Fondaparinux is a synthetic antithrombotic agent with specific anti-factor Xa activity. A population pharmacokinetic model of fondaparinux, based on data obtained in patients included in phase II/III trials, has been described. However, the validity of this model in everyday practice needed to be confirmed. This study was a multicenter, prospective cohort study in consecutive orthopedic patients treated with 2.5 mg of fondaparinux. Anti-Xa activities were recorded in 809 patients. Population parameters and inter-individual variability were estimated using NONMEM VI software. A two-compartment model with first-order absorption best described fondaparinux pharmacokinetics. Covariates partly explaining inter-individual variability were body weight, age and creatinine clearance estimated by the simplified Modification of Diet in Renal Disease formula. A body weight less than 50 kg and moderate renal failure increased drug exposure. These results suggest that fondaparinux pharmacokinetics need to be confirmed in patient populations not fully assessed in phase II/III studies

Inclusion Criteria:

• patient who need leg major orthopedic surgery
• age > 18 years
• patient who need prophylactic doses of fondaparinux

Exclusion Criteria:

• contra-indication to fondaparinux
• renal insufficiency

patients with major orthopedic surgery who need prophylactic doses of fondaprinux