FIB-4 (Fibrosis-4) Index

FIB-4 = (age in years x AST in U/L) / (platelet count in 10^9/L x square root of ALT in U/L). Two cutoff conventions are in simultaneous use and are both reported, explicitly labeled. The original derivation (Sterling 2006, HIV/HCV coinfection; also the WHO-referenced pair) uses 1.45 and 3.25: below 1.45 advanced fibrosis is unlikely (negative predictive value about 90%), above 3.25 it is likely (positive predictive value about 65%, specificity about 97%). The convention validated in MASLD/NAFLD cohorts (Shah 2009; used in AASLD guidance) uses 1.3 and 2.67: below 1.3 rules out advanced fibrosis with NPV about 90%, above 2.67 rules it in with PPV about 80%. Because age enters linearly, the index is less reliable in patients younger than about 35 years and older than 65 years; indeterminate results usually prompt elastography or a second-line serum test rather than biopsy.