APRI Score Formula
The APRI (AST-to-Platelet Ratio Index) is calculated as: (AST/upper limit of normal for AST) × 100/platelet count (×10⁹/L). 1
Formula Components
The calculation requires three specific values:
- AST level in IU/L or U/L 1
- Upper limit of normal (ULN) for AST at your laboratory (typically 37-40 IU/L) 1
- Platelet count in ×10⁹/L (or ×10³/μL) 1
Step-by-Step Calculation
- Divide AST by the ULN for AST at your specific laboratory to get the AST ratio 1
- Multiply this ratio by 100 1
- Divide the result by the platelet count (expressed as ×10⁹/L) 1
Example: If AST = 80 IU/L, ULN = 40 IU/L, and platelets = 150 × 10⁹/L:
- APRI = (80/40) × 100/150 = 2.0 × 100/150 = 1.33 1
Clinical Interpretation Thresholds
For Significant Fibrosis (≥F2):
- APRI >0.5 identifies most patients with significant fibrosis (sensitivity 72.9%, specificity 64.7%) 2
- APRI >1.5 has high specificity for significant fibrosis (AUROC 0.8) 1
For Cirrhosis (F4):
- APRI >1.0 identifies most patients with cirrhosis (sensitivity 59.4%, specificity 73.9%) 2
- APRI >2.0 has high specificity for cirrhosis (AUROC 0.89) 1
Important Caveats
The optimal cutoff values vary significantly based on baseline AST levels, which is a critical limitation often overlooked in clinical practice 3:
- When AST ≤37 IU/L: optimal cutoff for cirrhosis is APRI >0.6 3
- When 37 < AST ≤74 IU/L: optimal cutoff for cirrhosis is APRI >1.1 3
- When 74 < AST ≤148 IU/L: optimal cutoff for cirrhosis is APRI >2.2 3
- When AST >148 IU/L: optimal cutoff for cirrhosis is APRI >3.4 3
APRI performance is influenced by conditions affecting AST or platelets, including acute hepatitis, hemolysis, Gilbert syndrome, and any cause of thrombocytopenia unrelated to liver disease 2. The test performs best in chronic hepatitis C and NAFLD, but has limited accuracy in chronic hepatitis B 4, 5.