Biomarker: Alkaline Phosphatase
Overview
Alkaline phosphatase (ALP) is an enzyme measured in blood, with the largest circulating contributions typically originating from the liver and bones. It catalyzes the removal of phosphate groups from organic compounds and is relevant to bile flow and bone mineralization. High levels may indicate cholestatic or other hepatobiliary disease, increased bone turnover, bone disease, pregnancy, or normal growth, while low levels may occur with hypophosphatasia, malnutrition, zinc deficiency, hypothyroidism, or Wilson disease. ALP is clinically useful for evaluating liver and bone disorders, although total ALP alone does not identify its tissue source.
Clinical Use Cases
- Screening for or evaluating hepatobiliary disease, including cholestasis and biliary obstruction.
- Evaluating bone disorders associated with increased osteoblastic activity, including Paget disease, osteomalacia, healing fractures, and bone tumors.
- Monitoring known liver or bone disease and response to treatment.
- Supporting evaluation of an isolated or disproportionate ALP elevation alongside bilirubin, aminotransferases, gamma-glutamyl transferase, or 5′-nucleotidase.
- Distinguishing hepatic from nonhepatic sources through ALP isoenzyme or bone-specific ALP testing when clinically indicated.
- Supporting evaluation of low ALP in suspected hypophosphatasia, nutritional deficiency, hypothyroidism, or Wilson disease.
Specimen Types
- Serum from venous blood.
- Heparinized plasma, where validated by the laboratory.
- Serum or plasma for ALP isoenzyme testing.
Measurement Methods
- Automated spectrophotometric or photometric enzyme-activity assays.
- Colorimetric assays using phosphate ester substrates, commonly (p)-nitrophenyl phosphate, with results reported in international units per liter.
- Electrophoretic separation of ALP isoenzymes, sometimes combined with heat inactivation or other separation procedures.
- Immunochemical assays for bone-specific ALP.
Test Preparation and Influencing Factors
- Follow the ordering provider’s instructions. Fasting is often required when ALP is ordered with a comprehensive metabolic panel or other tests.
- An overnight fast, commonly at least 8 hours, may reduce the effect of a fatty meal on intestinal ALP, particularly in people with blood groups O or B.
- Do not stop prescription or nonprescription medicines unless instructed by a healthcare professional. Birth-control hormones and other medications may alter ALP results.
- Pregnancy can increase ALP because of placental ALP.
- Children and adolescents commonly have higher levels because of active bone growth.
- Age and sex affect expected reference intervals; laboratory-specific ranges should be used.
- Healing fractures and conditions that increase bone turnover can raise ALP.
- Liver injury, cholestasis, biliary obstruction, and some infiltrative liver diseases can increase ALP.
- Malnutrition, zinc or magnesium deficiency, hypothyroidism, and hypophosphatasia may lower ALP.
- Hemolysis, lipemia, inappropriate anticoagulants, and delays or unsuitable storage conditions can interfere with measurement.
Synonyms
- ALP
- Alkaline phosphatase
- Alkaline phosphatase activity
- Alk phos
- ALK
- PHOS
- Alkp
- Tissue-nonspecific alkaline phosphatase, when referring to the liver, bone, and kidney-associated isoenzyme
- Bone-specific alkaline phosphatase, for the bone-derived fraction
This information is provided for general educational purposes and is not medical advice. Biomarker information may describe testing methods, measurements, or interpretations that vary by laboratory or specific test. For details about a particular lab test, including biomarkers measured, specimen type, collection requirements, preparation, and turnaround time, please refer to the individual test description. Talk with a licensed health care provider about your results.
