BONE-SPECIFIC ALKALINE PHOSPHATASE (OSTASE)

• Clinical Applications

  Monitoring disease activity and the response to antiresorptive therapy in patients with Paget's disease.

  Monitoring the effectiveness of antiresorptive therapy in patients with osteoporosis.

  Determining the degree of bone turnover in patients with various forms of metabolic bone disease (renal osteodystrophy, hyperparathyroidism)

  
  
• Interpretation

  Bone ALP is a glycoprotein localised in the plasma membrane of osteoblasts. The precise role is unclear although it is essential for mineralisation. Total circulating ALP is also derived from liver, intestine, spleen, kidney, placenta (in pregnancy) or from various tumours. Bone ALP comprises approximately 50% of total circulating ALP in normal subjects. Measurement of bone ALP by IRMA reflects bone turnover more specifically and sensitively than total ALP. It is however important to note that the specificity for bone ALP is relative as 100 IU/L of liver ALP activity gives a bone ALP result of 2.5 to 5.8 mg/L in the bone ALP assay. Serum samples with significant elevations of liver ALP activity may therefore yield elevated results in this assay.

  The minimal significant change of bone ALP is 25%. Potential clinical uses include:

  1. Paget's disease - bone ALP (together with bCTX) is a useful marker of the activity of Paget's disease and the response to anti-resorptive therapy.
  2. Osteoporosis - in patients treated for osteoporosis the percentage change of bone ALP after effective anti-resorptive treatment is typically 50% decline at 3-6 months after initiating biphosphonate therapy. Furthermore, for a group of patients (but not necessarily for the individual patient) there is a close correlation between the reduction in bone turnover assessed at 3-6 months and the ultimate increase in bone density measured by DEXA at 1-2 years. The clinical utility of routinely monitoring the efficacy of anti-resorptive therapy in individual patients treated for osteoporosis is not firmly established. However, in patients with particularly severe osteoporosis or apparent resistance to anti-resorptive therapy there may be a role for monitoring bone turnover markers (bone ALP and bCTX) after initiating therapy to confirm the efficacy of anti-resorptive therapy.