Bisphosphonate whole body retention test: relations to bone mineralization rate, renal function and bone mineral content in osteoporosis and metabolic bone disorders

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Abstract
The bisphosphonate whole body retention test (WBR) has been used to estimate bone mineralization rate (bone turnover). Bisphosphonates given i.v. are taken up by bone or excreted in urine. The aim of the present investigation was to test the efficacy of WBR in estimating bone mineralization rate (m) and to evaluate the influence of renal function (ClCr) and bone mass (forearm bone mineral content; BMC) on WBR. The 24-h retention of 3.7 MBq 99mTc-HMBP (1-hydroxymethylene-1,1-bisphosphonate) (Osteoscan) given i.v. was measured by a medium sensitive whole body counter in thirty-one patients with hyperparathyroidisme (n = 14), hyperthyroidism (n = 8) or hypothyroidism (n = 9) (group 1) and in seventy-six females with postmenopausal spinal crush fracture osteoporosis (group 2). In the same individuals m was calculated from a 7-day 47Ca-kinetic study using the expanding calcium pool model. Multiple regression analysis of WBR vs. m and ClCr in group 1 disclosed that WBR correlated positively to m [rp = 0.49, P < 0.01 (rp = partial correlation coefficient)] and inversely to Clcr (rp = -0.44, P < 0.02). Inclusion of BMC in the analysis did not reveal any significant partial correlation between WBR and BMC (rp = -0.33, 0.05 < P < 0.10). In group 2 WBR correlated inversely to Clcr (rp = -0.48, P < 0.001) but showed no significant relation to m (rp = 0.10, NS). WBR was not related to BMC (rp = 0.09 NS), WBR was slightly inferior to serum alkaline phosphatase and renal hydroxyproline excretion in estimating bone mineralization rate in group 1 and, contrary to the biochemical markers, inapplicable in group 2. A model was developed that took into account that bisphosphonate is either taken up by bone or excreted in the urine. The model, however, did not improve the efficacy by which m could be predicted from simultaneous measurements of WBR and Clcr, probably because of an increase in the method error. We conclude that measurements of the whole body retention of HMBP is of no value in estimation bone turnover in the investigated metabolic states characterized by wide variations in bone turnover and no mineralization defects.