How to interpret liver function tests (LFTs)

What is in a liver function test?

Most Australian labs report ALT, GGT, ALP, total bilirubin, total protein, albumin and globulin as the standard LFT. Some leave AST out of the standard panel, so add it when you want the AST to ALT ratio. Globulin is calculated as total protein minus albumin.

Only albumin, and to a degree bilirubin, tells you how well the liver is working. The enzymes show liver cell injury or reduced bile flow, and several of them also come from muscle, bone or red cells.

LFT reference ranges and Vively optimal ranges

These are adult ranges from the Vively Biomarker Master List. Labs differ, so read each result against the range on your client’s report. ALP runs higher in teenagers and young adults while bones are still growing, and in pregnancy.

Vively optimal is a second reference for tracking change over time. It is not a diagnostic cut-off, and the lab range stays the primary reference. The ALT optimal bands sit close to the healthy upper limits Prati and colleagues proposed in 2002 (about 30 U/L in men and 19 U/L in women) after excluding people with fatty liver and metabolic risk.

What each marker mostly reflects

• ALT is made mainly in liver cells, so it is the most liver-specific of the enzymes.
• AST comes from the liver and also from skeletal muscle, heart muscle and red cells. It rises after hard training and in haemolysed samples.
• GGT sits in the bile ducts and is induced by alcohol, some medicines and fatty liver. Bone disease does not raise it, which makes it useful for working out where a high ALP is coming from.
• ALP comes from the bile ducts and from bone, and from the placenta in pregnancy.
• Bilirubin is the breakdown product of haemoglobin. The liver conjugates it and clears it into bile.
• Albumin is made only by the liver. A low result can reflect inflammation, low protein intake, losses through the kidney or gut, or reduced liver synthesis. A high result usually means dehydration.
• Globulin is mostly immune proteins. It rises with chronic inflammation and infection and, rarely, with a paraprotein.

Hepatocellular, cholestatic or mixed pattern

Compare how far ALT has risen with how far ALP has risen, each as a multiple of its upper limit. The R ratio puts a number on it: R = (ALT ÷ ALT upper limit) ÷ (ALP ÷ ALP upper limit). The American College of Gastroenterology guideline (Kwo et al., 2017) uses these bands:

• R above 5, hepatocellular. ALT and AST lead. Common causes are fatty liver (now called MASLD), alcohol, medicines and supplements, and viral hepatitis. Less common causes include haemochromatosis (check iron studies), coeliac disease, thyroid disease and muscle injury.
• R below 2, cholestatic. ALP and GGT lead, often with bilirubin. Common causes are gallstones, medicines, conditions of the bile ducts and pregnancy.
• R from 2 to 5, mixed. Features of both, often from medicines or infiltrative conditions.

Worked example: a man with ALT 160 U/L and ALP 80 U/L, on a lab with upper limits of 40 and 110. R = (160 ÷ 40) ÷ (80 ÷ 110) = 4 ÷ 0.73 = 5.5, a hepatocellular pattern. At four times the upper limit, his ALT also needs GP review.

The AST to ALT ratio

Divide AST by ALT. The ratio means most when at least one enzyme is raised. The Vively Master List uses 0.8 to 1.2 as the usual range.

• Below 1 (ALT higher than AST) is typical of fatty liver and of most viral and medicine-related liver injury.
• Above 2 with a raised GGT suggests alcohol-related liver injury (Cohen and Kaplan, 1979).
• Rising above 1 in a client with known fatty liver can mean fibrosis is progressing. The GP can calculate a FIB-4 score from age, AST, ALT and platelets to decide whether further assessment is needed.
• AST raised with a normal ALT points to muscle first. Check recent training, ask the GP about creatine kinase, and check whether the lab flagged the sample as haemolysed.

Isolated raised GGT

A GGT above the lab range with normal ALT, AST and ALP is common and usually not liver disease in itself. The usual reasons are regular alcohol, enzyme-inducing medicines such as phenytoin and carbamazepine, and fatty liver. Higher GGT, even inside the lab range, is also linked with metabolic and cardiovascular risk in large cohorts (Ruttmann et al., 2005), so read it with the metabolic markers below.

GGT falls slowly once alcohol is cut back, so retest after 4 to 8 weeks rather than days. A raised GGT alongside a raised ALP tells you the ALP is coming from the liver.

Isolated raised or low ALP

A raised ALP with a normal GGT usually comes from bone: growth in teenagers and young adults, a healing fracture, vitamin D deficiency or, in older clients, Paget’s disease. Pregnancy raises it too. A low ALP can go with zinc or magnesium deficiency, hypothyroidism and, rarely, the inherited condition hypophosphatasia.

Isolated raised bilirubin and Gilbert’s syndrome

A mildly raised total bilirubin, with normal ALT, AST, ALP and GGT and a normal full blood count, is most often Gilbert’s syndrome. It is a common inherited trait that slows bilirubin conjugation, and it is harmless. Bilirubin climbs further with fasting, illness, dehydration, stress and hard exercise, so a client who arrives fasted after a rough week can show a higher number than usual.

To confirm the pattern, the GP can request conjugated (direct) bilirubin. In Gilbert’s syndrome almost all of it is unconjugated. A raised conjugated bilirubin is a different picture and needs GP review, as does a raised bilirubin with a low haemoglobin, which can mean red cells are breaking down faster than usual.

Raised ALT with a high TG/HDL ratio: a metabolic signal

An ALT in the upper part of the range, or above Vively optimal, alongside high triglycerides and low HDL is one of the earliest blood signs of fatty liver linked to insulin resistance. That condition is now called metabolic dysfunction-associated steatotic liver disease, or MASLD (Rinella et al., 2023). The enzymes often stay inside the lab range for years.

• Look at direction first. A steady rise over successive tests, ideally from the same lab, matters more than one value.
• Read ALT with triglycerides, HDL and the TG/HDL ratio, fasting insulin or HOMA-IR, HbA1c and GGT. In clinic, add waist circumference and blood pressure.
• Triglycerides above the Vively optimal of 1.2 mmol/L with an ALT above optimal make the pattern more likely.
• Ask about alcohol, which raises the same markers.
• Raise a persistently high ALT, or an ALT above optimal with other metabolic risk, with the GP. They can check fibrosis risk with a FIB-4 score and arrange a liver ultrasound if needed.

Worked example: Sarah is a 44-year-old example client. Her ALT went 18, 24 then 31 U/L and her GGT went 14 then 24 U/L, all inside the lab range. Her ALT now sits above the women’s optimal of 10 to 20 U/L, and her fasting insulin, HbA1c and triglycerides rose over the same period. Taken one at a time, none of these is flagged. Read together, they point towards early metabolic dysfunction and possible fatty liver.

For the metabolic side of this pattern, read how to read insulin resistance in blood tests, and see the fasting insulin and HOMA-IR marker pages.

Before you read an LFT: exercise, alcohol and other traps

• Hard training. Heavy or unaccustomed resistance training can raise AST and ALT several-fold in healthy people, and the rise can last a week (Pettersson et al., 2008). Ask clients to skip heavy or new training for about a week before the test.
• Alcohol. A big weekend raises GGT and regular drinking keeps it up. Ask about the week before the test and the usual weekly intake.
• Medicines. Paracetamol, statins, amoxicillin with clavulanic acid, antiepileptics and methotrexate are common culprits. Statins often cause a small ALT rise.
• Supplements and herbs. Green tea extract, turmeric and curcumin, high-dose vitamin A, high-dose niacin, kava and some weight loss and bodybuilding products are recognised causes of liver injury (LiverTox). Ask about everything the client takes, including products you recommended.
• Fasting and illness. Both raise bilirubin in clients with Gilbert’s syndrome.
• Pregnancy and growth. ALP rises in pregnancy and while bones are growing, and albumin falls in pregnancy.
• Sample handling. A haemolysed sample raises AST. Check the lab comments before you read a lone high AST.

When to refer back to the GP

Optimal ranges do not replace red flags. Work within your own scope of practice.

Same day, urgent medical care:

• Jaundice (yellow skin or eyes), especially with dark urine or pale stools
• ALT or AST above 10 times the upper limit
• Abnormal LFTs with confusion, drowsiness, vomiting blood, black stools or severe abdominal pain

Back to the GP soon:

• ALT or AST above 3 times the upper limit
• ALP above 2 times the upper limit with a raised GGT, or any raised conjugated bilirubin
• Albumin below the lab range
• Globulin above the lab range without a clear reason
• Any raised enzyme that is still raised on a repeat test, or keeps rising
• Abnormal LFTs with unintended weight loss, itch, pain under the right ribs or fatigue that does not settle
• A raised ALT with high ferritin and high transferrin saturation

For red flags across other markers, see blood test red flags: when to refer a client back to the GP.

Ordering LFTs through Vively

Liver function tests are part of the Vively Baseline Health Check, and you can add them to any client panel. Results come back in the practitioner platform beside your client’s earlier results, so you can see the trend. You need to be a verified practitioner to refer tests. Request free practitioner access.

For clients: if a client wants to read up on a high GGT result, send them our plain-English GGT test page.

Parts of this page draw on Vively Practitioner Masterclass Part 1 with Dr Michelle Woolhouse. Watch the full session.

• Kwo PY, Cohen SM, Lim JK. ACG clinical guideline: evaluation of abnormal liver chemistries. American Journal of Gastroenterology. 2017.
• Newsome PN, Cramb R, Davison SM, et al. Guidelines on the management of abnormal liver blood tests. Gut. 2018.
• Prati D, Taioli E, Zanella A, et al. Updated definitions of healthy ranges for serum alanine aminotransferase levels. Annals of Internal Medicine. 2002.
• Cohen JA, Kaplan MM. The SGOT/SGPT ratio: an indicator of alcoholic liver disease. Digestive Diseases and Sciences. 1979.
• Ruttmann E, Brant LJ, Concin H, et al. Gamma-glutamyltransferase as a risk factor for cardiovascular disease mortality. Circulation. 2005.
• Pettersson J, Hindorf U, Persson P, et al. Muscular exercise can cause highly pathological liver function tests in healthy men. British Journal of Clinical Pharmacology. 2008.
• Rinella ME, Lazarus JV, Ratziu V, et al. A multisociety Delphi consensus statement on new fatty liver disease nomenclature. Hepatology. 2023.
• National Institute of Diabetes and Digestive and Kidney Diseases. LiverTox: clinical and research information on drug-induced liver injury.
• Royal College of Pathologists of Australasia. RCPA Manual: liver function tests.