Liver function tests explained: ALT, AST, ALP, bilirubin and albumin

What the liver does, and what “liver function tests” means

The liver processes nutrients and medicines, makes proteins such as albumin and clotting factors, produces bile that helps digest fat, and clears waste products including bilirubin. It is large, resilient and can be damaged for a long time without symptoms, so blood tests are an important way of noticing problems.

The name “liver function tests” (LFTs) is slightly misleading. Some of the tests show that liver cells are irritated or damaged, and only a few show how well the liver is performing its jobs. A report may also be titled liver panel or hepatic function panel. It usually contains:

• ALT and AST: enzymes released when liver cells are injured.
• ALP and GGT: enzymes linked to the bile ducts.
• Bilirubin, total and direct: a pigment handled by the liver and bile system.
• Albumin, total protein and sometimes the albumin-to-globulin (A/G) ratio: proteins that reflect the liver’s production.

The injury markers: ALT and AST

Alanine aminotransferase (ALT, previously SGPT) is found mainly in the liver, so a raised ALT is a fairly liver-specific signal. Aspartate aminotransferase (AST, previously SGOT) is also found in the liver but in heart and skeletal muscle and elsewhere, so it is less specific. Both are reported in units per litre (U/L, sometimes written IU/L).

Reference ranges differ with laboratory and sex, but an ALT of up to about 35 to 45 U/L and an AST of up to about 35 to 40 U/L are commonly printed as the upper limits. Some experts argue the upper limit for ALT should be lower than many laboratories print.

Doctors look at how high the values are, whether they are rising or falling, and the ratio of AST to ALT. As a very rough guide, a mild rise of ALT and AST is common and has many possible causes, such as fatty liver, medicines or a recent viral illness, while very high values suggest more acute injury. In some patterns, such as alcohol-related liver disease, AST is higher than ALT, but a ratio alone does not diagnose anything.

AST and ALT can also be raised by strenuous exercise, especially weight training or long runs, because muscle releases them too. If you worked out hard the day before a test, say so.

The bile-duct markers: ALP and GGT

Alkaline phosphatase (ALP) comes from the liver and bile ducts but also from bone, the intestine and, during pregnancy, the placenta. It is naturally higher in growing children and teenagers and in pregnancy. A raised ALP with a normal GGT often points to bone as the source, while a raised ALP with a raised GGT suggests the liver or bile ducts. GGT (gamma-glutamyl transferase) can also be raised by alcohol and by some medicines.

Typical adult ALP ranges are about 40 to 130 U/L, although they vary by laboratory and age.

Bilirubin

Bilirubin is a yellow-orange pigment produced when red blood cells are broken down. The liver processes it so it can be passed into bile. The report usually gives total bilirubin and “direct” (conjugated) bilirubin, with “indirect” bilirubin worked out by subtraction.

In India bilirubin is printed in mg/dL, and total bilirubin is typically about 0.3 to 1.2 mg/dL. Elsewhere it is in µmol/L: multiply mg/dL by 17.1, so 1.2 mg/dL is about 20.5 µmol/L.

A raised bilirubin can make the skin and whites of the eyes look yellow (jaundice), and it can come from several different places: faster red cell breakdown, a liver cell problem, or a blockage in the bile ducts. The split between direct and indirect bilirubin helps a doctor work out which. A mild, isolated rise in indirect bilirubin with otherwise normal liver tests is common and often harmless, such as in a condition called Gilbert’s syndrome, but a doctor must decide that. If you or your child develop jaundice, see a doctor rather than waiting for the next routine test.

Albumin and total protein

Albumin is the main protein made by the liver, and it keeps fluid inside blood vessels. A typical range is about 3.5 to 5.0 g/dL, or 35 to 50 g/L (multiply g/dL by 10). Because albumin has a half-life of around three weeks, a low value reflects a longer-standing problem, rather than something that happened yesterday.

A low albumin is not specific to the liver. It also falls in kidney disease that leaks protein into urine, in poor nutrition, in long-term illness and inflammation, and with dehydration or fluid overload changing blood concentration. For that reason, doctors interpret it alongside other results.

Total protein is albumin plus globulins. The A/G ratio compares them, and a changed ratio is a clue, not a diagnosis.

Putting the pattern together

Doctors often group results into broad patterns. This table is a simplified illustration, not a diagnostic tool.

Several other liver tests, such as the clotting test INR, are added when a doctor is concerned about how well the liver is functioning. If a raised ALT persists, doctors usually look for causes with further blood tests, such as viral hepatitis markers, and often an ultrasound scan.

Medicines, supplements and lifestyle

Many substances pass through the liver, so a doctor will ask about all of them, including:

• Prescription medicines, such as some antibiotics, anti-tuberculosis drugs, anti-seizure drugs, some cholesterol and diabetes medicines and others.
• Over-the-counter pain relievers and fever medicines. The safe limit for paracetamol in particular depends on the person, so only use it as directed.
• Herbal, Ayurvedic and other traditional products, and body-building or weight-loss supplements. Some of these have been linked to liver injury even though they are sold as natural.
• Alcohol, even in modest amounts if there is existing liver disease.
• Body weight and diet: fatty liver disease associated with excess weight, diabetes and high cholesterol is very common in India and is a frequent reason for mildly raised ALT.

Please do not stop a prescribed medicine because a liver test is high without speaking to the doctor who prescribed it. Often the benefit outweighs the risk, and doctors can monitor the test instead.

A worked example (example values)

Imagine an example report with ALT 78 U/L, AST 52 U/L, ALP 96 U/L, total bilirubin 0.8 mg/dL and albumin 4.3 g/dL, where the laboratory’s upper limit for ALT is 40 U/L.

ALT is almost twice the upper limit, AST is mildly raised and ALP, bilirubin and albumin are normal. The liver’s productive and bile-handling functions look intact, which is reassuring, while there is a sign of irritation of liver cells. Doctors would ask about weight, diabetes, alcohol, medicines and supplements, family history and recent illnesses, then often repeat the test after a few weeks and consider an ultrasound. They would not conclude anything from the numbers alone.

When the liver tests are repeated

A mildly raised enzyme in a person who feels well is often simply repeated after a few weeks to see whether it has settled, particularly if a recent illness, a new medicine or heavy exercise could explain it. Persistent abnormalities lead to a search for the cause, which usually includes a review of medicines and alcohol, blood tests for viral hepatitis and other conditions, and an ultrasound. If the numbers are rising, or if you have symptoms such as yellow eyes, dark urine, pale stools, severe itching, abdominal swelling or persistent vomiting, tell a doctor promptly instead of waiting for a scheduled test.

Vaccination against hepatitis B and, where advised, hepatitis A is a topic many doctors raise when liver tests are abnormal, and it is worth asking whether you are protected.

Questions worth asking your doctor

• Which of my liver results are outside the range, and how much?
• Could any of my medicines or supplements explain the result?
• Do I need an ultrasound or other tests, and when should I repeat this one?
• Is there anything I should avoid until the next test?
• What symptoms would mean I should contact you sooner?

Sources

• MedlinePlus: Liver Function Tests — overview of the panel
• MedlinePlus: ALT Blood Test — what ALT shows
• MedlinePlus: AST Test — what AST shows
• MedlinePlus: Alkaline Phosphatase — ALP sources and results
• MedlinePlus: Bilirubin Blood Test — total and direct bilirubin
• NHS: Liver disease — causes, symptoms and tests

A note on this guide

This guide is general information about liver test reports. It is not medical advice and cannot tell you whether your liver is healthy. Do not stop any medicine because of a result: confirm what it means with your doctor.