What is LFT?
LFT stands for "Liver Function Tests," though this term is technically a misnomer—these are actually liver blood tests that primarily measure hepatobiliary enzymes and markers of liver injury rather than true liver function. 1
Components of Liver Blood Tests
The standard panel includes several distinct categories of markers:
Hepatobiliary Enzymes (Markers of Injury)
- ALT (Alanine Aminotransferase): Most specific for liver injury, with normal levels 29-33 IU/L in men and 19-25 IU/L in women 2
- AST (Aspartate Aminotransferase): Less specific—can be elevated from heart, skeletal muscle, kidneys, brain, and red blood cells 2
- ALP (Alkaline Phosphatase): Elevated in cholestatic liver disease
- GGT (Gamma-Glutamyl Transferase): Marker of biliary injury
True Liver Function Markers
- Bilirubin (total, direct, indirect)
- Albumin
- INR/Prothrombin Time
- Platelet count (conveys information on fibrosis level) 1
Critical Distinction
The British Society of Gastroenterology guidelines explicitly recommend calling these "liver blood tests" rather than "liver function tests" because the predominant abnormality relates to elevations of hepatobiliary enzymes (indicating injury), not liver dysfunction. 1 When interpreted in isolation, hepatobiliary enzymes convey information on the level of ongoing injury, whereas bilirubin, albumin, and INR convey actual information on liver function. 1
Clinical Context and Limitations
These tests were developed in the 1950s and have significant limitations:
- Poor specificity: 30% of tests show at least one abnormal result outside reference range 1
- Low detection rate: Only 3.9% of patients with abnormal ALT/AST are diagnosed with significant liver disease within 5 years 1
- Not diagnostic tools: Used in isolation, liver blood tests are neither specific diagnostic tools nor specific exclusion tools 1
Common pitfall: Elevated transaminases don't always indicate liver disease—extrahepatic causes include muscular injury from strenuous exercise, hemolysis, myopathy, thyroid disease, and even vigorous physical activity. 3, 2
Severity Classification
Aminotransferase elevations are classified as:
- Mild: <5 times upper reference limit
- Moderate: 5-10 times upper reference limit
- Severe: >10 times upper reference limit 2
Pattern Recognition
Hepatocellular pattern: ALT and AST elevated much higher than ALP—indicates hepatocyte injury from causes like fatty liver disease, viral hepatitis, drug-induced injury, or ischemic hepatitis 2
Cholestatic pattern: ALP elevated with or without elevated bilirubin—suggests biliary obstruction or impaired bilirubin uptake 2
The AST:ALT ratio provides diagnostic clues: >2 suggests alcohol-induced liver disease, while <1 suggests metabolic disease-related fatty liver. 2