APRI (AST-to-Platelet Ratio Index)

APRI was derived by Wai and colleagues in 2003 as a bedside alternative to biopsy in chronic hepatitis C and requires only AST and a platelet count. The local laboratory upper limit of normal for AST is required because assays differ (commonly about 35-40 U/L). Interpretation bands follow the original paper and WHO usage: below 0.5 significant fibrosis (METAVIR F2-F4) is unlikely; 0.5 to below 1.5 is indeterminate; 1.5 or more suggests significant fibrosis (specificity about 98% in HCV cohorts). For cirrhosis, 1.0 is the sensitive WHO low cutoff and 2.0 or more indicates likely cirrhosis (specificity about 96%). No single cutoff set has universal consensus, so the score is reported with both labeled bands.