The present invention relates to isolated single-domain antibodies (sdAb) directed against Antithrombin (AT) to prolong the half-life of the proteins. Inventors have generated isolated single domain antibodies (sdAbs) directed against antithrombin. They observed that in amidolytic assays, sdAbs are incapable of blocking the inhibitory antithrombin activity towards thrombin and factor Xa in the presence of heparin. The different combinations of sdAb were able to block the inhibitory antithrombin activity towards thrombin and factor Xa in mice. Thus, the inventors propose to use different combinations of sdAb to block the inhibitory function of antithrombin in order to promote thrombin generation and thus treat haemophilia and other conditions that are associated with bleeding. Accordingly, the invention relates also to a method of preventing or treating bleeding disorders in a subject in need thereof, comprising administering to said subject a therapeutically effective amount of the single domain antibodies or the drug conjugate of the invention.