Freedom SOLO-Associated Thrombocytopaenia is Valve-Dependent and Not Due to In Vitro Pseudothrombocytopaenia.

Background: Use of the Freedom SOLO (FS) stentless aortic bioprosthesis is limited by a unique and as yet unexplained severe decrease in postoperative platelet count in the absence of FS-related excess bleeding or thromboembolism. We investigated whether anticoagulant-associated pseudothrombocytopaenia could explain this complication.

Methods: Thirty consecutive patients (mean age 75.4±7.7 years, 11 [36.7%] female) underwent elective aortic valve replacement (AVR) with either the stented bovine Mitroflow (MF, n=18) or the stentless bovine FS (n=12) aortic valve bioprostheses. Serial platelet counts were performed simultaneously with sampling tubes containing tripotassium (K-)-EDTA, trisodium (Na)-citrate, or novel alternative magnesium sulfate (MgSO, ThromboExact™)-based anticoagulant, respectively.

Results: Postoperative platelet counts decreased compared with preoperative values in all patients (p<0.001), but were significantly lower in patients receiving FS compared to MF at all measurement time points until the end of observation (day 9). Lowest platelet counts were seen on the first postoperative day for MF (mean reduction: -41.5%) and on the second postoperative day for FS (mean reduction: -59.9%). Postoperative platelet counts did not correlate with any of the anticoagulants, thereby indicating no pseudothrombocytopaenia in the study population. There was no interaction between anticoagulant and type of valve. Only 1% of variance in platelet counts was caused by the anticoagulant, 46% by the day of measurement relative to baseline, and 20% was caused by the type of valve.

Conclusions: The platelet-lowering effect in patients receiving the FS is valve-dependent and is not caused by systemic preanalytical (laboratory) measurement error such as anticoagulant-dependent pseudothrombocytaemia, particularly with EDTA and citrate.