An alkaline phosphatase, or ALP, blood test measures the body's level of ALP, which is a protein existing in all body tissues, explains MedlinePlus. The bones, bile ducts and liver are parts of the body with high amounts of ALP. The test involves obtaining a blood sample by drawing blood from a vein in the elbow area or the back of the hand.
Doctors order the ALP test to diagnose bone or liver disease or assess if current treatments for these diseases are effective, notes MedlinePlus. The exam is also part of a regular liver function test. Doctors sometimes order the test to diagnose conditions such as gallstones, biliary stricture, alcoholism or alcoholic liver disease. Normal results generally range from 44 to 147 international units per liter, but the values may differ depending on the laboratory conducting the test and factors such as age and gender.
High ALP levels may indicate liver disease, osteoblastic bone tumors, bone conditions or biliary obstruction, while lower levels may indicate Wilson's disease, protein deficiency, malnutrition or hypophosphatasia, according to MedlinePlus. Pregnant women and children experiencing growth spurts usually have high ALP levels.
Preparation for the ALP test involves fasting for at least six hours prior to the exam, states MedlinePlus. Doctors sometimes advise stopping or adjusting dosages of certain medications that can affect the results of the test.