Interpretation
Alkaline phosphatase (ALP) is present in a number of tissues including liver, bone, intestine, and placenta. The activity of ALP found in serum is a composite of isoenzymes from these sites.
Raised serum ALP occurs mainly with hepato-biliary disease (usually associated with GGT elevation). It also occurs in bone disease associated with increased osteoblastic activity and bone remodelling such as Paget’s disease, rickets, osteomalacia, fractures, and malignant tumours, where GGT is not usually elevated although bone turnover markers (P1NP) may be increased.
In most cases, it is usually possible to determine the origin of ALP elevation given the clinical context and whether other markers are elevated, eg GGT supporting liver disease or P1NP supporting bone disease.
Occasionally, the cause of ALP elevation may not be apparent and isoenzyme evaluation may be helpful in supporting a diagnosis. ALP isoenzyme determination is most likely to be helpful with sustained increase (>6 months) in total ALP (usually at least 2X elevated) when liver and bone markers are normal, and no cause for the increase is apparent clinically.
Discernible patterns in ALP Isoenzymes:
Liver ALP isoenzyme may be elevated in various liver diseases, including primary or secondary malignancy and other causes of biliary obstruction.
Bone ALP isoenzyme may be increased in osteoblastic bone tumours and bone remodelling (eg, Paget’s disease).
Intestinal ALP isoenzyme may be increased in patients with cirrhosis and in individuals who are blood group O or B secretors.
Placental ALP isoenzyme can be discerned usually in the 3rd trimester of pregnancy, although has been associated with some tumours.
In transient hyperphosphatasaemia of infancy, the ALP level may be very high (thousands), and typically persist for a few months. The pathogenesis is unknown, although has been suggested to follow viral infections. There is a characteristic appearance on ALP isoenzyme analysis that supports the diagnosis.
A typical appearance is also seen in macro-ALP, where there is a high molecular weight complex with ALP. Macro-ALP is suspected where there is persistent, otherwise unexplained elevation of ALP. This may be further supported by PEG precipitation and other studies, where the clinical laboratory will be able to assist.
Reference Intervals
A visual interpretation given indicating source of the enzyme i.e. bone, liver, intestinal, biliary, placental.
