Liver function test interpretation uses a small group of blood tests to identify patterns of liver injury, bile flow obstruction, and synthetic dysfunction. Students need this cheat sheet because single abnormal values rarely explain the whole clinical picture. A pattern-based approach helps separate hepatocellular injury, cholestatic injury, mixed injury, and impaired liver function.
It also supports safer decisions about urgency, follow-up testing, and possible causes.
The core tests include ALT, AST, alkaline phosphatase, bilirubin, albumin, and PT/INR. ALT and AST mainly reflect hepatocellular injury, while alkaline phosphatase and GGT support cholestasis or biliary obstruction. Bilirubin helps assess excretory function and hemolysis patterns, while albumin and PT/INR reflect hepatic synthetic function.
The R ratio, calculated as (ALT/ALT ULN) / (ALP/ALP ULN), helps classify the injury pattern.
Key Facts
- The R ratio is calculated as (ALT divided by ALT upper limit of normal) divided by (ALP divided by ALP upper limit of normal).
- An R ratio greater than 5 suggests a hepatocellular pattern of liver injury.
- An R ratio less than 2 suggests a cholestatic pattern of liver injury.
- An R ratio from 2 to 5 suggests a mixed hepatocellular and cholestatic pattern.
- ALT is more liver-specific than AST, while AST can also rise with muscle injury, hemolysis, or cardiac injury.
- A high ALP with a high GGT supports a hepatobiliary source, while a high ALP with normal GGT suggests bone or another non-hepatic source.
- Prolonged PT/INR can indicate impaired hepatic synthetic function and may signal severe acute or chronic liver disease.
- Low albumin usually reflects chronic synthetic dysfunction, malnutrition, inflammation, or protein loss rather than isolated acute liver injury.
Vocabulary
- ALT
- Alanine aminotransferase is an enzyme found mainly in hepatocytes and rises when liver cells are injured.
- AST
- Aspartate aminotransferase is an enzyme found in liver, muscle, heart, and red blood cells, so it is less liver-specific than ALT.
- ALP
- Alkaline phosphatase is an enzyme associated with bile ducts and bone that rises in cholestasis, obstruction, or bone disease.
- Bilirubin
- Bilirubin is a breakdown product of heme that increases with impaired conjugation, impaired excretion, biliary obstruction, or excess red blood cell breakdown.
- PT/INR
- Prothrombin time and international normalized ratio measure clotting function and can reflect the liver's ability to produce clotting factors.
- R ratio
- The R ratio compares ALT elevation to ALP elevation to classify liver injury as hepatocellular, cholestatic, or mixed.
Common Mistakes to Avoid
- Calling every abnormal liver panel liver failure is wrong because liver injury markers and liver synthetic function are different concepts.
- Interpreting AST as liver-specific is wrong because AST can rise from skeletal muscle injury, hemolysis, myocardial injury, or strenuous exercise.
- Ignoring the upper limit of normal when calculating the R ratio is wrong because the formula requires ALT/ALT ULN and ALP/ALP ULN, not raw values alone.
- Assuming high ALP always means bile duct disease is wrong because ALP can also come from bone, placenta, intestine, or other sources.
- Using albumin alone to diagnose acute liver failure is wrong because albumin has a long half-life and may stay normal early in acute injury.
Practice Questions
- 1 A patient has ALT 480 U/L with ALT ULN 40 U/L and ALP 120 U/L with ALP ULN 120 U/L. Calculate the R ratio and identify the injury pattern.
- 2 A patient has ALT 90 U/L with ALT ULN 45 U/L and ALP 480 U/L with ALP ULN 120 U/L. Calculate the R ratio and identify the injury pattern.
- 3 A patient has AST 210 U/L, ALT 95 U/L, ALP 105 U/L, bilirubin 1.0 mg/dL, and recent intense weightlifting. What non-hepatic source should be considered?
- 4 Why does a prolonged INR raise more concern for clinically significant liver dysfunction than an isolated mild ALT elevation?