Liver Function Test — Test Guide & Results Explained — Purpose, Procedure & Normal Values | MyMedicPlus

Liver function tests (LFTs) — also called a hepatic function panel, liver panel, or liver enzymes — are a group of coordinated blood tests that assess the structural integrity, metabolic function, and synthetic capacity of the liver. The standard LFT panel measures six to eight analytes reflecting different aspects of hepatic biology. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are cytoplasmic enzymes released into the bloodstream when hepatocytes (liver cells) are injured or destroyed — elevated levels indicate hepatocellular damage. Alkaline phosphatase (ALP) is expressed on the canalicular surface of bile duct cells and hepatocytes; its elevation primarily signals cholestasis (impaired bile flow) or bile duct obstruction, though it is also produced by bone, placenta, and intestine. Gamma-glutamyl transferase (GGT) is a membrane enzyme particularly sensitive to alcohol use, hepatic enzyme induction by drugs, and bile duct disease — its elevation alongside raised ALP confirms hepatic rather than bony origin. Total bilirubin (a breakdown product of haem from red blood cells) and direct (conjugated) bilirubin assess jaundice aetiology and severity. Albumin — synthesised exclusively by the liver — and total protein reflect the liver's protein synthetic reserve, declining only in chronic or severe liver disease when hepatic synthetic capacity is compromised. Prothrombin time (PT) or INR — though strictly a coagulation test — is included in comprehensive liver panels because the liver synthesises clotting factors I, II, V, VII, and X; prolongation indicates impaired hepatic synthesis. Together, these markers provide a composite assessment of hepatocellular injury, cholestatic disease, and liver synthetic reserve to guide diagnosis and monitoring.

LFTs are ordered to evaluate a broad range of hepatobiliary presentations and clinical indications. Symptomatic presentations include jaundice (yellowing of skin and sclerae) with or without pruritis (itch), right upper quadrant or epigastric abdominal pain suggesting liver or gallbladder pathology, nausea, anorexia, fatigue, abdominal distension from ascites, peripheral oedema, and signs of hepatic encephalopathy including confusion and sleep disturbance. Asymptomatic liver enzyme elevation is frequently detected incidentally on routine blood testing or during investigation of unrelated conditions. Specific diagnostic categories include: alcoholic liver disease (ALD); non-alcoholic fatty liver disease (NAFLD/MASLD, now the most common liver disease worldwide); viral hepatitis B and C (chronic infection); autoimmune hepatitis; primary biliary cholangitis (PBC); primary sclerosing cholangitis (PSC); drug-induced liver injury (DILI) from statins, paracetamol/acetaminophen, anti-tuberculosis drugs (isoniazid, rifampicin), methotrexate, amiodarone, herbal and dietary supplements; liver cirrhosis and its complications (portal hypertension, variceal bleeding, hepatorenal syndrome); and hepatocellular carcinoma (HCC) surveillance in at-risk patients. Pre-operative LFTs assess surgical risk. Serial LFTs monitor treatment response in hepatitis B (antiviral therapy) and autoimmune hepatitis (corticosteroids, azathioprine) and identify recurrent drug toxicity.

Fasting for 8 hours before the blood draw is preferred, as a fatty meal elevates postprandial triglycerides that can interfere with bilirubin measurement by turbidity in some laboratory assay systems. Water and essential medications are permitted during the fasting period. Do not drink alcohol for at least 24–48 hours before testing — even moderate intake of 2–3 units can substantially raise GGT and AST and may affect bilirubin, producing misleading results that suggest liver disease or confound monitoring of abstinent patients. Inform your doctor of all medications and supplements, including over-the-counter paracetamol (acetaminophen), NSAIDs, herbal remedies (kava, black cohosh, comfrey, pyrrolizidine alkaloids), anabolic steroids, oral contraceptives, statins, antibiotics, anti-tuberculosis drugs, antifungals, and anticonvulsants, as the list of drugs causing hepatotoxicity is extensive. Avoid strenuous resistance exercise the day before the test — muscle damage from intense exercise releases AST (and to a lesser extent ALT) from damaged muscle cells, raising enzyme levels that may be misinterpreted as hepatic origin without context. For repeat LFTs to monitor a known liver condition or drug effect, sampling at a consistent time of day reduces biological variation.

A phlebotomist applies a tourniquet to the upper arm, cleans the antecubital fossa with an antiseptic swab, and draws 5–10 mL of blood from a forearm vein into a serum separator tube (gold or red-top) — most laboratories require only one serum tube for the full LFT panel, as all analytes are measured from the same separated serum sample. Some laboratories also use a lithium heparin (green-top) tube depending on their analyser requirements. The tube is centrifuged to separate serum from red blood cells, then placed on an automated chemistry analyser. ALT and AST are measured by kinetic enzymatic methods (IFCC-standardised); ALP and GGT by enzymatic colorimetric assays; bilirubin by diazo reaction or enzymatic method; albumin by bromocresol green or bromocresol purple dye-binding; and total protein by the biuret method. All LFT components are measured from a single serum sample simultaneously. The procedure requires only 5 minutes and causes brief, minor needle discomfort. Results are available within 2–8 hours in most hospital laboratories.

Normal adult reference ranges (approximate; laboratories set population-specific ranges): ALT — 7–56 U/L (men may have slightly higher upper limits than women; AASLD recommends 29–33 U/L as the upper limit of normal for optimal sensitivity); AST — 10–40 U/L; ALP — 44–147 U/L (physiologically elevated in pregnancy, growing children, and bone disease); GGT — 9–48 U/L; Total bilirubin — 0.1–1.2 mg/dL (1.7–20.5 μmol/L); Direct (conjugated) bilirubin — 0–0.3 mg/dL; Albumin — 3.5–5.0 g/dL; Total protein — 6.3–8.2 g/dL. Interpretation by pattern is more clinically useful than individual values: hepatocellular pattern (ALT/AST predominantly elevated) — suggests hepatitis (viral, autoimmune, drug-induced, ischaemic, or alcoholic); ALT/AST elevation >10 times the upper limit of normal (ULN) indicates acute hepatocellular necrosis. Cholestatic pattern (ALP elevation ≥3x ULN with raised GGT) — suggests bile duct obstruction (gallstones, pancreatic head cancer, stricture), drug-induced cholestasis, PBC, or PSC. Mixed pattern — many conditions including NAFLD and infiltrative diseases cause both patterns simultaneously. AST/ALT ratio >2:1 — strongly suggests alcoholic liver disease; ratio >3:1 is highly specific for alcoholic hepatitis. Low albumin (<3.0 g/dL) combined with prolonged prothrombin time (INR >1.5) indicates severe hepatic synthetic dysfunction consistent with advanced cirrhosis or acute liver failure. Isolated raised bilirubin with all other LFTs normal may indicate Gilbert's syndrome — a benign inherited condition affecting 5–10% of the population — particularly if unconjugated (indirect) bilirubin is elevated.

The blood draw is very low risk — minor bruising, soreness, or a small haematoma at the venepuncture site are the only common adverse effects. LFTs have clinically important limitations that must be appreciated for accurate interpretation. Mild enzyme elevation (1–3x ULN) is common and frequently caused by benign or reversible conditions — hepatic steatosis (fatty liver), modest alcohol consumption, recently started medication, obesity, or vigorous exercise — rather than significant liver disease, and does not mandate immediate invasive investigation. Conversely, normal or near-normal LFTs do not exclude serious or advanced liver disease: patients with well-compensated cirrhosis — even those with significant portal hypertension and oesophageal varices — often have only mildly elevated or entirely normal transaminases because the bulk of the hepatocyte mass has been replaced by fibrosis and the remaining cells are not acutely inflamed. ALP elevation requires differentiation of hepatic from bony origin: GGT elevation confirms hepatic origin (bone does not produce GGT), while isolated ALP elevation with normal GGT in a growing adolescent or patient with bone disease (Paget's disease, fractures, bone metastases) indicates skeletal ALP. Mildly elevated transaminases without a clear cause after initial investigation warrant ultrasound of the liver to exclude steatosis, focal lesions, or biliary dilatation, and serological testing for viral hepatitis. LFTs do not quantify fibrosis (liver scarring) or cirrhosis — liver stiffness measurement (Fibroscan/transient elastography), MR elastography, or liver biopsy is required to stage fibrosis degree (F0–F4 Metavir scale) in patients with chronic liver disease.