Alkaline Phosphatase (ALP)

What is alkaline phosphatase (ALP)?

Alkaline phosphatase is an enzyme found in many tissues of the body; most of the ALP in the blood comes from the liver and bones. The enzyme takes its name from the fact that it works best in an alkaline, or basic, environment. It is also produced in smaller amounts in the intestine, in the kidneys and, during pregnancy, in the placenta. In the liver it is concentrated particularly in the cells lining the bile ducts, and in bone in the osteoblasts, the cells that build new bone tissue.

The ALP test measures total enzyme activity in blood serum, and the result is reported in U/L. When bile flow is blocked or bone formation speeds up, more of this enzyme passes into the blood and its level rises. The test therefore offers clues about both the liver and bile ducts and bone health; on its own, however, it does not tell which tissue the increase comes from.

When is the alkaline phosphatase test ordered?

ALP is usually ordered as part of the liver function tests, together with GGT, bilirubin, AST and ALT. When bone disease is suspected, it is evaluated together with tests such as calcium, phosphorus, vitamin D and parathyroid hormone. It is also frequently included in the biochemistry panels ordered during routine health check-ups.

In people diagnosed with liver or bone disease, ALP may be repeated at regular intervals to monitor the course of the disease and the response to treatment. It may also be among the follow-up tests for people who take medicines that affect bone structure over a long period. Your doctor may also use this test to monitor certain types of cancer that can spread to the bones.

• Jaundice, dark urine or pale stools
• Upper right abdominal pain, itching or suspected gallstones
• Unexplained bone pain or bones that break easily
• Monitoring of bone diseases such as Paget's disease
• Monitoring while taking medicines that may affect the liver

How should you prepare for the alkaline phosphatase test?

Fasting for 8–12 hours is generally recommended for ALP, because, especially after a fatty meal, ALP from the intestine can pass into the blood and raise the result slightly. This effect may be more pronounced in people with certain blood groups. You can drink water while fasting; giving the blood sample in the morning makes it easier to complete the fasting period.

Some antibiotics, epilepsy medicines and hormone-containing medicines can affect ALP levels, so tell your doctor about the medicines and supplements you take; do not stop any medicine on your own. It is also important to mention if you have recently had a broken bone, because ALP can rise during healing. The blood sample is taken from a vein in the arm, and the procedure is completed within a few minutes. If you wish, our nurse can take this sample at your home or workplace.

What does the alkaline phosphatase result mean?

High ALP may be associated with bile duct obstruction, liver diseases, the effects of certain medicines or conditions in which bone formation is accelerated. On the bone side, the main causes include Paget's disease, fracture healing, softening of the bones due to vitamin D deficiency and an overactive parathyroid gland. To tell whether the source is the liver or the bone, your doctor may order additional tests such as GGT or ALP isoenzymes.

In growing children and in late pregnancy, high values are mostly considered normal. Low ALP is less common; it may be associated with severe malnutrition, zinc or magnesium deficiency, or rare inherited conditions such as hypophosphatasia. The reference range differs considerably by age, sex and laboratory; on this page you can see the range of the laboratory that performs the analysis. The final assessment rests with your doctor, who considers your result together with your symptoms and other tests.