Background & Aims Non-alcoholic fatty liver disease (NAFLD) is a common liver disorder with non-alcoholic steatohepatitis (NASH) being a more progressive phenotype associated with progression to cirrhosis. Type III collagen is a main component of the fibrotic extracellular matrix. PRO-C3 is a biomarker for detectison of moderate/severe fibrosis and the test is further improved when incorporated into the ADAPT algorithm. Here, we validated PRO-C3 and ADAPT within the CENTAUR screening population. Methods PRO-C3 was assessed in plasma from the screening population of the phase IIb CENTAUR study (NCT02217475) in adults with NASH and liver fibrosis. The relation between PRO-C3 and histologic features of NASH was evaluated, as well as the demographics of patients with high and low levels of PRO-C3. The diagnostic ability of PRO-C3 as a stand-alone marker or incorporated into ADAPT to identify patients with F≥2 and NASH was estimated using ROC analysis and logistic regression models. Results 517 subjects with matched biopsy and PRO-C3 test were included. Patients with PRO-C3 levels ≥20.2 ng/mL showed increased levels of insulin, HOMA-IR, ALT, AST, alkaline phosphatase, and platelet count compared to patients with low PRO-C3 (p<0.05). PRO-C3 increased stepwise with increasing liver fibrosis, lobular inflammation, hepatocyte ballooning, steatosis, and NAS (p<0.05), and could separate NAFL from NASH (p<0.0001). PRO-C3 was independently associated with fibrosis and NASH when adjusted for clinical confounders. ADAPT outperformed FIB4, APRI, and AST/ALT ratio as predictor of advanced fibrosis and NASH (p<0.001). Conclusion PRO-C3 was associated with NAS and fibrosis. ADAPT outperformed other non-invasive scores for detecting NASH. These data support the use of PRO-C3 and ADAPT as diagnostic tools to identify patients with NASH eligible for inclusion in clinical trials.