APRI grades liver fibrosis using only an AST value and a platelet count, both already sitting on routine lab reports. The World Health Organization endorsed it for settings where biopsy and elastography are out of reach.
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Fibrosis math from two cheap numbers
As liver fibrosis advances, two routine numbers drift in opposite directions. AST climbs as damaged cells leak enzyme, while platelets fall because a stiff liver produces less thrombopoietin and an enlarged spleen traps more of what remains. Wai and colleagues divided one trend by the other in 2003 and produced APRI, a score that tracks biopsy proven fibrosis surprisingly well for something built from a basic metabolic panel and blood count.
Reading the bands
The formula expresses AST as a multiple of its upper normal limit, multiplies by one hundred, and divides by platelet count. Scores below 0.5 make significant fibrosis unlikely, while scores above 1.5 point toward advanced fibrosis or cirrhosis. The wide middle band is honestly inconclusive, which is where second line tools such as FIB-4, elastography, or specialist referral come in. The score performs best in chronic hepatitis C where it was validated, and reasonably in hepatitis B and fatty liver disease.
Why the WHO backed it
Much of the world lacks FibroScan machines and hepatology clinics, but nearly every district laboratory can measure AST and count platelets. The World Health Organization hepatitis B guidelines formally recommend APRI for deciding who needs treatment when fancier tests are unavailable. That endorsement turned a statistical convenience into a frontline tool across entire continents.
Its blind spots
Anything that shakes platelets or AST for non liver reasons distorts the score. A muscle injury raises AST, an immune platelet disorder lowers the count, and both bend APRI without any fibrosis at all. Acute hepatitis flares also inflate it temporarily. Like every fibrosis score, it reads best in stable chronic disease with the whole clinical picture in view.