Ans: D i.e. anti-factor Xa activity Laboratory monitoring of anticoagulant LMW heparin selectively inhibit factor Xa with little effect on Ha. So LMW heparins have smaller effect on aPTT and whole blood clotting time. Since aPTT/clotting times are not prolonged, laboratory monitoring is not needed LMWH therapy is monitored by the anti-factor Xa assay, measuring anti-factor Xa activity.