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Here is a thorough overview of Liver Function Tests (LFTs):
Liver Function Tests (LFTs)
What LFTs Actually Measure
The term "liver function tests" is somewhat misleading. Most of the enzymes (AST, ALT, ALP, GGT) are markers of hepatocyte injury rather than true liver function. True functional markers include albumin, total protein, PT/INR, and ammonia, which only fall abnormal when >80% of liver tissue is destroyed.
The liver can be thought of as three systems tested by LFTs:
- Chemical-metabolic system - tested by AST, ALT
- Reticuloendothelial (Kupffer cell) system - involved in hemoglobin/bilirubin metabolism
- Biliary secretion system - tested by ALP, GGT, bilirubin
Individual Tests - Normal Ranges & Significance
1. Alanine Aminotransferase (ALT)
- Normal: ~7-56 U/L (men slightly higher; ~7-40 U/L women)
- Primarily liver-specific (also in kidney, muscle)
- Best marker of hepatocyte injury
- Rises linearly with BMI
- ALT > AST in most liver diseases
2. Aspartate Aminotransferase (AST)
- Normal: ~10-40 U/L (men); ~9-32 U/L (women)
- Found in liver, cardiac muscle, skeletal muscle, kidney, brain, lung, pancreas - less specific than ALT
- ~80% of AST is mitochondrial
- Strenuous exercise and rhabdomyolysis can raise AST
AST:ALT Ratio (DeRitis Ratio)
| Pattern | Meaning |
|---|---|
| ALT > AST | Most liver diseases (viral hepatitis, NAFLD) |
| AST > ALT (>2:1) | Alcoholic liver disease, cirrhosis, Wilson disease |
| AST/ALT >3:1 | Strongly suggests alcoholic hepatitis |
3. Alkaline Phosphatase (ALP)
- Normal: 44-147 IU/L (adults); higher in children and pregnancy
- Sources: liver (biliary epithelium), bone, intestine, placenta
- Best marker of cholestasis and biliary obstruction
- Most sensitive marker of hepatic metastases
- Elevated in Paget's disease (bone source)
- ALP must be interpreted with GGT - if GGT is normal, elevated ALP is likely of bone origin
4. Gamma-Glutamyl Transferase (GGT)
- Normal: 9-48 U/L (varies by sex)
- From biliary epithelial cells and hepatocytes
- Sensitive indicator of hepatobiliary injury (especially biliary tract)
- Elevated by alcohol, warfarin, barbiturates, valproate, methotrexate
- Elevated in up to 70% of chronic alcoholics (2-3x ULN in heavy drinkers)
- Used as a marker of recent alcohol consumption
5. Total Bilirubin
- Normal: 0.1-1.2 mg/dL
- Direct (conjugated) bilirubin: 0-0.3 mg/dL
- Indirect (unconjugated) bilirubin: 0.2-0.8 mg/dL
| Type elevated | Suggests |
|---|---|
| Indirect (unconjugated) | Pre-hepatic (hemolysis), Gilbert's syndrome, Crigler-Najjar |
| Direct (conjugated) | Post-hepatic obstruction, hepatocellular disease |
| Both | Severe hepatitis, cirrhosis, fulminant failure |
6. Albumin
- Normal: 3.5-5.0 g/dL
- Synthesized exclusively in the liver
- Reflects chronic liver function (half-life ~20 days)
- Falls only when >80% of liver is destroyed (cirrhosis, fulminant failure)
- Low albumin = chronic or severe liver disease
7. Total Protein
- Normal: 6.0-8.3 g/dL
-
90% synthesized by the liver
- Low in severe liver failure; may be elevated in chronic infection/inflammation (globulins)
8. Prothrombin Time (PT) / INR
- Normal PT: 9.4-12.5 seconds; INR: 0.9-1.1
- Liver synthesizes most clotting factors (I, II, V, VII, IX, X)
- PT/INR is a sensitive early marker of acute liver failure
- Prolonged PT not correctable with vitamin K = hepatocellular disease
9. Ammonia
- Normal: 15-45 mcg/dL
- The liver is the only organ that detoxifies ammonia (via urea cycle)
- Elevated only when >80% of liver tissue is destroyed
- High ammonia leads to hepatic encephalopathy
10. Lactate Dehydrogenase (LDH)
- Widely distributed, nonspecific
- LD4 and LD5 isoenzymes suggest liver or skeletal muscle damage
- Isolated elevation of ALP + LD suggests space-occupying lesions (e.g., hepatic metastases, HCC)
LFT Patterns in Disease
| Condition | AST/ALT | ALP | GGT | Bilirubin | Albumin | PT |
|---|---|---|---|---|---|---|
| Acute Hepatitis | Very high (>10x) | Mildly elevated | Elevated | Both elevated | Normal | Normal |
| Cirrhosis | Normal or mildly elevated | Elevated | Elevated | Both elevated | Low | Prolonged |
| Biliary Obstruction | Normal | Very high | Very high | Direct elevated | Normal | Normal |
| Hepatic Metastases | Mild elevation | Markedly elevated | Elevated | Normal/mild | Normal | Normal |
| Alcoholic Hepatitis | Elevated, AST>ALT (>3:1) | Elevated | Markedly elevated | Elevated | Normal/low | May be prolonged |
| Fulminant Liver Failure | Very high (>10,000 IU/L) | Elevated | Elevated | Both elevated | Very low | Very prolonged |
When to Order LFTs
- Suspected liver disease (jaundice, hepatomegaly, ascites, spider naevi)
- Monitoring patients on hepatotoxic drugs (statins, methotrexate, antituberculous therapy, paracetamol/acetaminophen)
- Screening for alcohol-related liver disease
- Pre-operative assessment
- Evaluation of fatigue, weight loss, anorexia with suspected hepatic cause
Key Clinical Tips
- Isolated GGT elevation with normal ALP in an otherwise well patient - think alcohol or enzyme-inducing drugs
- ALP elevated, GGT normal - bone source (Paget's, bone mets, growing children, pregnancy)
- Very high transaminases (>1000 IU/L) - viral hepatitis, ischemic hepatitis ("shock liver"), drug-induced liver injury, or autoimmune hepatitis
- Low albumin + prolonged PT + elevated bilirubin = severe hepatocellular dysfunction
- Jaundice with direct hyperbilirubinemia + high ALP = biliary obstruction until proven otherwise
Sources: Quick Compendium of Clinical Pathology 5th ed.; Henry's Clinical Diagnosis and Management by Laboratory Methods
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SGOT (AST) - Aspartate Aminotransferase
What is SGOT?
SGOT = Serum Glutamic-Oxaloacetic Transaminase - this is the old name for AST (Aspartate Aminotransferase). Both terms refer to the exact same enzyme and test. Modern labs and guidelines use "AST," but "SGOT" is still widely used in clinical practice.
Similarly, SGPT is the old name for ALT (Alanine Aminotransferase).
Basic Facts
- SGOT/AST is an enzyme found primarily in the liver, heart, skeletal muscle, kidney, brain, lung, and pancreas (in decreasing concentration)
- About 80% of AST is mitochondrial; the rest is cytoplasmic
- It is released into the bloodstream when these tissues are injured or die
- It is a marker of hepatocyte injury, not true liver function
- Less liver-specific than ALT/SGPT because it is present in many other organs
Normal Range
| Population | Normal SGOT/AST |
|---|---|
| Adult males | 10-40 U/L |
| Adult females | 9-32 U/L |
| Reference standard (Henry's Lab Methods) | 8-33 U/L at 37°C |
Values vary slightly between laboratories.
Causes of Elevated SGOT/AST
Graded by Degree of Elevation
| Level | Elevation | Common Causes |
|---|---|---|
| Mild | 1-3x ULN | Fatty liver (NAFLD), hypothyroidism, strenuous exercise, celiac disease, medications |
| Moderate | 3-10x ULN | Chronic hepatitis B or C, alcoholic liver disease, cardiac ischemia, cirrhosis |
| High | >10x ULN | Acute viral hepatitis, ischemic hepatitis ("shock liver"), drug-induced liver injury, autoimmune hepatitis |
| Very high | >10,000 IU/L | Fulminant hepatic failure, severe ischemic hepatitis, acetaminophen overdose |
Liver Causes
- Acute viral hepatitis (A, B, C, D, E)
- Alcoholic hepatitis
- Non-alcoholic fatty liver disease (NAFLD/NASH)
- Drug/toxin-induced liver injury (paracetamol/acetaminophen, statins, isoniazid, alcohol)
- Cirrhosis
- Hepatic metastases or primary liver cancer (HCC)
- Autoimmune hepatitis
- Wilson disease
- Hemochromatosis
Non-Liver Causes (important - AST is not liver-specific!)
- Cardiac: Myocardial infarction, myocarditis (heart is rich in AST)
- Skeletal muscle: Rhabdomyolysis, polymyositis, dermatomyositis, strenuous exercise
- Other: Hemolysis, renal infarction, pancreatitis, thyroid disease
- Drugs: Anabolic steroids, statins, antituberculous drugs, methotrexate, valproate
SGOT vs SGPT (AST vs ALT) - The DeRitis Ratio
The AST:ALT (SGOT:SGPT) ratio is diagnostically important:
| Ratio | Pattern | Interpretation |
|---|---|---|
| ALT > AST (ratio <1) | SGPT > SGOT | Most viral hepatitis, NAFLD - typical pattern |
| AST > ALT (ratio >2:1) | SGOT > SGPT | Alcoholic hepatitis, cirrhosis, Wilson disease |
| AST/ALT >3:1 | SGOT >> SGPT | Strongly suggests alcoholic hepatitis |
| Isolated AST elevation | SGOT raised, ALT normal | Think non-liver source: heart, muscle, hemolysis |
Key point: If SGOT is elevated but SGPT (ALT) is normal, always suspect a non-hepatic cause - particularly myocardial infarction or muscle injury.
Causes of LOW SGOT/AST
Low values are rarely clinically significant, but may be seen in:
- Acute renal failure
- Chronic renal dialysis
- Vitamin B6 (pyridoxine) deficiency (AST requires B6 as a cofactor)
- Diabetic ketoacidosis
- Pregnancy
Clinical Interpretation Tips
- SGOT/AST alone is not diagnostic - always interpret alongside SGPT/ALT, ALP, GGT, bilirubin, albumin, and PT
- In suspected MI, also check troponin and CK since AST rises in cardiac injury
- AST starts rising within 6-12 hours of hepatocyte injury, peaks at 24-72 hours, and returns to normal within days-weeks
- Strenuous exercise can raise AST - ask about physical activity before interpreting
- Alcoholic hepatitis classically shows: AST usually <300 U/L, AST:ALT >2:1, with elevated GGT
Sources: Quick Compendium of Clinical Pathology 5th ed.; Morgan & Mikhail's Clinical Anesthesiology 7th ed.; Henry's Clinical Diagnosis and Management by Laboratory Methods; Harrison's Principles of Internal Medicine 22nd ed.
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