentration of CaCI2 (l-3mM) and was only slightly reduced by the addition of 0.25 U/ml heparin. Different flow velocities were compared and slightly increased D-dimer generation was observed on the stents and in the perfused blood at flow of lOOml/min as compared to 50 and 10 ml/mm. Conclusions: We suggest that the evaluation of clot formation on grafts by D-dimer assay has potential advantage over the established platelet adhesion determination.