Bone alkaline phosphatase (BAP) is the bone-specific isoform of alkaline phosphatase. A glycoprotein that is found on the surface of osteoblasts, BAP reflects the biosynthetic activity of these bone-forming cells. Serum levels of BAP are believed to reflect the metabolic status of osteoblasts. An accurate assessment of bone metabolism is critical for determining the severity of metabolic bone disease and responses to therapy. Measurement of serum levels of BAP has been shown to be useful in evaluating patients with Paget's disease, osteomalacia, primary hyperparathyroidism, renal osteodystrophy, osteoporosis and metastases to bone. BAP concentration is high in Paget disease and osteomalacia.
Osteoporosis is a metabolic bone disease characterized by low bone mass and abnormal bone microarchitecture. It can result from a number of clinical conditions including states of high bone turnover, endocrine disorders (primary and secondary hyperparathyroidism and thyrotoxicosis), osteomalacia, renal failure, gastrointestinal diseases, long-term corticosteroid therapy, multiple myeloma, and cancer metastatic to the bones.
Paget disease is another common metabolic bone disease caused by excessive rates of bone remodeling resulting in local lesions of abnormal bone matrix. These lesions can result in fractures or neurological involvement. Anti-resorptive therapies are used to restore the normal bone structure.
Males:<2 yrs: 25-221 ug/L2-9 yrs: 27-148 ug/L10-13 yrs: 35-169 ug/L14-17 yrs: 13-111 ug/LAdult: <=20
Females: <2 yrs: 28-187 ug/L2-9 yrs: 31-152 ug/L10-13 yrs: 29-177 ug/L14-17 yrs: 7-41 ug/LAdult Pre-Menopause: <=14Adult Post-Menopause: <=22
The BAP assay is not intended as a screening test for osteoporosis. The BAP results should be interpreted in light of the total clinical presentation of the patient, including: symptoms, clinical history, data from additional tests and other appropriate information.
Anti-resorptive therapies lower BAP from baseline measurements in Paget disease, osteomalacia, and osteoporosis. Several studies have shown that anti-resorptive therapies for management of osteoporosis patients should result in at least a 25% decrease in BAP within 3 to 6 months of initiating therapy. BAP also decreases following anti-resorptive therapy in Paget disease.
When used as a marker for monitoring purposes, it is important to determine the critical difference (or least significant change). The critical difference is defined as the difference between 2 determinations that may be considered to have clinical significance. The critical difference for this method was calculated to be 25% with a 95% confidence level. Therefore, changes in BAP concentrations that exceed 25% may be attributed to changes in bone remodeling.
Liver-derived alkaline phosphatase (ALP) has some cross-reactivity in this assay: 100 U/L of liver ALP activity gives a result of 2.5 g/L to 5.8 g/L. Accordingly, serum specimens with significant elevations of liver ALP activity may yield elevated results.