What is alkaline phosphatase?
Alkaline phosphatase (AP) is a metabolic enzyme that occurs in the cells of a wide variety of tissues in the body – for example, in the bones, liver and bile ducts.
There are various subforms (isoenzymes) of alkaline phosphatase. With one exception, these occur specifically in a particular tissue, for example bone-specific phosphatase in skeletal tissue.
If the AP level is measured in the blood, it is all the subforms of the enzyme together (total AP). If necessary, the levels of various isoenzymes can also be determined in the laboratory.
When is alkaline phosphatase determined?
The concentration of alkaline phosphatase in the blood (total AP) is determined primarily when bone or liver diseases are suspected. AP is present in large amounts in both tissues (bone, liver).
Alkaline phosphatase: reference values
|
Age |
AP Normal Value |
|
until 1 day |
< 250 U/l |
|
2 to 5 days |
< 231 U/l |
|
6 days to 6 months |
< 449 U/l |
|
7 to 12 months |
< 462 U/l |
|
1 to 3 years |
< 281 U/l |
|
4 to 6 years |
< 269 U/l |
|
7 to 12 years |
< 300 U/l |
|
13 to 17 years |
< 187 U/l for women < 390 U/l for men |
|
over 18 years |
35 – 105 U/l for women 40 – 130 U/l for men |
When is alkaline phosphatase low?
Very rarely, alkaline phosphatase is too low. This occurs, for example, in cases of pronounced hypothyroidism or anemia.
Alkaline phosphatase can also be too low in the rare hereditary disease Wilson’s disease, which manifests itself in disturbed copper metabolism. Other possible causes are the metabolic disorders achondroplasia and hypophosphatasia, which are also rare.
When is alkaline phosphatase elevated?
If alkaline phosphatase is elevated, this may be due to the following reasons, for example:
- bone diseases such as bone tumors, Paget’s disease, rickets (vitamin D deficiency), osteomalacia, osteomyelitis, hyperparathyroidism (HPT)
- certain forms of kidney weakness (renal insufficiency)
- chronic inflammatory bowel diseases
- acromegaly (growth hormone excess)
- Cushing’s syndrome (cortisol excess)
- parathyroid hyperfunction
During pregnancy and fetal growth, alkaline phosphatase is naturally increased in the blood. Elevated AP levels are also possible in individuals with blood type B or 0.
Taking various medications such as allopurinol (gout medication), antiepileptic drugs or the contraceptive pill can also increase the AP level.
