Background: TEG6S® and TEG5000® (Haemonetics Corp, USA) are haemostasis analysers that measure viscoelasticity properties of whole blood. Both use different mechanisms to assess similar components of the coagulation process. The aim of this study was to assess agreement and interchangeability between the TEG6S and TEG5000 analysers. Methods: 3.5 mL whole blood was collected from 25 adult patients in a tertiary intensive care unit (ICU). Analysis was performed using TEG6S and TEG5000 haemostatic platforms. Agreement between platforms was measured using Lin's concordance coefficient (Lin's CC), further validated using intraclass correlation coefficients and reduced major axis regression (RMAR). Results: Sixteen (64%) patients were male; mean (range) age: 59yo (23-86). TEG6S and TEG5000 systems were broadly interchangeable. The majority of TEG variables demonstrated almost perfect or substantial agreement and minimal proportional bias (maximum amplitude demonstrated a fixed bias). LY30%, however, demonstrated poor agreement and a proportional bias. Lin's CC coefficients (95% CI, RMAR slope, intercept) between TEG6S and TEG5000 variables were: R time: 0.78 (0.64-0.92, 0.76, 0.92); K time: 0.82 (0.69-0.94, 1.30, - 0.93); alpha angle: 0.79 (0.64-0.95, 1.04, - 1.43); maximum amplitude (MA): 0.90 (0.83-0.96, 0.99, - 5.0); LY30%: 0.34 (0.1-0.58, 0.43, 0.04). Conclusions: Adult patients with critical illness demonstrate almost perfect agreement in the R time and MA, substantial agreement in K time and alpha angle, but poor agreement in LY30%, as measured by the TEG6S and TEG5000 analysers. With the exception of LY30%, the TEG6S and TEG5000 platforms appear interchangeable. This has important implications for use in clinical practice and multi-site research programs. Trial registration: ANZCRT number: 12617000062325, registered 12/Jan17. Retrospectively registered.