ALT, AST, ALP, and GGT: What Each Liver Marker Means
Last updated: 16 Sep 2026
You had a liver function test as part of your health screening. The results arrived with a set of abbreviations you do not recognise: ALT, AST, ALP, GGT. Some are within range. One or two might be flagged. You are not sure whether to worry, what the abbreviations mean, or what your doctor will do about them.
These four markers are enzymes found in your liver cells. When your liver is healthy, they stay inside the cells, and only small amounts appear in your blood. When liver cells are damaged, inflamed, or under stress, these enzymes leak into the bloodstream, and their levels rise.
But they are not interchangeable. Each one tells your doctor something slightly different about what is happening in your liver, and the pattern of which enzymes are elevated often matters more than any single number.
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ALT (Alanine Aminotransferase): The Liver-Specific Marker
ALT is the most liver-specific of the four. It is found predominantly in liver cells, with much smaller amounts in the kidneys, heart, and muscles. When ALT is elevated, the most likely source is the liver itself.
What it indicates when elevated: liver cell damage or inflammation. The most common causes of elevated ALT in Singapore include metabolic dysfunction-associated steatotic liver disease (MASLD, previously called non-alcoholic fatty liver disease), which is driven by visceral fat, insulin resistance, and excess refined carbohydrate intake. Alcohol-related liver damage. Viral hepatitis (hepatitis B is particularly relevant in Singapore, where the carrier rate remains significant). Medication side effects (certain painkillers, statins, antibiotics, and herbal supplements can elevate ALT). And, less commonly, autoimmune hepatitis.
What your doctor is thinking when ALT is elevated: the degree of elevation matters. Mildly elevated ALT (1 to 3 times the upper limit of normal) is extremely common and is most frequently caused by fatty liver or alcohol. Moderately elevated ALT (3 to 10 times) suggests more active liver inflammation. Markedly elevated ALT (more than 10 times) raises concern for acute hepatitis, drug-induced liver injury, or other acute liver conditions.
ALT is the single most important screening marker for fatty liver, which is why it is included in every comprehensive health screening at Regis Medical. A Singapore community study found that 40 per cent of participants had fatty liver on ultrasound. Many of them would have had mildly elevated ALT as their only clinical clue.
AST (Aspartate Aminotransferase): The Less Specific Partner
AST is found in the liver, but also in significant amounts in the heart, muscles, kidneys, brain, and red blood cells. This means elevated AST does not automatically point to the liver. It can also rise after intense exercise, muscle injury, a heart attack, or haemolysis (breakdown of red blood cells).
What it indicates when elevated: liver damage (in combination with elevated ALT) or non-liver sources of cellular injury.
The ALT-to-AST ratio. This is where the relationship between the two markers becomes diagnostically useful.
When ALT is higher than AST (ALT/AST ratio greater than 1), the pattern is most consistent with fatty liver disease, metabolic liver injury, or viral hepatitis. This is the most common pattern seen in metabolically driven liver conditions.
When AST is higher than ALT (AST/ALT ratio greater than 1, particularly greater than 2), the pattern raises concern for alcohol-related liver disease. Chronic alcohol consumption preferentially damages mitochondria, which releases more AST relative to ALT. This pattern is one of the clinical clues that may prompt your doctor to discuss alcohol intake, as we covered in our blog on alcohol and men's health.
When AST is elevated but ALT is normal, the source may be non-hepatic: recent intense exercise, muscle damage, cardiac injury, or a laboratory artefact.
Your doctor interprets AST alongside ALT rather than in isolation. The ratio between them provides diagnostic direction that neither number alone can offer.
ALP (Alkaline Phosphatase): The Bile Duct Marker
ALP is found in the liver (specifically in the cells lining the bile ducts), but also in bone, the intestines, kidneys, and placenta. Its presence across multiple tissues means that elevated ALP does not always indicate a liver problem.
What it indicates when elevated in the liver context: obstruction or disease of the bile ducts (the tubes that carry bile from the liver to the intestine). Conditions that elevate ALP through a hepatic mechanism include gallstones obstructing the bile duct, bile duct inflammation or infection (cholangitis), infiltrative liver diseases, and certain medications.
What it indicates from non-liver sources: bone disorders are the most common non-hepatic cause. Growing children and adolescents normally have elevated ALP because of active bone growth. In adults, elevated ALP with normal ALT and AST may indicate bone conditions such as Paget's disease, osteomalacia (vitamin D deficiency affecting bone mineralisation), or bone metastases.
How your doctor distinguishes the source: if ALP is elevated and ALT or GGT is also elevated, the source is most likely the liver (biliary system). If ALP is elevated but ALT, AST, and GGT are all normal, the source is more likely bone. Additional testing (GGT confirmation, vitamin D levels, or bone-specific ALP) can clarify.
GGT (Gamma-Glutamyl Transferase): The Sensitivity Marker
GGT is found primarily in the liver and bile ducts. It is the most sensitive liver enzyme, meaning it responds to liver stress earlier and at lower levels of damage than ALT or AST. However, this sensitivity comes at the cost of specificity: GGT can be elevated by many conditions, making it less precise as a standalone marker.
What it indicates when elevated: GGT rises in response to bile duct obstruction (like ALP, but more sensitive), fatty liver and metabolic liver disease, alcohol consumption (GGT is particularly responsive to alcohol, even moderate intake), medications that induce liver enzymes (including some anticonvulsants and certain supplements), and obesity and insulin resistance.
The alcohol connection. GGT is the marker most closely associated with alcohol intake. Elevated GGT in a patient with normal ALT and AST may be the earliest biochemical signal that alcohol is placing stress on the liver, even before fatty liver or hepatitis develops. If your GGT is elevated and you drink regularly, the connection is likely causal.
GGT and metabolic health. Beyond alcohol, GGT is increasingly recognised as a marker of broader metabolic dysfunction. Elevated GGT has been associated with insulin resistance, metabolic syndrome, cardiovascular risk, and Type 2 diabetes risk, independent of other liver markers. Some researchers consider it a marker of oxidative stress throughout the body, not just in the liver.
How your doctor uses GGT: GGT is most valuable in combination with other markers. GGT elevated alongside ALP confirms a biliary (liver) source for the ALP elevation. GGT elevated with normal ALP, ALT, and AST may indicate early metabolic stress or alcohol effect. GGT elevated alongside ALT suggests active liver injury from fatty liver, alcohol, or both.
What the Patterns Mean Together
Your doctor does not diagnose liver conditions from a single enzyme. The pattern of elevation across all four markers, interpreted alongside your medical history, alcohol intake, medications, weight, and metabolic health, provides the diagnostic direction.
ALT elevated, AST mildly elevated, ALP normal, GGT mildly elevated: most consistent with fatty liver disease. The most common pattern in Singaporean adults with metabolic risk factors.
AST greater than ALT, GGT elevated, ALP mildly elevated: raises concern for alcohol-related liver disease.
ALP and GGT elevated, ALT and AST normal or mildly elevated: suggests a biliary (bile duct) cause. Warrants further investigation, potentially including ultrasound imaging.
ALT and AST markedly elevated (more than 10 times), ALP and GGT variable: suggests acute liver injury. Requires urgent evaluation for viral hepatitis, drug-induced liver injury, or other acute causes.
All markers normal: your liver is functioning well. No further investigation needed.
When Elevated Liver Markers Deserve Follow-Up
Mildly elevated liver enzymes are extremely common, and the majority of cases are explained by fatty liver, alcohol, medication effects, or recent intense exercise. Most do not indicate serious liver disease.
However, elevated markers should not be dismissed. They deserve context. At our GP clinic, your doctor will interpret your liver function results alongside your metabolic markers (blood sugar, cholesterol), your alcohol history, your medications, your weight, and your waist circumference. If fatty liver is the likely cause, the same lifestyle interventions that improve metabolic health, weight loss, reduced refined carbohydrate intake, increased exercise, and reduced alcohol, are the primary treatment.
If markers remain elevated or worsen on repeat testing, further investigation (viral hepatitis screening, autoimmune markers, liver ultrasound) may be warranted.
A liver function test is part of every comprehensive health screening at Regis Medical. Understanding what each marker means helps you engage with your results as an informed participant in your own health rather than a passive recipient of abbreviations you do not recognise.
Your liver does not produce symptoms when it is under strain. These four enzymes are its way of telling you.
Liver Function Test for peace of mind
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