AST/ALT ratio: what the liver enzyme ratio can and cannot tell you

Liver blood tests report two transaminase enzymes, ALT (alanine aminotransferase) and AST (aspartate aminotransferase). Both leak into the blood when cells are injured. The ratio is AST divided by ALT, and some laboratories print it on the report while others leave it for the clinician to work out. It carries no unit.

The reason a ratio can be informative lies in where the enzymes are found. Lab Tests Online UK describes ALT as an enzyme mainly found in the liver and the best test for detecting hepatitis, while AST is found in the liver and a few other places, particularly the heart and other muscles.

When both enzymes are raised, comparing them helps decide how the liver is being injured. According to Lab Tests Online UK, liver disease is more likely when the values of AST and ALT are higher, with ALT rising more than AST in acute liver damage such as hepatitis. The same source says comparing the results helps determine which type of liver disease might be present, and that a high AST to ALT ratio may help predict long-term complications such as cirrhosis in people with chronic viral hepatitis, chronic alcoholism or non-alcoholic fatty liver disease.

The ratio is therefore best seen as one of several patterns, together with ALP, GGT, bilirubin and albumin, rather than a stand-alone test.

Very high values, more than ten times the upper limit, usually point to acute hepatitis, often viral, and the enzymes may stay high for one to two months, sometimes three to six months, before settling, as Lab Tests Online UK explains. Chronic hepatitis tends to give lower, fluctuating values, often less than four times the upper limit, and moderate rises can accompany high alcohol intake, diabetes or raised triglycerides, which cause fatty liver.

In some conditions, especially blocked bile ducts, cirrhosis or liver cancer, ALT can be close to normal. That is why a normal result does not exclude significant liver disease.

AST in particular can come from muscle. Lab Tests Online UK notes that when there is doubt about the source of high enzyme levels, a raised creatine kinase suggests muscle damage and troponin shows whether the heart is damaged. Strenuous exercise or intramuscular injections can raise both enzymes, as can some medicines and herbal products, and haemolysis can raise AST.

Lab Tests Online UK also says many conditions including shock, burns, severe infections, trauma, muscle damage from severe exercise and pregnancy can make liver blood tests abnormal without liver disease, and conversely normal results can be found in people with serious liver disease.

No special preparation is needed. Tell the clinician about all prescription and over-the-counter medicines, herbal and dietary supplements, alcohol intake, and recent heavy exercise or injections. Paracetamol overuse, particularly with alcohol, can cause severe liver damage, so be honest about doses.

Abnormal enzymes are usually repeated and followed by tests for hepatitis viruses, an ultrasound of the liver and a review of medicines and alcohol. Depending on the picture, doctors may request iron studies, autoimmune markers, AFP or tests for inherited conditions such as haemochromatosis, Wilson's disease and alpha-1 antitrypsin deficiency.

Scoring tools and scans that assess scarring may be used when chronic liver disease is suspected. Seek urgent help for yellowing of the skin or eyes with confusion, vomiting blood or black stools; call 999 or go to A&E.