One limitation of LMWHs is that they are predominately cleared by the kidneys so their biologic half-life may be pro- longed in patients with renal failure.127,128 1.2.3 Monitoring Antithrombotic Effect: LMWHs are typically administered in xed or weight-adjusted doses for thromboprophylaxis and in weight-adjusted doses for therapeutic purposes. Coagulation moni- toring is not generally necessary, but some authorities suggest that monitoring be done in obese patients and in those with renal insuf ciency.129-131 Monitoring may also be advisable when treatment doses of LMWH are given during pregnancy.132 Although some studies reported that high anti-Xa levels were associated with an increased bleeding risk,55,133 several other studies failed to show a rela- tionship between anti-Xa levels and bleeding.134-136 A randomized controlled trial comparing monitored and unmonitored dalteparin therapy for treatment of VTE showed no bene t of monitoring.137 Monitoring is thus not recommended for the majority of patients; if monitoring is required, the anti-Xa level is the rec- For treatment of VTE, a conservative peak anti-Xa level with twice-daily enoxaparin or nadroparin is 0.6 to 1.0 units/mL.132,138-140 The target range for peak anti-Xa levels (measured 4 h after dosing) with once- daily enoxaparin is likely to be above 1.0 units/mL,138 whereas it is 0.85 units/mL with tinzaparin and 1.3 units/mL and 1.05 units/mL with nadroparin and dalteparin, respectively.140 The suggested peak target ranges for several LMWHs are shown in Table 6. 1.2.4 Dosing and Monitoring in Special Situations: With enoxaparin, anti-Xa activity is increased to appropriate levels when the drug is administered to obese patients in doses based on total body weight up to a weight of 144 kg.141 The same is true for dalteparin142,143 and tinzaparin144 in patients weighing up to 190 and 165 kg, respectively. 