Liver Function Tests: ALT, AST, Bilirubin & Abnormal Results

What Are Liver Function Tests?

Liver function tests (LFTs) are a group of blood tests that help assess the health of your liver. Despite the name, they do not directly measure how well the liver functions — rather, they detect enzymes and proteins that are released into the blood when liver cells are damaged or when bile flow is obstructed.

The liver is one of the largest and most versatile organs in the body. It processes nutrients, produces bile to aid digestion, stores energy, detoxifies harmful substances, manufactures proteins essential for blood clotting, and regulates cholesterol. Because the liver is so resilient and can function even when significantly damaged, LFTs are often the first indicator that something is wrong.

ALT (Alanine Aminotransferase)

ALT is an enzyme found predominantly in liver cells. When these cells are damaged or inflamed, ALT leaks into the bloodstream, causing levels to rise. ALT is considered the most liver-specific of the standard LFT enzymes.

• Normal range — Approximately 7 to 56 U/L (varies by laboratory and sex).
• Common causes of raised ALT — Non-alcoholic fatty liver disease (NAFLD), alcohol-related liver disease, viral hepatitis (B and C), medications (including statins and paracetamol), autoimmune hepatitis, and coeliac disease.

AST (Aspartate Aminotransferase)

AST is found in the liver, heart, muscles, kidneys, and brain. Because it is not exclusively found in liver cells, raised AST can indicate damage to any of these organs. However, when AST is elevated alongside ALT, the liver is the most likely source.

• Normal range — Approximately 10 to 40 U/L.
• AST:ALT ratio — A ratio greater than 2:1 can suggest alcohol-related liver damage. In most other liver conditions, ALT is higher than AST.

ALP (Alkaline Phosphatase)

ALP is an enzyme found in the liver, bile ducts, and bone. Raised ALP levels may indicate obstruction of the bile ducts (cholestasis), bone disease, or liver disease. It is also normally raised during pregnancy and in growing children.

• Normal range — Approximately 30 to 130 U/L.
• When ALP is raised — If raised alongside GGT, the source is likely the liver or bile ducts. If raised with normal GGT, the source is more likely bone.

GGT (Gamma-Glutamyl Transferase)

GGT is an enzyme found in the liver, bile ducts, and pancreas. It is particularly sensitive to alcohol consumption and is often used as an indicator of alcohol-related liver damage. Even moderate alcohol intake can raise GGT.

• Normal range — Approximately 8 to 61 U/L for men; 5 to 36 U/L for women.
• Common causes of raised GGT — Alcohol use, medications (including anticonvulsants), bile duct obstruction, fatty liver, and pancreatitis.

Bilirubin

Bilirubin is a yellow pigment produced when old red blood cells are broken down. The liver processes bilirubin and excretes it in bile. Raised bilirubin causes jaundice — a yellowing of the skin and whites of the eyes.

• Normal range — Approximately 3 to 21 µmol/L.
• Causes of raised bilirubin — Liver disease, bile duct obstruction, haemolytic anaemia (excessive red cell breakdown), and Gilbert's syndrome — a harmless inherited condition affecting around 5 to 10% of the population that can cause mildly elevated bilirubin, especially during illness, fasting, or stress.

Albumin

Albumin is a protein made by the liver. It plays a key role in maintaining blood volume and transporting substances through the blood. Low albumin can indicate chronic liver disease, malnutrition, or kidney disease (where albumin is lost in the urine).

• Normal range — Approximately 35 to 50 g/L.
• Low albumin — Can indicate advanced liver disease (cirrhosis), nephrotic syndrome, malabsorption, or prolonged illness.

Common Causes of Abnormal LFTs

The most common causes of abnormal liver function tests in the UK are:

• Non-alcoholic fatty liver disease (NAFLD) — Now the most common liver condition in the UK, affecting up to 30% of the population. Linked to obesity, type 2 diabetes, and metabolic syndrome.
• Alcohol-related liver disease — The second most common cause. Even modest regular drinking can cause elevated GGT and ALT.
• Medications — Statins, antibiotics, antifungals, and paracetamol can all cause temporary LFT elevation. Your GP will monitor this when starting new medications.
• Viral hepatitis — Hepatitis B and C can cause chronic liver inflammation. Screening is recommended for people born in high-prevalence regions.

When Should You Have LFTs?

Your GP may request liver function tests if you have symptoms such as abdominal pain, jaundice, dark urine, pale stools, fatigue, or loss of appetite. LFTs are also routinely checked when:

• Starting medications that can affect the liver (such as statins or methotrexate).
• Monitoring known liver conditions.
• As part of a general health check or well-person screen.
• Investigating abnormal results from other tests.

Getting Tested at Home

Liver function tests are straightforward and do not require fasting. A mobile phlebotomist from Lola Dispatch can collect your sample at home and send it to an accredited laboratory. This is ideal for routine monitoring, medication checks, or if you simply want a convenient way to keep an eye on your liver health.

Book a home blood test today and receive your results within 24 to 48 hours.