Test Catalog

Test ID: BAP    
Bone Alkaline Phosphatase, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Diagnosis and assessment of severity of metabolic bone disease including Paget disease, osteomalacia, and other states of high bone turnover


Monitoring efficacy of antiresorptive therapies including postmenopausal osteoporosis treatment


The assay is not intended as a screening test for osteoporosis.


Measurements of bone turnover markers are not useful for the diagnosis of osteoporosis; diagnosis of osteoporosis should be made on the basis of bone density.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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. BAP has been shown to be a sensitive and reliable indicator of bone metabolism.(1)


Normal bone is constantly undergoing remodeling in which bone degradation or resorption is balanced by bone formation. This process is necessary for maintaining bone health. If the process becomes uncoupled and the rate of resorption exceeds the rate of formation, the resulting bone loss can lead to osteoporosis and, consequently, a higher susceptibility to fractures.


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.(2)


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. Antiresorptive therapies are used to restore the normal bone structure.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.


<2 years: 25-221 mcg/L

2-9 years: 27-148 mcg/L

10-13 years: 35-169 mcg/L

14-17 years: 13-111 mcg/L

Adults: < or =20 mcg/L


<2 years: 28-187 mcg/L

2-9 years: 31-152 mcg/L

10-13 years: 19-177 mcg/L

14-17 years: 7-41 mcg/L


Premenopausal: < or =14 mcg/L

Postmenopausal: < or =22 mcg/L

Interpretation Provides information to assist in interpretation of the test results

Bone alkaline phosphatase (BAP) concentration is high in Paget disease and osteomalacia.(3)


Antiresorptive therapies lower BAP from baseline measurements in Paget disease, osteomalacia, and osteoporosis. Several studies have shown that antiresorptive therapies for management of osteoporosis patients should result in at least a 25% decrease in BAP within 3 to 6 months of initiating therapy.(4,5) BAP also decreases following antiresorptive therapy in Paget disease.(6)


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.(1)

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Assay results should only be used in conjunction with information available from the clinical evaluation of the patient and other diagnostic procedures.


Human antimouse or other heterophile antibodies may be present in patient specimens. Although the assay has been specifically formulated to minimize their effects on the assay, results from patients known to have these antibodies should be carefully evaluated.


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 mcg/L to 5.8 mcg/L. Accordingly, serum specimens with significant elevations of liver ALP activity may yield elevated results.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Kress BC: Bone alkaline phosphatase: methods of quantitation and clinical utility. J Clin Ligand Assay 1998;21(2):139-148

2. Kuo TR, Chen CH: Bone biomarker for the clinical assessment of osteoporosis: recent developments and future perspectives. Biomark Res 2017;5:18 Published 2017 May 18. doi:10.1186/s40364-017-0097-4

3. Sharma U, Pal D, Prasad R: Alkaline phosphatase: an overview. Indian J Clin Biochem 2014;29(3):269–278 doi:10.1007/s12291-013-0408-y

4. Kress BC, Mizrahi IA, Armour KW, et al: Use of bone alkaline phosphatase to monitor alendronate therapy in individual postmenopausal osteoporotic women. Clin Chem 1999;45(7):1009-1017

5. Garnero P, Darte C, Delmas PD: A model to monitor the efficacy of alendronate treatment in women with osteoporosis using a biochemical marker of bone turnover. Bone 1999;24(6):603-609

6. Raisz L, Smith JA, Trahiotism M, et al: Short-term risedronate treatment in postmenopausal women: Effects on biochemical markers of bone turnover. Osteoporos Int 2000;11:615-620