ALT

Upper limits vary by lab from about 33 to 55, and some labs print a lower limit for women than men; use the range printed.

ALT is an enzyme found mainly inside liver cells. When liver cells are irritated or damaged, some of the enzyme leaks into the blood, so the level is a marker of liver cell injury rather than of how well the liver is working. The test measures the enzyme's activity in a blood sample.

It is the most widely used liver blood test and is part of the routine 'liver function tests' or 'LFT' panel. Doctors use it in general health checks, when starting or monitoring certain medicines, and when looking into tiredness, abdominal symptoms or a fatty liver.

• Fatty liver linked to weight, diabetes or alcohol
• Viral hepatitis and other infections
• Alcohol, especially in the days before the test
• Many medicines, including statins, some antibiotics, painkillers and herbal remedies
• Very hard exercise, which releases enzyme from muscle
• Coeliac disease and some thyroid conditions

• Low vitamin B6
• Older age and low body weight, by a small amount
• Long-standing kidney disease

• A hard workout or a heavy weekend of alcohol before the test can raise it for a few days
• A small rise is common in the general population and often settles on repeat
• Lab upper limits differ, so the same value can be flagged by one lab and not another

No fasting needed. Avoiding hard exercise and alcohol for a couple of days beforehand gives a truer reading. A single mild result is usually repeated before anything else is done.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.