ALP blood test – a liver test for biliary tract and skeleton
ALP, alkaline phosphatase, is an enzyme found in several parts of the body. The largest amounts are found in the liver, biliary tract and skeleton. Smaller amounts are also found in, among other things, the intestine and placenta.
An ALP blood test is mainly used to investigate possible effects on the liver and biliary tract or to assess changes in skeletal metabolism. Elevated ALP can, for example, occur in cases of obstruction of bile flow, inflammation of the biliary tract, certain liver diseases or conditions that affect the skeleton.
The test is often called P-ALP, which means that the enzyme activity is measured in blood plasma. ALP can be ordered as a separate blood test or included with other liver tests in a health check.
What does an ALP test show?
An ALP test shows the activity of the enzyme alkaline phosphatase in the blood. Since ALP primarily comes from the liver, bile ducts and bones, an abnormal value can indicate an impact in one of these areas.
ALP is particularly useful in assessing the bile ducts. If bile flow is obstructed or the bile ducts are affected, ALP can rise in the blood. The test can also rise when bone formation or breakdown is increased.
However, an elevated ALP value alone does not indicate where the increase comes from or what disease is behind it. The result therefore needs to be assessed together with other blood tests, symptoms, medical history and sometimes imaging tests.
Common causes of elevated ALP
An elevated ALP value can have several causes. A minor or temporary increase does not necessarily mean a serious illness, but a clear or long-term elevated value may need to be investigated further.
- Cholestasis or obstruction of the bile ducts: Gallstones, strictures or other effects that make it difficult for bile to pass can raise ALP.
- Liver and biliary tract diseases: Inflammation or other effects on the bile ducts and liver can cause elevated values.
- Increased bone turnover: ALP can rise when the skeleton builds up or breaks down bone tissue at a higher rate than normal.
- Bone fractures: During the healing of a fracture, ALP can temporarily increase.
- Vitamin D deficiency: Pronounced vitamin D deficiency and effects on bone mineralization can cause elevated ALP.
- Pregnancy: ALP can be elevated during pregnancy because the placenta produces a form of the enzyme.
- Growth in children and adolescents: Children and teenagers often have higher ALP than adults because their bones are growing.
If ALP is above the reference range for a long time or is clearly elevated, additional tests may be needed to determine whether the increase is mainly from the liver, biliary tract, or bones.
What can cause a long-term high ALP?
A long-term elevated ALP value can occur in conditions that affect the biliary tract, liver, or bones. How the result should be interpreted depends, among other things, on how much the value is elevated and which other blood tests are abnormal.
- Bile duct obstruction: For example, gallstones, narrowing or pressure on the bile ducts.
- Bile duct inflammation: Various inflammatory diseases can affect the bile ducts and raise ALP.
- Liver disease: Certain liver changes can cause elevated ALP, especially when bile flow is affected.
- Liver metastases or other tumor changes: Elevated ALP can occur in tumor diseases that affect the liver or bile ducts, but the test alone cannot diagnose cancer.
- Skeletal diseases: Conditions with increased bone turnover can raise ALP.
- Vitamin D deficiency and osteomalacia: Insufficient mineralization of the skeleton can result in elevated values.
- Paget's disease: An uncommon bone disease that can cause greatly elevated ALP.
- Skeletal metastases: Tumor disease affecting the skeleton can in some cases cause an increase, but ALP is not a specific cancer test.
ALP alone cannot confirm or rule out liver, biliary tract or skeletal disease. An abnormal result always needs to be interpreted in its medical context.
What is a normal ALP value?
The reference range for ALP varies between laboratories, analysis methods and age groups. The result is usually given in µkat/L. A value just outside the range does not necessarily mean serious disease. The interpretation should also take into account other liver tests, symptoms, medications, age, pregnancy and any skeletal problems.
P-ALP high – what does an elevated value mean?
A high P-ALP means that the activity of alkaline phosphatase in the blood plasma is above the laboratory's reference range. The elevation may originate from the liver and biliary tract or from the skeleton.
If ALP is elevated at the same time as GT, it is more likely that the elevation originates from the liver or biliary tract. If ALP is elevated but GT is normal, a skeletal-related cause may be more likely, but this needs to be should be assessed together with other tests and clinical data.
A slightly elevated value may be transient. A severely or persistently elevated ALP should be followed up, especially if other liver tests are abnormal or if symptoms are present.
Slightly elevated ALP
A slightly elevated ALP can be seen, among other things, during growth, pregnancy, after a bone fracture or in the event of a mild impact on the liver or biliary tract. In some cases, a new test is recommended to see if the value normalizes or persists.
Severely elevated ALP
A severely elevated ALP may occur in the event of a more obvious obstruction in the biliary tract or in conditions that cause a significantly increased bone turnover. The result then needs to be assessed together with other blood tests, symptoms and possible further investigation.
P-ALP low – is a low value dangerous?
A low P-ALP is less common than a high value and often has no clinical significance. A single low result therefore does not necessarily mean that something is wrong.
Low ALP values can in some cases occur with malnutrition, lack of certain nutrients, low metabolism or more unusual congenital conditions. The result especially needs to be assessed further if the value is clearly low in repeated tests or if other symptoms and abnormal test results are present.
A low ALP value cannot be used alone to make a diagnosis.
How is ALP interpreted together with other liver tests?
ALP is often analyzed together with other liver tests. The combination of different test values can help distinguish between effects on the liver cells, the biliary tract and the skeleton.
ALP and GT
GT is often used together with ALP to assess whether an elevated ALP is likely to come from the liver or the biliary tract. If both ALP and GT are elevated, it more often indicates liver or biliary tract involvement.
If ALP is elevated while GT is within the reference range, a cause outside the liver, for example in the skeleton, may be more likely. However, the results are not unambiguous and always need to be assessed together with other information.
ALP and ALT
ALT is primarily a marker for effects on the liver cells, while ALP is more linked to the biliary tract and the skeleton. If both ALP and ALT are elevated, there may be a mixed effect on the liver and biliary tract.
ALP and AST
AST can increase when liver cells are affected but also occur in muscles and other tissues. An elevated ALP together with AST and ALT needs to be interpreted based on which sample is most markedly elevated.
ALP and bilirubin
Bilirubin is handled by the liver and excreted via the bile. If both ALP and bilirubin are elevated, it may indicate that bile flow is affected, for example in the case of an obstruction in the bile ducts.
ALP, calcium and vitamin D
If an elevated ALP is suspected of coming from the skeleton, it may be relevant to also check calcium, phosphate, vitamin D and other tests that reflect the function of the skeleton and mineral metabolism.
When may it be relevant to test ALP?
A blood test for ALP may be relevant as part of a health check, when following up on previously abnormal liver values or when there is suspicion of an impact on the bile ducts or the skeleton.
For example, it may be relevant to check ALP in the following cases:
- previously elevated liver values
- suspected obstruction or inflammation of the bile ducts
- yellow skin or yellow whites of the eyes
- itching without a clear explanation
- dark urine or light stools
- pain under the right rib cage
- suspected or previously diagnosed liver disease
- bone pain or suspected bone disease
- suspected vitamin D deficiency or disturbed bone turnover
- follow-up after a bone fracture or during treatment of bone disease
Symptoms that may occur with elevated ALP
An elevated ALP value does not in itself cause any symptoms. The symptoms depend instead on the underlying cause.
When the liver or biliary tract is affected, the following symptoms may occur:
- yellow skin or yellow whites of the eyes
- dark-colored urine
- unusually light-colored stools
- itching
- nausea or decreased appetite
- pain or discomfort under the right rib cage
- fever or chills in case of inflammation or infection
When the bones are affected, symptoms may instead be:
- persistent bone or joint pain
- muscle weakness
- increased tendency to fracture
- pain after a fracture that is healing
These symptoms can have many other causes. An ALP blood test can provide information but is not a substitute for a medical evaluation.
How an ALP blood test is done
ALP is measured through a regular blood sample taken from a one in the arm. The sample is usually taken quickly and ALP can be ordered separately or included in a larger liver test together with, for example, ALT, AST, GT and bilirubin.
Blood samples are usually taken from a vein in the arm and are used both in the investigation and follow-up of various conditions.
Always follow the instructions before taking the sample and inform if necessary about medications, dietary supplements, pregnancy or recent bone fractures, as this may be relevant to the interpretation.
Do you need to fast before an ALP blood test?
A single ALP test usually does not require fasting. However, if ALP is part of a larger health check, other analyses in the same sample may require you to fast.
Therefore, always follow the instructions that are part of the blood test or health check you have ordered.
Order a blood test for ALP
By testing ALP, you will receive information about a blood value that is used to assess the liver, biliary tract and skeleton. The test may be relevant when following up on previously abnormal liver values or as part of a broader check of liver and biliary tract health.
After you have ordered the test, you will submit the blood sample to an affiliated sampling clinic. When the analysis is complete, the result will be available digitally together with the laboratory's reference range.
