What a liver function test measures
MedlinePlus explains that the most common liver function tests measure:
- Enzymes: ALT (alanine transaminase, often printed as SGPT), AST (aspartate aminotransferase, called SGOT), ALP (alkaline phosphatase) and GGT (gamma-glutamyl transferase). These are proteins made mainly in the liver.
- Bilirubin: a waste product made when old red blood cells break down. The liver removes most of it from the body.
- Proteins: albumin, a protein made in the liver, and total protein, which includes albumin and globulins.
- Sometimes more: prothrombin time (how long blood takes to clot) or LDH.
Some of these show how well the liver is working, while others show whether it may be damaged. MedlinePlus stresses that liver function tests alone usually cannot diagnose a specific disease. At our centre, the diagnostics catalogue lists the Liver Function Test (LFT) as well as separate SGOT (AST), SGPT (ALT), bilirubin total and direct, and neonatal bilirubin tests.
Liver function test normal range table
These are the common adult ranges given in the MedlinePlus Medical Encyclopedia. Units differ between labs, so check the unit as well as the number.
| Test (other name) | Common normal range (adults) |
|---|---|
| ALT (SGPT) | 4 to 36 U/L |
| AST (SGOT) | 8 to 33 U/L |
| Alkaline phosphatase (ALP) | 40 to 129 IU/L (varies with age and sex) |
| Gamma-glutamyl transferase (GGT) | 5 to 40 U/L |
| Total bilirubin | 0.1 to 1.2 mg/dL (1.71 to 20.5 µmol/L) |
| Direct (conjugated) bilirubin | Less than 0.3 mg/dL (less than 5.1 µmol/L) |
| Albumin | 3.4 to 5.4 g/dL (34 to 54 g/L) |
| Total protein | 6.5 to 8.1 g/dL (65 to 81 g/L) |
MedlinePlus notes that ALP is normally higher in children going through growth spurts and in pregnant women, so an adult range does not apply to them.
SGPT (ALT) and SGOT (AST)
ALT is found in high levels in the liver, and AST in the liver, heart and muscles. When liver cells are injured, these enzymes are released into the blood. MedlinePlus lists possible reasons for raised levels, including hepatitis (a swollen, inflamed liver), fatty liver, scarring of the liver (cirrhosis), too much iron in the body, reduced blood flow to the liver, liver tumours, and medicines or drugs that are toxic to the liver, such as alcohol or acetaminophen (paracetamol). AST can also rise after a heart attack or with muscle injury, which is why doctors read it together with ALT.
A raised ALT is more likely to point to liver disease when the other liver tests are also abnormal.
Bilirubin and jaundice
Bilirubin is left in the blood after old red blood cells are removed, and the liver helps clear it in the stool. MedlinePlus says a blood bilirubin level of about 2.0 mg/dL can cause jaundice, the yellow colour of the skin and eyes, which is the most common reason the test is ordered.
A high bilirubin level can come from liver problems such as hepatitis or cirrhosis, from gallstones or a narrowed bile duct, from conditions in which red blood cells break down faster than normal, or from Gilbert disease, in which the liver processes bilirubin differently. Newborn babies normally have higher bilirubin in the first few days of life, and their doctor judges the level using the baby's age, whether the baby was born early and how fast the level is rising.
ALP and GGT
ALP is found in the liver and in bone. MedlinePlus lists blocked bile ducts, hepatitis and other liver disease, but also bone disease, a healing fracture and an overactive parathyroid gland, among the causes of a high ALP. That is why a high ALP on its own does not prove a liver problem.
GGT is mainly a liver enzyme. MedlinePlus says the GGT test may also be used to screen for or monitor alcohol use, and that alcohol and medicines that are toxic to the liver can raise it.
Albumin and total protein
Albumin keeps fluid inside the blood vessels and carries substances such as bilirubin and some medicines. MedlinePlus says a low albumin may be a sign of liver disease or kidney disease, or of the body not getting or absorbing enough protein. Total protein is also checked for nutrition problems, kidney disease and liver disease. Because the kidneys also affect albumin, your doctor may order a kidney function test at the same time.
Who needs a liver function test?
Many liver tests are part of routine check-ups. According to MedlinePlus, you may need liver function tests if you have symptoms such as nausea and vomiting, loss of appetite, tiredness or weakness, jaundice, swelling or pain in the abdomen, swelling in the ankles and legs, dark urine or pale stools, or frequent itching.
Testing may also be advised without symptoms if you have a higher risk of liver damage, for example a family history of liver disease, alcohol use disorder, obesity, diabetes, exposure to hepatitis, or a medicine that can affect the liver. People already treated for liver disease are tested to follow progress.
MedlinePlus adds that if you have been ill recently, some abnormal results may be temporary, and your doctor may simply repeat the test later. Your doctor may also order a CBC test when jaundice or tiredness is being investigated.
Is fasting needed for an LFT? How to prepare
MedlinePlus says your provider will tell you how to prepare and that you will probably need to fast (no food or drink) for 10 to 12 hours before a liver panel. Some single tests, such as ALT and AST, need no special preparation, so follow your own lab's instructions:
- Confirm the fasting time. Ask the lab whether to fast. If you are told to fast for 10 to 12 hours, an overnight fast with an early-morning sample is the simplest way; our fasting blood test guide explains how to manage it.
- Avoid alcohol. Avoid alcohol before the test unless your doctor says otherwise, because alcohol affects GGT and other liver enzymes.
- List your medicines and supplements. Tell the doctor about every medicine, herbal product and supplement you take. Many medicines change liver test results, but do not stop any without medical advice.
- Mention recent illness. Tell the doctor if you have recently had a fever or other illness, since some changes in liver tests can be temporary.
- Review the whole report with a doctor. Ask a doctor to read all the values together; the pattern matters more than any single number.