Paediatric patients who require anticoagulation with therapeutic doses of low-molecular weight heparin are at risk of having a residual anticoagulant effect at the time of surgery, even if managed according to current peri-operative guidelines. Testing for residual effect is not currently recommended in such circumstances. A 15-year-old child with a mechanical aortic valve replacement requiring long-term warfarin treatment, as well as underlying coagulation defects, was administered low-molecular weight heparin for bridging anticoagulation before kyphoscoliosis surgery. Thromboelastography was used intra-operatively to diagnose residual heparinisation, which was demonstrated by a prolonged reaction (R) time of 16.0 min in the plain cup, compared with 9.2 min in the heparinase cup. Subsequently, thromboelastography was also used to monitor haemostatic therapy, which consisted of protamine 2−1 and 500 IU cryoprecipitate. Thromboelastography was used intra-operatively to allow rapid testing of coagulation status and guide therapy, thereby minimising use of blood products and reducing complications.