Patients with multiple myeloma (MM) are at high risk of venous thromboembolism (VTE) and these patients require adequate thromboprophylaxis. Following the publication of the AVERT clinical study, Apixaban is strongly recommended as a prophylactic treatment option for patients with cancer, based on high quality of evidence and a favorable efficacy/safety profile. A multicenter and ancillary study - APIXABOR - that measured the plasmatic concentration of Apixaban in patients with MM and treated with preventive dose has been conducted. The peak drug concentration was superior in MM plasma, as compared to non-myeloma patients under prophylaxis. Therefore, the present study evaluated whether differences in pharmacokinetics have an impact on pharmacodynamics (i.e. decrease in coagulability in MM patients as compared to non-MM patients undergoing a surgery for total knee replacement who also have VTE prophylaxis). In fine, this study may inform to better manage thromboprophylaxis in MM patients.

Inclusion Criteria:

• Age ≥ 18 years old,
• Signed informed consent,
• Patient covered by a social security scheme.
• Group 1 : patient with a diagnosis of de novo multiple myeloma, with an indication of thromboprophylaxis with Apixaban,
• Group 2 : patient requiring a surgery for total knee replacement, , with an indication of thromboprophylaxis with Apixaban.

Exclusion Criteria:

• Curative doses of anticoagulation treatment,
• Contra-indication to Apixaban,
• Pregnant or breastfeeding woman,
• Refusal to sign consent,
• Patient under legal protection.