AST to Platelet Ratio Index (APRI)

Introduction

The APRI (AST to Platelet Ratio Index) is a simple calculation that estimates the degree of liver scarring (fibrosis) using two routine blood tests: aspartate aminotransferase (AST) and platelet count. Chun-Tao Wai, MD, and colleagues, including hepatologist Anna S. Lok, MD, developed it in 2003 for patients with chronic hepatitis C. It was designed to amplify the opposing effects of liver fibrosis on AST and platelet count: as the liver scars, AST tends to rise and platelets tend to fall. Because it uses inexpensive, widely available tests, it can help reduce the need for a liver biopsy, especially where specialized testing isn’t available.

• Intended for patients with hepatitis C being considered for liver biopsy to determine chance of fibrosis or cirrhosis.

• This index is among the best validated methods for predicting HCV progression.

• The AST to Platelet Ratio Index (APRI) estimates the likelihood of hepatic fibrosis and cirrhosis in patients with chronic liver disease, using a routine AST result and platelet count.

• Enter the AST value, the upper limit of normal for AST used by your laboratory, and the platelet count.

What it needs: Three values: the patient’s AST (U/L), the lab’s upper limit of normal for AST (U/L), and the platelet count (× 10⁹/L, the same as × 10³/μL). The upper limit varies between labs, and sometimes by sex, so use the value from the lab that ran the test.

How it’s calculated: APRI = (AST level / upper normal limit of AST) / platelet count (10⁹/L) × 100. For example, a patient with an AST of 80 U/L, a lab upper limit of 40 U/L, and a platelet count of 100 × 10⁹/L has an APRI of (80 ÷ 40) ÷ 100 × 100 = 2.0. Higher values suggest more advanced fibrosis.

Result Cut Points

1. APRI Score (<0.7) - Not sensitive or specific enough to determine level of fibrosis or cirrhosis.

2. APRI Score (≥0.7 to <1.0) - Significant Fibrosis: 77% sensitive & 72% specific

3. APRI Score (≥1 to <1.5) - Severe Fibrosis (61% sensitive & 64% specific) & Cirrhosis (76% sensitive & 72% specific)

4. APRI Score (≥1.5 to <2) - Significant Fibrosis (27% sensitive & 93% specific); Severe Fibrosis (br/>50% sensitive & 84% specific); Cirrhosis (76% sensitive & 72% specific).

5. APRI Score (≥2) - Severe Fibrosis (36% sensitive & 93% specific) & Cirrhosis (46% sensitive & 91% specific)

Management Considerations:

• Source: Meta-analysis Lin et al (2011)

• Interpret alongside the full clinical picture.

• At extreme values (very high or very low) the APRI can avoid further invasive testing in a substantial proportion of patients. Findings in validation studies suggest only moderate predictive value.

• Most experts recommend using the AST Platelet Ratio Index in conjunction with other non-invasive testing especially with intermediate values, as sensitivity and specificity may not be sufficient at these values.

• The FIB-4 Score may also be useful in these patients as well.