Focus on Diagnosis: The Alkaline Phosphatase Level: Nuances of a Familiar Test

Abstract

1. Sarah D. Corathers, MD* 1. *Chief Resident for Internal Medicine, University of Cincinnati, Cincinnati, Ohio. Dr Corathers wrote this article while a resident in internal medicine and pediatrics at the Cincinnati Children’s Medical Center Alkaline phosphatase is a commonly encountered laboratory value that is included in the panel of liver function tests. Although elevated concentrations generally are attributed to either liver or bone sources, the enzyme has been identified as a biomarker for a diverse range of diseases and physiologic processes. Elevated alkaline phosphatase may have implications ranging from indicating periodontal disease to predicting preterm labor. Functionally, alkaline phosphatase removes a phosphate group from nucleotides and proteins. As the name suggests, the enzyme works optimally at basic pH levels. It is found throughout the body in a variety of isoenzymes unique to the tissue of origin. Highest concentrations are in liver and bone, but the enzyme is present in lesser amounts in placenta, kidney, intestines, and leukocytes. Increased concentrations of alkaline phosphatase are derived from tissues that are either functionally disturbed (obstructed liver) or greatly stimulated (growing bone). Abnormally low concentrations are far less common and more likely are related to a genetic condition or nutritional deficiency. Normal levels can vary with age, sex, hormonal status, and blood type. In children, serum alkaline phosphatase concentrations are considerably higher than in adults and correlate with the rate of bone growth. In adults, values are slightly higher in men than in women, but after age 60 years, the enzyme value is equal or higher in women. Concentrations are increased during puberty and pregnancy and after menopause. Measurement of alkaline phosphatase activity in the laboratory involves the release of 1 mmol/min of phosphate from a standard compound (p-nitrophenylphosphate). The total amount of enzyme from all …