LFTs Decoded: What's Actually "Liver" and What Isn't
LFTs Decoded: What's Actually "Liver" and What Isn't
The AST/ALT ratio, the hepatocellular vs. cholestatic pattern, and the muscle connection everyone misses.
"Liver function tests" is a misnomer that has confused generations of clinicians. Most of the tests in the panel don't measure liver function at all—they measure liver injury. True liver function is reflected by albumin, bilirubin, PT/INR, and the liver's synthetic capacity. The aminotransferases (AST and ALT) tell you about hepatocyte damage, not about how well the liver is doing its job.
Let's sort out what each test actually means.
The Tests: Injury Markers vs. Function Markers
| Test | What It Measures | Key Interpretation |
|---|---|---|
| ALT (Alanine Aminotransferase) | Hepatocyte injury marker. More liver-specific than AST. | Elevated in hepatitis (viral, autoimmune, drug-induced, MASLD). The most specific "liver enzyme." |
| AST (Aspartate Aminotransferase) | Found in liver AND muscle, heart, kidney, brain, RBCs | Elevated in liver disease AND muscle injury, hemolysis, MI. Not liver-specific. |
| ALP (Alkaline Phosphatase) | Found in bile ducts, bone, placenta, intestine | Elevated in cholestasis (biliary obstruction) AND bone disease. Confirm hepatic origin with GGT. |
| GGT (Gamma-Glutamyl Transferase) | Biliary/hepatic enzyme | Confirms that elevated ALP is hepatic (not bone). Also elevated with alcohol use and enzyme-inducing medications. |
| Bilirubin (total and direct) | Product of heme breakdown | Direct (conjugated) elevated: hepatocellular disease or biliary obstruction. Indirect (unconjugated) elevated: hemolysis or Gilbert's syndrome. |
| Albumin | TRUE liver function marker (synthetic capacity) | Low in chronic liver disease, malnutrition, nephrotic syndrome, inflammation. Half-life 21 days—doesn't change acutely. |
| PT/INR | TRUE liver function marker (synthetic capacity) | Elevated when liver can't produce clotting factors. Responds within hours to days (unlike albumin). The best acute marker of liver synthetic failure. |
The Two Patterns: Hepatocellular vs. Cholestatic
This is the most important framework for interpreting LFTs. When you see elevated liver enzymes, classify the pattern:
| Pattern | AST/ALT | ALP/GGT | Think About |
|---|---|---|---|
| Hepatocellular | ↑↑↑ Markedly elevated | Normal or mildly elevated | Viral hepatitis, drug-induced liver injury (DILI), autoimmune hepatitis, ischemic hepatitis ("shock liver"), Wilson's disease, acetaminophen toxicity |
| Cholestatic | Normal or mildly elevated | ↑↑↑ Markedly elevated | Biliary obstruction (gallstones, tumor), primary biliary cholangitis, primary sclerosing cholangitis, drug-induced cholestasis |
| Mixed | ↑ Elevated | ↑ Elevated | Drug-induced, infiltrative diseases, granulomatous hepatitis, some viral hepatitis |
To formalize the pattern: R = (ALT/ALT ULN) ÷ (ALP/ALP ULN). R >5 = hepatocellular. R <2 = cholestatic. R 2–5 = mixed. This calculation helps when the pattern isn't immediately obvious.
The AST/ALT Ratio: Hidden Clues
- AST/ALT >2:1: Classic for alcoholic liver disease. The ratio occurs because alcohol selectively depletes hepatic pyridoxal-5-phosphate (needed for ALT production) and also causes mitochondrial injury (releasing more AST).
- AST/ALT ~1:1: Most other liver diseases (viral hepatitis, MASLD, drug-induced)
- ALT > AST: Typical of MASLD/MASH (fatty liver), chronic hepatitis C
- AST markedly elevated with relatively normal ALT: Think non-hepatic source—muscle injury (check CPK!), hemolysis, myocardial injury
Elevated AST with normal or mildly elevated ALT + elevated CPK = muscle, not liver. This was covered in the myositis antibody post and the Bellocchi autoimmune review. A patient referred for "elevated transaminases" who also has proximal weakness needs a CPK and a myositis workup—not a liver biopsy. Always check CPK when AST is disproportionately elevated.
The Magnitude Matters
| ALT/AST Level | Differential |
|---|---|
| <5× ULN (mild) | MASLD/MASH, medications, alcohol, celiac disease, thyroid disease, hemochromatosis, alpha-1 antitrypsin deficiency |
| 5–15× ULN (moderate) | Acute viral hepatitis, autoimmune hepatitis, drug-induced, alcoholic hepatitis, biliary obstruction |
| >15× ULN (severe / >1000) | Acetaminophen toxicity, ischemic hepatitis ("shock liver"), acute viral hepatitis (A, B, rarely C), autoimmune hepatitis flare, acute Budd-Chiari, Wilson's disease (acute presentation) |
The Workup: What to Order Next
For Persistent Mild Elevation (Most Common Primary Care Scenario)
- Confirm persistence: repeat in 3–6 months with fasting labs
- Check: hepatitis B panel (HBsAg, anti-HBs, anti-HBc), HCV antibody, iron studies + ferritin (hemochromatosis), ANA + IgG level (autoimmune hepatitis), CPK (muscle source), TSH, celiac panel (tTG-IgA + total IgA), fasting lipid panel + HbA1c (metabolic syndrome/MASLD)
- Imaging: RUQ ultrasound (assess for fatty liver, gallstones, structural lesions)
- Review medications and supplements (statins, NSAIDs, acetaminophen, herbals, antibiotics)
- Assess alcohol use honestly
For Acute Severe Elevation (>1000)
This is urgent. Check acetaminophen level, hepatitis A IgM, hepatitis B core IgM, HCV RNA, autoimmune hepatitis panel (ANA, anti-smooth muscle, IgG), toxicology screen, PT/INR (synthetic function), and Doppler ultrasound (Budd-Chiari, portal vein thrombosis). Consider Wilson's disease in young patients (ceruloplasmin, 24-hour urine copper).
In children with unexplained elevated transaminases, the differential includes everything above plus: Wilson's disease (screen any child >3 years with unexplained liver disease), alpha-1 antitrypsin deficiency, Reye syndrome (ASA + viral illness—now rare), metabolic liver diseases (galactosemia, tyrosinemia in infants), and celiac disease. Remember that neonatal "physiologic jaundice" is indirect bilirubinemia from immature conjugation—a direct bilirubin >1 mg/dL in a newborn is ALWAYS pathologic and warrants urgent evaluation (biliary atresia must be ruled out).
The Pitfalls
- Calling them "liver function tests" when AST/ALT/ALP are injury markers. True function = albumin + PT/INR + bilirubin.
- Assuming elevated AST = liver disease. AST is in muscle, heart, kidney, and RBCs. Check CPK.
- Not using the hepatocellular vs. cholestatic framework. The pattern determines the workup.
- Ignoring an isolated elevated ALP. Confirm hepatic origin with GGT. If GGT is normal, the ALP is from bone (Paget's, growing children, pregnancy, bone metastases).
- ALP is normally elevated in growing children (from bone growth plates)—a mildly elevated ALP in a healthy child is usually bone, not liver.
- Forgetting MASLD as the most common cause of mild transaminitis. It's now the most prevalent liver disease worldwide.
- Not checking celiac disease in unexplained transaminitis. Celiac causes cryptogenic elevation in up to 9% of cases.
- Stopping statins reflexively for mild ALT elevation. ALT <3× ULN on a statin can be monitored without discontinuation per current guidelines.
Bottom Line
LFTs are not one test—they're a panel of injury markers and function markers that tell very different stories. Classify the pattern (hepatocellular vs. cholestatic vs. mixed), consider the magnitude, always check CPK when AST is disproportionately high, and remember that albumin and PT/INR are the true measures of how well the liver is actually working. And never forget the two great masqueraders of liver disease: muscle injury and celiac disease.
Stay sharp out there.
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