Interpretive comments: Dosing protocols for Arixtra (fondaparinux) have been validated by empiric clinical trials. The relationship between pharmacokinetic parameters and either efficacy or safety (bleeding) is not well defined. Peak levels are generally drawn 3-4 hours after drug administration. Fondaparinux plasma concentrations have been reported to range from 0.1 to 0.5 mg/L for adult patients on thromboprophylaxis and from 0.6 to 1.5 mg/L for adults on therapeutic doses (Depasse F, et al, J Thromb Haemost 2004; 2:346-8). A therapeutic level of 0.5-1.0 mg/L has been suggested in a pediatric study (Young G, et al. Pediatric Blood Cancer 2011; 57:1049-54).