The feasibility of implementing this intervention may also be affected by the following barriers reported by observational studies: (1) anti factor Xa tests are not widely available97 ; (2) anti factor Xa tests are poorly standardized between laboratories, making proper LMWH dose adjustments impossible in some cases97 ; and (3) anti factor Xa test reproducibility is poor.98-100 Conclusions and research needs for this recommendation. The guideline panel considered the net benefit associated with adjusting LMWH doses based on the results of anti factor Xa monitoring very uncertain. Because of concerns relating to anti factor Xa test standardization and reproducibility and weak correlation between bleeding events and anti factor Xa concentrations,101 the panel suggests against adjusting LMWH doses based on anti factor Xa concentration monitoring. Seven panel members preferred making a strong recommendation against the intervention, but this majority was not sufficiently large to satisfy the criterion for a strong recommendation (80% of the panel). The panel identified the following additional research questions. (1) What are the anti factor Xa concentration cutoffs (determined in a manner that ensures accuracy and reproducibility) that correlate with risk of recurrent VTE and bleeding events? (2) What percentage change in LMWH dose in response to an out-of- range anti factor Xa concentration is optimal to return the concentration to the therapeutic range? (3) What is the compar- ative effectiveness of adjusting LMWH doses based on the results of anti factor Xa concentrations (performed in a manner that ensures accuracy and reproducibility) vs no such monitoring for patients with estimated creatinine clearance values of ,30 mL/min requiring treatment of VTE? 