Liver Function Tests Explained: ALT, AST, GGT and What They Mean

Your liver is one of the hardest-working organs in your body. It filters toxins, produces bile for digestion, stores energy, synthesises proteins, and plays a central role in metabolism. Yet most of us rarely think about our liver until something goes wrong. Liver function tests (LFTs) are a group of blood tests that provide a window into how well your liver is performing — and understanding them is essential for maintaining long-term health.

What Are Liver Function Tests?

Liver function tests, sometimes called a "liver panel" or "hepatic function panel," measure a combination of enzymes and proteins in your blood. Despite their name, LFTs don’t directly measure liver function — they primarily detect liver damage or inflammation. The key markers included in a standard Australian LFT panel are alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transferase (GGT), alkaline phosphatase (ALP), and bilirubin. Some panels also include total protein and albumin, which do reflect the liver’s synthetic function.

ALT (Alanine Aminotransferase)

ALT is an enzyme found predominantly in liver cells. When liver cells are damaged or inflamed, ALT leaks into the bloodstream, causing levels to rise. This makes ALT one of the most specific markers for liver injury. The normal reference range in most Australian laboratories is approximately 5–40 U/L, though some labs have recently lowered the upper limit to 25 U/L for women and 33 U/L for men based on updated evidence.

Common causes of elevated ALT include non-alcoholic fatty liver disease (NAFLD), viral hepatitis (hepatitis B and C), alcohol-related liver disease, certain medications (including paracetamol and some statins), and autoimmune hepatitis. Mildly elevated ALT is extremely common in Australia — studies suggest that up to 20% of the adult population may have abnormal liver enzymes, largely driven by the rising prevalence of NAFLD.

AST (Aspartate Aminotransferase)

AST is an enzyme found in the liver but also in the heart, muscles, kidneys, and brain. Because it’s less specific to the liver than ALT, an elevated AST alone doesn’t necessarily indicate liver disease — it could also reflect muscle damage from intense exercise, a recent injury, or even a heart attack. The typical reference range is 5–40 U/L.

The ratio of AST to ALT can provide important diagnostic clues. In most forms of liver disease, ALT is higher than AST. However, when AST exceeds ALT (an AST:ALT ratio greater than 2:1), this pattern is suggestive of alcohol-related liver disease. This ratio is a simple but powerful tool that clinicians use to differentiate between causes of liver enzyme elevation.

GGT (Gamma-Glutamyl Transferase)

GGT is an enzyme found in the liver, bile ducts, pancreas, and kidneys. It is one of the most sensitive markers for liver and bile duct disease, but its lack of specificity means it can be elevated by a wide range of factors. The reference range is typically 5–50 U/L for women and 5–60 U/L for men.

GGT is particularly sensitive to alcohol consumption — even moderate drinking can cause levels to rise. It is also commonly elevated in NAFLD, bile duct obstruction, pancreatitis, and with the use of certain medications including anticonvulsants and some antibiotics. Importantly, elevated GGT has been associated with increased cardiovascular risk independent of alcohol consumption, making it a marker worth monitoring even if you don’t drink.

ALP (Alkaline Phosphatase)

ALP is an enzyme found in the liver, bones, kidneys, and intestines. In the context of liver health, elevated ALP typically suggests bile duct obstruction or cholestatic (bile flow) problems. The reference range is generally 30–110 U/L, though this varies by age and sex. Elevated ALP in isolation is often related to bone conditions rather than liver disease — particularly in growing adolescents, pregnant women, and individuals with vitamin D deficiency or Paget’s disease.

When ALP is elevated alongside GGT, this strongly suggests a hepatobiliary (liver and bile duct) cause rather than a bone-related cause. Your doctor can order a GGT specifically to help differentiate between these possibilities.

Bilirubin

Bilirubin is a yellow pigment produced when red blood cells break down. The liver processes bilirubin and excretes it in bile. Elevated bilirubin can cause jaundice — a yellowing of the skin and eyes. The normal range is approximately 2–20 µmol/L in most Australian laboratories.

Mildly elevated bilirubin is most commonly caused by Gilbert’s syndrome, a benign genetic condition affecting approximately 5–10% of the population. In Gilbert’s syndrome, bilirubin levels rise during fasting, illness, or stress but cause no liver damage. Higher elevations may indicate hepatitis, bile duct obstruction, or haemolytic anaemia (excessive red blood cell breakdown).

Albumin and Total Protein

Albumin is a protein synthesised by the liver, and it’s one of the few LFT markers that actually reflects liver function rather than damage. Low albumin (below 35 g/L) can indicate chronic liver disease, malnutrition, kidney disease, or chronic inflammation. Total protein levels provide additional context but are less specific. In advanced liver disease (cirrhosis), the liver’s ability to produce albumin declines, making this a marker of disease severity.

What to Do if Your LFTs Are Abnormal

If your liver function tests come back elevated, the first step is not to panic. Mild, isolated elevations are common and often transient. Your doctor will typically consider your medical history, alcohol intake, medications, body weight, and risk factors for viral hepatitis before deciding on next steps.

Common follow-up investigations include repeating the LFTs after two to four weeks, hepatitis B and C serology, a liver ultrasound, and iron studies (to exclude haemochromatosis). In many cases, lifestyle modifications — such as reducing alcohol intake, improving diet, increasing physical activity, and losing excess weight — can normalise liver enzymes within months.

Tracking your LFTs over time with a platform like Yearly allows you to see how your liver health responds to these changes, providing motivation and accountability. If your enzymes remain persistently elevated despite lifestyle modifications, your GP may refer you to a hepatologist (liver specialist) for further assessment.

Protecting Your Liver Health

The good news is that the liver has remarkable regenerative capacity. Even in cases of significant fatty liver disease, weight loss of just 5–10% of body weight has been shown to substantially improve liver enzymes and reduce liver fat. Limiting alcohol consumption, maintaining a healthy weight, staying physically active, and avoiding unnecessary medications are the cornerstones of liver health. Annual LFTs are a simple and effective way to ensure your liver is coping with the demands you place on it.

References

1. Gastroenterological Society of Australia (GESA). "Australian guidelines for the assessment and management of non-alcoholic fatty liver disease (NAFLD)." 2023.
2. Prati, D. et al. "Updated definitions of healthy ranges for serum alanine aminotransferase levels." Annals of Internal Medicine, vol. 137, no. 1, 2002, pp. 1–10.
3. Whitfield, J.B. "Gamma glutamyl transferase." Critical Reviews in Clinical Laboratory Sciences, vol. 38, no. 4, 2001, pp. 263–355.
4. Adams, L.A. et al. "The natural history of nonalcoholic fatty liver disease: a population-based cohort study." Gastroenterology, vol. 129, no. 1, 2005, pp. 113–121.
5. The Royal Australian College of General Practitioners (RACGP). "Guidelines for preventive activities in general practice." 10th Edition, 2025.