A liver function test (LFT) — also called a hepatic panel — measures six blood markers that together assess how well your liver is managing its two main jobs: processing and filtering (enzymes, bilirubin) and manufacturing proteins (albumin, total protein). Abnormal results don't diagnose a disease on their own, but their pattern — which markers are elevated, by how much, and in what ratio — points clearly toward specific conditions.
What an LFT Measures: Reference Ranges
| Marker | Normal Range | What It Reflects |
|---|---|---|
| AST (aspartate aminotransferase) | 10–40 IU/L | Liver cell damage (also present in heart, muscle) |
| ALT (alanine aminotransferase) | 7–56 IU/L | Liver cell damage; more liver-specific than AST |
| ALP (alkaline phosphatase) | 44–147 IU/L | Bile duct function; also bone turnover |
| Total bilirubin | 0.1–1.2 mg/dL | Red blood cell breakdown + liver processing capacity |
| Albumin | 3.5–5.0 g/dL | Liver synthetic function; nutritional status |
| Total protein | 6.3–8.2 g/dL | Combined albumin and globulin; immune and liver function |
Understanding Each Marker
AST and ALT — markers of liver cell injury
AST and ALT are enzymes inside liver cells (hepatocytes). When hepatocytes are damaged or die, these enzymes leak into the bloodstream. ALT is the more liver-specific marker — it is found primarily in liver cells. AST is also present in heart muscle, skeletal muscle, kidneys, and brain, so AST elevation without ALT elevation often points away from the liver.
The AST:ALT ratio is diagnostically useful:
- Ratio >2:1 (AST twice ALT): classic pattern for alcoholic liver disease
- Ratio <1:1 (ALT greater than AST): more typical of non-alcoholic fatty liver disease (NAFLD), viral hepatitis
- Both >10× normal: acute hepatitis (viral, drug-induced, or ischemic)
ALP — bile ducts and bone
ALP elevation in isolation — without proportional AST/ALT rise — typically indicates bile duct obstruction rather than hepatocellular damage. Common causes: gallstones blocking the common bile duct, primary biliary cholangitis, drug-induced cholestasis. ALP is also elevated in growing children (bone growth), pregnancy (placental ALP), and Paget's disease of bone. If ALP is elevated and liver disease is suspected, gamma-glutamyltransferase (GGT) is often measured alongside it — elevated GGT confirms a liver/biliary source rather than bone.
Total bilirubin — the jaundice marker
Bilirubin is the yellow byproduct of red blood cell breakdown. The liver processes ("conjugates") bilirubin for excretion in bile. Total bilirubin above 1.2 mg/dL is biochemically elevated; visible jaundice (yellowing of skin and whites of eyes) typically appears when levels exceed 2.5–3.0 mg/dL.
Bilirubin rises in three main scenarios: the liver can't process bilirubin fast enough (liver disease, cirrhosis), the bile ducts are blocked (obstruction), or red blood cells are being destroyed faster than normal (hemolysis). A mild isolated bilirubin elevation (1.2–3.0 mg/dL) with normal AST/ALT/ALP is often Gilbert syndrome — a benign genetic variant affecting 5–10% of people that causes unconjugated bilirubin to accumulate, especially during fasting or illness.
Albumin — the long-term liver report card
Albumin is the most abundant protein the liver makes, with a half-life of about 20 days. Unlike AST/ALT, which spike acutely with liver injury, albumin reflects the liver's synthetic function over the past several weeks. Low albumin (<3.5 g/dL) suggests chronic liver dysfunction — cirrhosis, long-standing hepatitis — or malnutrition, nephrotic syndrome, or protein-losing enteropathy. It doesn't fall rapidly in acute liver injury (there's still old albumin circulating), which is why it complements the acute enzyme markers rather than replacing them.
How to Read Elevation Patterns
| Pattern | What's Elevated | Likely Cause |
|---|---|---|
| Hepatocellular (mild) | AST/ALT 1–3× ULN | Fatty liver (NAFLD/AFLD), statin use, thyroid disease, mild viral hepatitis |
| Hepatocellular (severe) | AST/ALT >10× ULN | Acute viral hepatitis, drug-induced liver injury (DILI), ischemic hepatitis |
| Cholestatic | ALP elevated, AST/ALT mildly elevated | Bile duct obstruction (gallstone, stricture), primary biliary cholangitis |
| Mixed | AST, ALT, ALP all elevated | Drug reactions, primary sclerosing cholangitis, infiltrative disease |
| Synthetic failure | Low albumin ± elevated bilirubin | Cirrhosis, acute liver failure, severe chronic hepatitis |
| Isolated bilirubin elevation | Bilirubin 1.2–3.0, others normal | Gilbert syndrome (benign), hemolysis |
Common Liver Conditions and Their LFT Fingerprint
- Non-alcoholic fatty liver disease (NAFLD): most common cause of mild LFT abnormality in the US; AST/ALT mildly elevated (1–3×), ALT often higher than AST, ALP normal
- Alcoholic liver disease: AST:ALT ratio >2:1 is characteristic; GGT often markedly elevated
- Viral hepatitis (A, B, C): AST/ALT can reach 10–50× ULN in acute hepatitis; ALP relatively less elevated
- Cirrhosis: AST/ALT may normalize as hepatocytes are replaced by scar tissue; low albumin and elevated bilirubin become the dominant findings
- Drug-induced liver injury (DILI): pattern varies by drug; many statins, NSAIDs, and antibiotics can cause hepatocellular or cholestatic patterns
Getting a Liver Function Test at Home
An LFT requires a simple blood draw and can be collected through mobile phlebotomy — a licensed phlebotomist comes to you, and the sample is processed at a certified reference lab. Fasting is not required for the liver markers, though if your panel also includes glucose, an 8-hour fast is recommended. Results are typically available within 24–48 hours.
GGT: the often-overlooked liver marker
Gamma-glutamyl transferase (GGT) is frequently included in a hepatic panel but less often explained. It is an enzyme found in the liver, bile ducts, and kidneys, and is particularly sensitive to:
- Alcohol use — GGT is one of the most sensitive markers for alcohol-related liver injury. It rises with even moderate habitual alcohol consumption and returns to normal within 2–4 weeks of abstinence. An elevated GGT with an AST:ALT ratio above 2:1 strongly suggests alcohol as the cause.
- Bile duct obstruction — GGT rises in parallel with ALP when bile flow is obstructed. GGT elevation alongside ALP confirms a hepatobiliary source for the ALP rise (since ALP also increases with bone turnover, a normal GGT with elevated ALP points toward bone disease rather than liver disease).
- Medication effects — GGT is induced by many drugs including phenytoin, barbiturates, rifampin, and some antidepressants, even without causing actual liver damage.
Normal GGT range: 9–48 U/L (men); 9–32 U/L (women). Values vary by laboratory.
Medications that commonly elevate liver enzymes
Drug-induced liver injury (DILI) is among the most common reasons for an unexpected LFT abnormality in an otherwise healthy patient. The pattern of elevation matters:
- Statins (atorvastatin, rosuvastatin, simvastatin) — cause mild transaminase elevation in up to 3% of users. Clinically significant liver injury is rare but possible. Most guidelines no longer require routine LFT monitoring in asymptomatic patients on statins, but a baseline and follow-up if symptomatic is prudent.
- Acetaminophen (Tylenol) — the most common cause of acute liver failure in the U.S. at high doses or in the context of alcohol use. Even therapeutic doses (4g/day) can cause transaminase elevation in fasting individuals. Acetaminophen overdose produces a distinctive pattern: AST and ALT rapidly exceed 1,000–10,000 IU/L within 24–72 hours.
- NSAIDs (ibuprofen, naproxen, diclofenac) — diclofenac in particular has a known association with hepatocellular injury. Generally mild and reversible, but requires monitoring in patients with pre-existing liver disease.
- Amoxicillin-clavulanate (Augmentin) — the most common antibiotic cause of cholestatic hepatitis; elevates ALP and bilirubin more than AST/ALT. Typically occurs 1–6 weeks after the course ends.
- Methotrexate — used for rheumatoid arthritis, psoriasis, and cancer; causes hepatic fibrosis with cumulative dosing. Requires regular LFT monitoring and periodic liver biopsy at high cumulative doses.
- Herbal and dietary supplements — green tea extract, kava, anabolic steroids, and many traditional herbal remedies are documented causes of DILI. Always report supplements to your physician when discussing an abnormal LFT — they are often overlooked.
What happens after an abnormal liver function test?
The next steps depend on the degree of elevation and the clinical picture:
- Mild transaminase elevation (1–3× normal), no symptoms: Repeat LFT in 4–6 weeks after reviewing medications, alcohol use, and supplement intake. If values normalize, no further workup is usually needed.
- Persistent mild elevation or moderate elevation (3–10× normal): Expanded workup including hepatitis B surface antigen (HBsAg), hepatitis C antibody (anti-HCV), antinuclear antibody (ANA) for autoimmune hepatitis, iron studies and ferritin for hemochromatosis, ceruloplasmin in younger patients for Wilson's disease, and abdominal ultrasound to assess liver echogenicity and rule out obstruction.
- Severe elevation (>10× normal): Acute liver injury requiring urgent evaluation. Causes include acute viral hepatitis (A, B, E), ischemic hepatitis (shock liver), acute drug overdose (especially acetaminophen), or autoimmune flare. Emergency evaluation is appropriate.
- Isolated ALP elevation with normal transaminases: Rule out bone source with GGT; if GGT is also elevated, right upper quadrant ultrasound to evaluate bile ducts.
Sources and further reading
Liver Function Panel at Home
ALT, AST, bilirubin, alkaline phosphatase — your liver panel can be drawn at home by a certified phlebotomist. No fasting required for most markers. Same certified reference lab.
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