When a person has evidence of liver disease, very high ALP levels can tell the doctor that the person’s bile ducts are somehow partially or totally blocked or inflamed. Often, ALP is high in people who have cancer that has spread to the liver or the bones, and doctors can do further testing to see if this has happened. If a person with bone or liver cancer responds to treatment, ALP levels will decrease. When a person has high levels of ALP, and the doctor is unsure why, ALP isoenzyme tests might be requested to try to determine the cause.

ALP is generally part of a routine laboratory test profile called liver function tests. It is usually requested with several other tests if a patient has symptoms of a liver or bone disorder.

Raised levels of ALP are usually due to a disorder of either the bone or liver. If other liver function tests such as bilirubin, gamma-glutamyl transferase (GGT) or alanine aminotransferase (ALT) are also raised, this usually indicates that the ALP is coming from the liver. However if calcium and phosphate measurements are abnormal, this suggests that the ALP might be coming from bone. In some forms of liver disease, such as hepatitis, ALP is usually much less elevated than AST or ALT. However, when the bile ducts are blocked (for example by gallstones, scars from previous gallstones or surgery, or by a tumour), ALP and bilirubin may be increased much more than either AST or ALT. ALP can also be raised in bone diseases such as Paget’s disease (where bones become enlarged and deformed), or in certain cancers that spread to bone.

Pregnancy can increase ALP levels. Children have higher ALP levels because their bones are growing, and ALP is often very high during the 'growth spurt' which occurs at different ages in males and females.

Eating a meal can increase the ALP level slightly for a few hours in some people. Ideally the test should be done after fasting overnight. Some drugs may increase ALP levels, especially some of the drugs used to treat psychiatric problems, but significant increases are rare.