Muscles and bones
Botspecifiek alkalisch fosfatase (Ostase)
The part of alkaline phosphatase that comes from bone; a measure of the rate of bone formation, not a test for osteoporosis.
Everything about this testWhat it measures
Alkaline phosphatase is an enzyme found mainly in the liver (the bile ducts) and in the bone-forming cells. Bone-specific alkaline phosphatase (Ostase) is the form that comes from bone. A high value shows that a lot of bone is being formed. Because only the bone form is measured, the test also helps to show whether a raised total alkaline phosphatase comes from bone or from the liver. The result says something about the rate of bone formation, not about how much bone there is.
- A raised rate of bone formation, for example with Paget's disease or a vitamin D deficiency
- A clue as to whether a raised total alkaline phosphatase comes from bone or from the liver
- A change in the rate of bone formation during treatment for osteoporosis
High
A high value fits a high rate of bone formation. This is normal in children and young people who are still growing and in the months after a broken bone. A high value is also seen with a vitamin D deficiency that softens the bone (osteomalacia), with Paget's disease (locally disordered bone remodeling), with an overactive parathyroid gland and with bone changes due to reduced kidney function. In the years after menopause the value is often slightly higher. A high value does not mean that you have osteoporosis.
Low
A low value fits a low rate of bone formation. This is seen during treatment that slows bone breakdown and with long-term use of corticosteroids. A low value does not mean that your bones are strong or weak: the amount of bone is measured with a bone density scan (DXA). Rarely: an inherited shortage of alkaline phosphatase (hypophosphatasia), in which total alkaline phosphatase is also low.
If your result is abnormal
With a raised value, the GP looks at calcium, phosphate, vitamin D, kidney function and the parathyroid gland, and if needed requests an X-ray or bone scan for symptoms in one place. With a question about osteoporosis, a risk assessment follows and, if needed, a bone density scan.
Reliability
In an individual, a single value says little about the amount of bone or the risk of a broken bone. Osteoporosis is diagnosed with a bone density scan; when looking for a cause, the guideline on osteoporosis and fracture prevention lists ordinary alkaline phosphatase, not the bone-specific form. The test is useful for telling bone from liver with a raised total alkaline phosphatase, and for following a bone disease. The test method also reacts to a small extent to the liver form, so a strongly raised liver form can raise the result slightly.
Limitations
- Growth in children and young people and a recent broken bone raise the value, without there being a bone disease.
- The test method also reacts to a small extent to alkaline phosphatase from the liver.
- A change in the value is only meaningful if it is larger than the natural variation; preferably compare results from the same laboratory.
- There are no agreed cut-off values; laboratories use different methods and reference values.
How common is this
Osteoporosis is common, especially in women after menopause and in older people. Osteoporosis is diagnosed with a bone density scan, not with bone markers in blood. Paget's disease occurs mainly in older people and often causes no symptoms.
Who is this useful for
Useful with a raised total alkaline phosphatase where it is unclear whether it comes from bone or the liver, with a known bone disease such as Paget's disease, or when following treatment for osteoporosis. It is not a test for osteoporosis: in people without symptoms the result adds nothing to estimating the risk of a broken bone.
Downsides and risks
- A high value can wrongly be read as osteoporosis, leading to worry and unnecessary further tests.
- A normal value can be falsely reassuring, while it does not measure the risk of a broken bone.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results