Developmental haemostasis is a concept, now universally accepted, introduced by Andrew et al. in the late 1980’s. However, coagulation analysers and reagents have changed significantly over the past 15 years. Coagulation testing is known to be sensitive to changes in individual reagents and analysers. We hypothesised that the reference ranges developed by Andrew et al. may not be appropriate for use in a modern coagulation laboratory. Our study was designed to determine whethera current day coagulation testing system (STA Compact analyser and Diagnostica Stago reagent system) was sensitive to agerelated changes in coagulation assays. This is the first large scale study since Andrew et al. to determine the age associated numerical changes in coagulation proteins. Our results confirm the concepts of developmental haemostasis elucidated by Andrew et al. However, our results clearly demonstrate that the absolute values of reference ranges for coagulation assays in neonates and children vary with analyser and reagent systems. The results confirm the need for coagulation laboratories to develop age-related reference ranges specific to their own testing systems. Without this, accurate diagnosis and management of neonates and children with suspected bleeding or clotting disorders is not possible. Finally we present age related reference ranges for D-dimers, TFPI, and endogenous thrombin potential, previously not described.