AP (alkaline phosphatase)

Alkaline phosphatase (AP, also ALP in the literature) is an enzyme found mainly in the liver, bile ducts and bones. The test measures total alkaline phosphatase in blood and is most often performed as part of liver tests or when bone disease is suspected. A raised value shows that one of these tissues is more active or under strain; a doctor identifies which one alongside other results.

Who the test is for

• people having a general health examination, as part of liver tests, and when liver or biliary disease is suspected;
• people with yellowing skin and eyes, itchy skin, dark urine, pale stools or pain under the right rib margin;
• people with bone pain and when Paget’s disease or softening of the bones due to vitamin D deficiency is suspected;
• people being treated for liver or bone disease and those taking medicines that may affect the liver;
• people with suspected spread of malignancy to the liver or bones, as part of a doctor-led investigation.

What alkaline phosphatase is and what it shows

In adults, almost all alkaline phosphatase in the blood comes from the liver and bones, approximately half from each, with a smaller amount from the intestine. During pregnancy, a placental form contributes, while the bone component predominates in growing children.

In the liver, the enzyme is found in cells lining the bile canaliculi and rises most when bile flow is obstructed. In bone, it is produced by osteoblasts, cells that build new bone, so it rises when bone formation increases. The test alone does not show the source of the rise, so it is interpreted alongside other results, primarily GGT.

Preparation and sample

Blood is taken from a vein. Fasting is not essential for alkaline phosphatase, but if glucose or lipids are being tested from the same collection, come fasting. In some people, more often those with blood group 0 or B, a fatty meal temporarily raises the intestinal form of alkaline phosphatase, so avoid a fatty meal before blood collection. Tell staff which medicines you take, particularly anti-epileptic medicines, and whether you are pregnant or have recently had a fracture.

How to interpret the result

The reference interval depends primarily on age, then on sex and method, and is printed on the report. In children, the result is always compared with values for their age. A doctor interprets the result alongside other tests and symptoms.

When a higher value is expected

• Children and adolescents: because of bone growth, alkaline phosphatase is several times higher than in adults, particularly during growth spurts. A very high value without disease is sometimes found in young children (transient hyperphosphatasaemia), returning to normal by itself within a few months.
• Pregnancy: in the third trimester alkaline phosphatase rises due to the placental form and returns to its usual level a few weeks after birth.
• Fracture healing temporarily raises the bone component of the enzyme.

Raised alkaline phosphatase and the liver

If GGT is raised alongside alkaline phosphatase, the source is probably the liver or bile ducts. The most common causes are:

• stones or narrowing of the bile ducts;
• chronic inflammatory bile duct diseases, such as primary biliary cholangitis;
• medicines that slow bile flow, such as some antibiotics, and anti-epileptic medicines;
• fatty liver, alcohol and hepatitis, where the increase is usually mild and AST and ALT are more prominent;
• tumour deposits (metastases) in the liver.

The doctor then usually also requests bilirubin, AST and ALT and abdominal ultrasound, which shows whether the bile ducts are dilated (abdominal ultrasound is available only in Split).

Raised alkaline phosphatase and bones

If GGT and other liver results are normal, bones are the more likely source. Possibilities then include:

• Paget’s disease of bone, in which values can be very high;
• osteomalacia in adults and rickets in children, most often due to long-term vitamin D deficiency;
• overactive parathyroid glands (hyperparathyroidism);
• malignant tumour deposits in the bones.

A bone source can be confirmed with the bone ALPtest, and the doctor usually also requests calcium, phosphate, vitamin D and PTH.

Low alkaline phosphatase

A low value is less often clinically important. It can occur with malnutrition, zinc or magnesium deficiency and an underactive thyroid. Persistently very low values, particularly with bone pain or frequent fractures, may indicate the rare inherited disease hypophosphatasia.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Prices for all tests and panels are available in the service price list. Alkaline phosphatase is part of the Extended liver health panel, which also includes AST, ALT, GGT, total bilirubin and total protein.

Frequently asked questions

What does mildly raised alkaline phosphatase mean?

A mild increase is often of little significance, for example after a fatty meal, during fracture healing or in pregnancy. If GGT is also raised, the doctor looks for the cause in the liver and bile ducts; otherwise, in the bones. The test is usually repeated without a preceding fatty meal, alongside other liver tests.

Why is alkaline phosphatase raised in pregnancy?

The placenta produces its own form of alkaline phosphatase, so the level in the third trimester may be several times higher than before pregnancy, which is expected. Nevertheless, mention itchy skin in pregnancy, particularly on the palms and soles, to your gynaecologist because it may indicate cholestasis of pregnancy, which is checked using other tests.

Why do children have high alkaline phosphatase?

In children and adolescents, bone-building cells are very active in growth zones, making alkaline phosphatase several times higher than in adults. It falls to adult levels when growth ends.

Is raised alkaline phosphatase a sign of cancer?

Usually not. Much more common causes are biliary disease, medicines, growth, pregnancy and vitamin D deficiency. Deposits in the liver or bones can raise alkaline phosphatase, but it is not a tumour marker and is not used to detect cancer.