Alkaline phosphatase

What is being tested?
Alkaline phosphatase is an enzyme, a protein involved in the vital processes of cells. It is found in high concentrations in liver cells and bone cells. In the liver it is found in the membranes of the cells lining the bile ducts, small channels that gradually widen and carry bile from the liver to the intestine; bile is used to digest dietary fats. Small amounts of ALP are found in the placenta of pregnant women and in the intestine. These different tissues produce different forms of ALP, called isoenzymes.

How is the sample collected for testing?
A blood sample is taken from a vein in the arm.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
In liver disease, high ALP levels indicate that bile is not flowing properly through the bile ducts because something is obstructing it. In patients with cancer, high ALP values may be due to the tumour having spread to the liver or bone; in this case further investigations will be needed. If a patient with liver or bone cancer responds to chemotherapy, ALP values fall. When a rise in ALP cannot be explained, the isoenzyme fractions may be typed to find out where it comes from.

When is it requested?
The ALP enzyme is generally requested together with other specific tests to assess liver disease (the so-called “liver profile”). It is also requested when a patient shows signs of liver or bone disease.

What does the test result mean?
High ALP levels usually indicate liver or bone damage. If other liver profile tests such as bilirubin, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are also high, the raised alkaline phosphatase comes from the liver. If blood calcium and phosphate measurements are abnormal, the ALP probably comes from bone.

In some liver diseases, such as hepatitis, ALP usually rises less than AST and ALT. When the bile ducts are blocked (by stones, scarring from stones or surgery, or a tumour), ALP and bilirubin rise much more than AST or ALT. In rare liver diseases there is an isolated rise in ALP.

In some bone diseases, such as Paget’s disease (in which the bones become enlarged and deformed) or some cancers that spread to the bones, the rise in ALP may be isolated. When it is difficult to tell where the ALP comes from, the doctor may order other tests to find the exact cause of the rise. In these cases another enzyme, GGT, may be measured: it is found in the same places as ALP in the liver but is absent from bone.

1. What other tests are used to assess liver disease?
2. Who is at risk of liver disease?
3. What signs suggest liver disease?

1. What other tests are used to assess liver disease?
Other commonly used liver function tests include several enzymes found in liver cells, such as alanine aminotransferase (ALT), aspartate aminotransferase (AST) and bilirubin (a yellow pigment removed by the liver).

2. Who is at risk of liver disease?
Although many agents can potentially damage the liver, the main causes of liver disease are viral infections and alcohol abuse. In rare cases some medicines can damage the liver: in these cases your doctor will order liver function tests to monitor any harmful effect over time. There are also rare inherited liver diseases.

3. What signs suggest liver disease?
When the liver is severely damaged in a short time (a condition called acute hepatitis), the skin and the whites of the eyes turn yellow, the urine turns brownish and the stools become pale. In the late stages of liver disease there may be mental confusion, a tendency to bleed, abdominal swelling and vomiting of blood (haematemesis).