The activated clotting time (ACT) was invented as a whole blood test to detect coagulopathy, but nowadays is almost exclusively used to guide heparin anticoagulation. Although the ACT provides a fairly reliable and fast bedside test of the coagulation status, only a few studies have focused on its use to monitor pre- or intraoperative coagulation status as an early marker of impaired haemostasis or increased bleeding tendency. The aim of this study was to compare intraoperative i-STAT(®) ACT values with commonly used thresholds of standard coagulation tests for the diagnosis of coagulopathy during paediatric non-cardiac surgery. We performed a prospective, observational study in a University Children's hospital and included 50 paediatric patients who underwent major elective, non-cardiac, surgery. The i-STAT(®) kaolin ACT test was obtained intraoperatively and compared to the commonly used threshold of standard coagulation tests (PT/INR, aPTT, and plasma fibrinogen level). A total of 181 blood samples were taken from 50 pediatric patients. Moderate correlation was found between ACT and aPTT (r = 0.694; p < 0.001), and all other coagulation tests. The median ACT values remained within the normal range throughout the entire surgical phase, while standard coagulation tests were mostly abnormal during surgery. Intraoperative measurement of ACT did not provide comparable thresholds of normal haemostasis as compared to standard coagulation testing.