Alkaline phosphatase of bone origin in hemodialyzed patients. 110 assays

Objective: To evaluate the value of plasma bone-specific alkaline phosphatase (bAP) in the diagnosis and the prediction of the type of renal osteodystrophy in hemodialysis patients. To examine whether bAP correlated with the two classical biochemical markers of bone turnover in dialysis patients which are total alkaline phosphatases (tAP) and intact PTH (iPTH).

Methods: One hundred adult uremic patients undergoing regular hemodialysis therapy were included in the study. Plasma bAP was determined using a radioimmunometric assay. Intact PTH (1-84) was measured using a radioimmunometric assay, Allegro Intact PTH, Nichols Institute, CA, USA. Other laboratory tests including calcium, phosphorus, and tAP were performed by automated methods.

Results: Based on previous biochemical and histological data obtained in the same type of patients, we arbitrarily divided plasma bAP levels in normal (10-20 ng/ml), low (< 10 ng/ml), and high (> 20 ng/ml). We found here that bAP levels were normal in 27 patients, high in 36, and low in 37. In the group with normal bAP, 8/27 patients had iPTH levels higher than 200 pg/ml, the other 19 patients had an iPTH within the normal range. In the 36 patients with high bAP, 10 showed markedly high iPTH and clinical signs of severe hyperparathyroidism leading to the indication of a surgical parathyroidectomy in 7 patients and i.v. calcitriol therapy in the other 3. However, 9 of these 36 patients had an iPTH lower than 200 pg/ml and 17 between 200-500 pg/ml. In the 37 patients with low bAP, iPTH levels were also low in 15 of them, but 12 patients had iPTH levels higher than 200 pg/ml. There was a good correlation between bAP, tAP, and iPTH. However, bAP correlated better with iPTH than tAP. A bAP > 20 ng/ml had a sensitivity of 56%, specificity of 92% and positive predictive value of 80% in the prediction of an PTH higher than > 400 pg/ml and therefore of the biological diagnosis of hyperparathyroidism. A bAP < 10 ng/ml had a sensitivity of 70%, specificity of 56% and a positive predictive value of 48% in predicting a normal-low iPTH (< 100 pg/ml).

Conclusions: In the absence of bone biopsy, plasma bAP self provides useful information about bone remodeling in hemodialysis patients. The combination of bAP and iPTH measurements further define the type of bone turnover. either elevated plasma bAP or iPTH alone are no always anonymous with secondary hyperparathyroidism.