Interpreting Liver Function Tests: Patterns, Causes and Emergencies

Interpreting Liver Function Tests (LFTs): Patterns, Causes, Algorithms and Emergency Findings

Study level: Clinical medicine, emergency medicine and clinical chemistry | Topic: Liver tests

Learning objectives

• Identify what each common liver panel test measures and what it cannot prove.
• Classify results as hepatocellular, cholestatic or mixed using the R-ratio concept.
• Interpret bilirubin, albumin, INR, ALP, GGT, ALT and AST together.
• Build a stepwise differential and investigation plan.
• Recognise acute liver failure, cholangitis, severe drug injury and other emergencies.

1. The liver panel: injury versus function

Test	What it reflects	Important limitations
ALT	Leakage from hepatocytes; relatively liver-specific.	Can rise in muscle/other disease; magnitude does not equal functional failure.
AST	Hepatocyte and muscle injury; mitochondrial and cytosolic enzyme.	Less liver-specific; also rises in skeletal/cardiac muscle and haemolysis.
ALP	Bile-duct/ductular injury and cholestasis; also bone, placenta and intestine.	Physiologically higher in children, pregnancy and healing bone.
GGT	Supports hepatobiliary source of an elevated ALP; inducible enzyme.	Alcohol and many drugs raise it; isolated GGT is nonspecific.
5′-nucleotidase	Alternative confirmation of hepatic ALP source.	Less available; does not identify the exact cause.
Total/direct bilirubin	Heme breakdown, hepatic uptake/conjugation and biliary excretion.	Fractionation is essential; haemolysis and fasting also alter levels.
Albumin	Longer-term hepatic protein synthesis and nutrition/inflammation.	Long half-life; low in renal/GI loss, inflammation and malnutrition.
PT/INR	Short half-life clotting-factor synthesis and vitamin K status.	Also altered by warfarin, vitamin K deficiency, DIC and anticoagulants.
Total protein/platelets	Globulins and portal-hypertension/marrow clues.	Supportive, not specific.

2. Classify the biochemical pattern

2.1 R-ratio

Use the upper limit of normal (ULN) for the laboratory:

R = (ALT ÷ ALT ULN) ÷ (ALP ÷ ALP ULN)

• Hepatocellular: R ≥5; ALT/AST predominate.
• Mixed: R >2 and <5.
• Cholestatic: R ≤2; ALP/GGT and often bilirubin predominate.

Patterns can evolve. In severe hepatocellular injury, aminotransferases may fall as hepatocytes are lost while INR/bilirubin worsen; a falling ALT is not automatically recovery.

3. Hepatocellular pattern

3.1 Common causes

Cause	Clues	Initial tests
Viral hepatitis	Exposure, fever, malaise, jaundice, very high ALT/AST.	HAV IgM, HBsAg/anti-HBc IgM, HCV antibody/RNA; HEV where relevant.
Drug-induced liver injury (DILI)	New medicine, supplement or dose change; variable latency.	Full medication timeline, exclusion of viral/autoimmune/obstructive causes.
Ischemic/hypoxic hepatitis	Shock, hypoxia, cardiac failure; abrupt very high AST/ALT and LDH.	Haemodynamics, lactate, LDH, ECG/echo and renal tests.
Autoimmune hepatitis	Female predominance, other autoimmunity, high IgG.	ANA, ASMA, anti-LKM where appropriate, IgG and specialist assessment.
Metabolic fatty liver/MASH	Obesity, diabetes, dyslipidaemia; mild chronic ALT elevation.	Metabolic profile, ultrasound/elasticity and fibrosis risk assessment.
Wilson disease	Young patient, neuropsychiatric symptoms, haemolysis or acute liver failure.	Ceruloplasmin, urinary copper, slit-lamp exam and specialist testing.
Muscle injury	Myalgia, weakness, trauma, seizures or strenuous exercise; AST often prominent.	Creatine kinase, renal function and urine.

3.2 AST:ALT ratio

An AST:ALT ratio above about 2 supports alcohol-associated liver disease in the right context, but it is neither sensitive nor specific. AST predominance also occurs in cirrhosis, muscle injury and advanced mitochondrial injury.

4. Cholestatic pattern

4.1 Intrahepatic versus extrahepatic

• Extrahepatic obstruction: gallstones, pancreatic/ampullary cancer, biliary stricture, cholangitis or postoperative injury.
• Intrahepatic cholestasis: sepsis, drug injury, primary biliary cholangitis, primary sclerosing cholangitis, infiltrative disease, pregnancy-related cholestasis.

4.2 Diagnostic sequence

1. Confirm hepatic source of ALP with GGT or 5′-nucleotidase and repeat if unexpected.
2. Ask about jaundice, pale stool, dark urine, itch, fever, RUQ pain, drug exposure and inflammatory bowel disease.
3. Perform ultrasound first in many settings to assess ducts, gallstones, liver texture and portal flow.
4. If ducts are dilated, escalate to MRCP/CT/endoscopic assessment and urgent intervention when cholangitis/obstruction is likely.
5. If ducts are not dilated, consider autoimmune cholestatic disease, DILI, sepsis, infiltrative disease and pregnancy-related causes.

4.3 Cholangitis emergency

Fever/rigors, RUQ pain and jaundice suggest ascending cholangitis; hypotension and altered mental state indicate severe disease. Start sepsis management, cultures and antibiotics per local protocol, urgent imaging and biliary drainage consultation.

5. Bilirubin interpretation

Pattern	Mechanism	Examples
Unconjugated predominant	Overproduction or impaired conjugation.	Haemolysis, Gilbert syndrome, neonatal physiology, ineffective erythropoiesis.
Conjugated predominant	Hepatocyte excretion failure or obstruction.	Hepatitis, cholestasis, obstruction, sepsis, drug injury.
Mixed	Combined hepatocellular and biliary dysfunction.	Severe hepatitis, advanced cirrhosis and mixed DILI.

Urine bilirubin is present when conjugated bilirubin is water-soluble. Dark urine with pale stools and itch suggests cholestasis, whereas isolated unconjugated hyperbilirubinaemia without liver injury suggests haemolysis or impaired conjugation.

6. Synthetic function and severity

• INR/PT: rises quickly when clotting-factor synthesis is impaired. An unexpectedly prolonged INR may also reflect vitamin K deficiency, warfarin, DIC or anticoagulant contamination.
• Albumin: low albumin suggests chronic disease or nonhepatic loss; normal albumin does not exclude acute liver failure.
• Glucose: hypoglycaemia is a severe hepatic failure warning, especially with encephalopathy.
• Ammonia: may support encephalopathy assessment but correlates imperfectly with severity; treat the patient, not the number alone.
• Platelets/spleen: thrombocytopenia and splenomegaly support portal hypertension but have many other causes.

7. Acute liver failure: emergency recognition

• Jaundice plus confusion, asterixis, drowsiness or coma.
• Rising INR/PT, hypoglycaemia, metabolic acidosis, renal injury or lactate elevation.
• Very high aminotransferases from acetaminophen/toxin, viral hepatitis or ischemia; the cause may be clinically silent initially.
• Urgent actions: ABCDE, bedside glucose, stop hepatotoxic drugs, contact a liver unit, test for acetaminophen and viral/autoimmune causes, manage cerebral oedema/sepsis/renal failure and avoid unnecessary procedures.

8. Interpreting common combinations

Pattern	Likely considerations	Next step
ALT/AST high, ALP mild, bilirubin variable	Hepatitis, ischemia, DILI, autoimmune hepatitis, muscle injury.	Repeat/trend, medication and exposure history, viral/autoimmune tests, CK and ultrasound.
ALP/GGT high with direct bilirubin	Obstruction, cholangitis, cholestatic DILI, PBC/PSC.	Ultrasound, urgent assessment if pain/fever, cholangiography when indicated.
Isolated ALP high, GGT normal	Bone growth/disease, fracture, vitamin D disorders, pregnancy.	Bone-specific ALP/other tests and clinical evaluation.
Isolated GGT high	Alcohol, enzyme-inducing drugs, metabolic disease.	Review context; do not diagnose liver disease from GGT alone.
Low albumin + high INR + thrombocytopenia	Advanced chronic liver disease, but also nutrition, DIC or anticoagulant effect.	Assess synthetic function, portal hypertension and competing causes; urgent if bleeding/encephalopathy.

9. Medication and toxin review

• Record every prescription, over-the-counter drug, herbal remedy, supplement and recreational substance with start/stop dates and dose changes.
• Acetaminophen overdose can cause severe hepatocellular injury even before symptoms; obtain a timed level and follow local antidote protocol.
• Amoxicillin–clavulanate, antiepileptics, antituberculous drugs, statins, methotrexate and many herbal products can cause variable DILI.
• Do not stop essential medicines blindly; weigh alternatives and involve pharmacy/hepatology when the patient is stable.

10. Pregnancy and children

Reference intervals and differential diagnoses differ in pregnancy and childhood. Consider hyperemesis, intrahepatic cholestasis of pregnancy, pre-eclampsia/HELLP, acute fatty liver of pregnancy, biliary disease, viral hepatitis and metabolic disease. In children, ALP is physiologically higher and biliary atresia or inherited disorders may present early; use age-specific intervals and specialist pathways.

11. Clinical cases

Case 1: very high transaminases

A patient has AST/ALT above 3000 IU/L and shock after a cardiac event. Ischemic hepatitis is likely, but check acetaminophen, viral hepatitis, ultrasound/Doppler, lactate and INR. Treat hypoxia/shock and monitor for acute liver failure.

Case 2: cholangitis

Fever, RUQ pain, jaundice, hypotension and confusion with high ALP/bilirubin indicate severe cholangitis. Begin sepsis care, cultures and urgent antibiotics, and arrange biliary drainage; do not wait for a complete outpatient work-up.

Case 3: isolated GGT

An asymptomatic patient has high GGT with normal ALT, AST, ALP and bilirubin. Review alcohol, drugs and metabolic risk; an isolated GGT does not establish hepatobiliary disease.

12. Quick self-test

1. Which tests mainly indicate hepatocellular injury?
2. What is the R-ratio and how does it classify injury?
3. Why is GGT useful with ALP but nonspecific alone?
4. What does INR tell you, and what nonhepatic factors alter it?
5. List four acute liver failure red flags.
6. What is the first imaging study for many cholestatic patterns?

Answers

1. ALT and AST.
2. R = (ALT/ALT ULN) ÷ (ALP/ALP ULN); ≥5 hepatocellular, ≤2 cholestatic, 2–5 mixed.
3. It supports a hepatic source of ALP, but alcohol, drugs and other factors can raise GGT without significant liver disease.
4. INR reflects short-lived clotting-factor synthesis; warfarin, vitamin K deficiency, DIC and anticoagulants also prolong it.
5. Encephalopathy, rising INR, hypoglycaemia, metabolic acidosis/lactate, renal failure and rapidly progressive jaundice.
6. Ultrasound to assess ducts, gallstones, liver and portal structures.

Key take-home points

• Separate injury markers (ALT/AST/ALP/GGT) from function markers (INR, albumin, glucose and clinical encephalopathy).
• Classify the pattern before listing causes; use trends and the R-ratio.
• Cholestasis with fever, pain, jaundice, hypotension or confusion is an emergency until drainage/sepsis issues are addressed.
• A falling aminotransferase does not guarantee recovery when INR, bilirubin or mental state worsen.
• Always review medications, toxins, alcohol, viral exposures, pregnancy and nonhepatic sources of enzyme elevation.

Selected references

• NCBI Bookshelf: Liver Function Tests
• MedlinePlus: Liver Function Tests
• AASLD: How to approach elevated liver enzymes
• AASLD: Defining and managing acute liver failure
• NCBI Bookshelf: RUCAM and liver injury patterns