Alkaline Phosphatase (ALP) is an enzyme found throughout the body and is a common component of routine blood tests, such as the Comprehensive Metabolic Panel. Measuring ALP levels in the bloodstream provides physicians with initial information about the health of several major organ systems. The result is reported in Units per Liter (U/L) or International Units per Liter (IU/L). A reading like 132 U/L must be interpreted within the context of a person’s age, gender, and overall health profile.
The Primary Functions of Alkaline Phosphatase
The biochemical job of Alkaline Phosphatase is to act as a catalyst that removes phosphate groups from various molecules, a process called dephosphorylation. This enzymatic action is crucial because phosphate groups are often used by the body to inhibit or activate other molecules. Therefore, ALP is involved in numerous regulatory pathways. Its activity is dependent on cofactors, specifically the minerals zinc and magnesium, which are necessary for the enzyme to function properly.
One of its most significant roles is in bone mineralization, which is the proper formation of bone tissue. ALP is highly active in osteoblasts, the cells responsible for building bone, where it hydrolyzes inorganic pyrophosphate (PPi). PPi is a potent natural inhibitor of bone growth. By breaking it down, ALP effectively clears the way for the deposition of calcium and phosphate crystals that give bone its strength.
The enzyme also functions outside of the skeletal system, particularly in the liver and the lining of the digestive tract. In the liver, ALP is concentrated in the walls of the bile ducts, where it aids in the transport of fats and other substances. High concentrations in the intestinal lining suggest a role in the absorption of nutrients, including certain vitamins and fats.
Distinguishing the Sources of ALP
Alkaline Phosphatase is not a single entity, but a collection of distinct forms, known as isoenzymes, that originate from specific tissues. The major sources that contribute to the circulating level in the blood are the liver, bone, intestines, and the placenta during pregnancy. Determining the source of an elevated reading is the next step in interpreting the test result.
For a typical adult, the ALP reference range usually falls between 40 to 129 U/L, though this range varies slightly by laboratory. A result of 132 U/L is near the upper limit for many adults and may be considered mildly elevated. For adult women, whose upper limit is often lower (around 104 U/L), 132 U/L is more clearly elevated. Conversely, this reading is well within the normal range for a rapidly growing child or adolescent.
To pinpoint the origin of an elevated ALP reading, physicians often order a Gamma-Glutamyl Transferase (GGT) test alongside it. GGT is an enzyme found almost exclusively in the liver and bile ducts, but not in bone tissue. If both ALP and GGT levels are elevated, the source is most likely the liver or biliary system. If ALP is high but the GGT level is normal, the elevation strongly suggests a bone-related issue.
Causes of High Alkaline Phosphatase Readings
Elevated ALP levels are primarily associated with conditions affecting the liver or the bones. In the liver, the most common cause is the obstruction of bile flow, known as cholestasis. When bile ducts are blocked by gallstones, tumors, or inflammation, the resulting pressure causes liver cells to produce and release more ALP into the bloodstream.
Other liver conditions, such as cirrhosis (extensive scarring) and hepatitis (inflammation), can also result in elevated ALP readings. Damage to the liver tissue impairs its normal function, leading to the leakage of enzymes, including ALP, into circulation. Additionally, certain medications can induce liver stress and cause a transient rise in the enzyme’s level.
In bone tissue, high ALP levels are a marker of increased osteoblast activity, meaning new bone formation is occurring at a faster-than-normal rate. This is why children and adolescents naturally have higher levels during periods of rapid growth, which can sometimes push a reading above the adult reference range. Other non-disease causes include the normal healing process after a bone fracture. A high ALP reading in a pregnant woman is also typical, due to the placental isoenzyme, particularly in the third trimester.
Pathological bone conditions also cause significant increases in ALP. Paget’s disease of the bone, a disorder of excessive bone breakdown and regrowth, is a classic example that causes marked elevation. Bone tumors or cancer that has spread to the bones (metastasis) also stimulate osteoblasts to attempt repair, leading to enzyme release.
Causes of Low Alkaline Phosphatase Readings
While less common than elevated levels, a significantly low Alkaline Phosphatase reading can also signal underlying health concerns. The most serious, though rare, cause is a genetic disorder called Hypophosphatasia. This condition is characterized by a deficient activity of the tissue-nonspecific ALP isoenzyme, which severely impairs the body’s ability to mineralize bone.
Low ALP levels can be a marker for nutritional deficiencies, particularly a lack of necessary mineral cofactors like zinc and magnesium. Insufficient intake or absorption of these minerals can result in a measurable drop in ALP activity. Protein-calorie malnutrition can similarly lead to reduced enzyme production and lower blood levels.
Certain endocrine and hematological conditions are also associated with reduced ALP activity. An underactive thyroid gland, or hypothyroidism, can contribute to lower readings due to a general slowing of metabolic processes, including enzyme production. Additionally, some forms of severe anemia, such as pernicious anemia, have been linked to a decrease in circulating Alkaline Phosphatase.