[Study of renal osteodystrophy by bone biopsy. Age as an independent factor. Diagnostic value of bone remodeling markers].

The spectrum of bone disease in uremic patients on hemodialysis has changed in the last years. Undecalcified bone biopsy with histomorphometric measurements and tetracycline labelling remains the gold standard for diagnosis of the different forms of renal osteodystrophy. But because of its invasive nature and complicated laboratory processing a number of non-invasive biochemical parameters have been proposed. The aim of our study was to determine the prevalence of the different forms of renal osteodystrophy in our patients in hemodialysis. Moreover we analyse the correlation between several biochemical parameters and the histological findings and evaluate their diagnostic and predictive value. Transiliac bone biopsies were performed in seventy three uremic patients (31 males) on chronic hemodialysis and static and dynamic parameters were measured. Serum levels of intact parathyroid hormone (iPTH), osteocalcin (OC), total alkaline phosphatase (FAT) and bone alkaline phosphatase (FAO) were determined. High-bone remodelling (50 pts, 68.5%) predominates over low-bone remodelling (23 pts, 31.5%). The distribution of the different types of bone disease was: Mild hyperparathyroidism 8 pts, Osteitis fibrosa 37 pts, Mixed lesions 5 pts, Adynamic bone disease 21 pts and Osteomalacia 2 pts. Six of our 73 patients were diabetics and they had adynamic bone disease (4 pts), osteomalacia (1 pt) and osteitis fibrosa (1 pt). Patients older than 50 years presented lower cellular activity (osteoblast surface, ObS/BS) and lower bone formation rate (BFR/BS). iPTH showed different correlation with these parameters of bone formation in patients above and below 50 years old suggesting that older patients need higher levels of PTH to obtain a determined level of bone formation. iPTH, OC, FAT and FAO correlated with the majority of histomorphometric indices of bone formation and resorption, though the best correlations were those with iPTH. The diagnostic and predictive value of these bone markers is better with high-bone remodelling. Serum levels of FAT > 300 U/l, OC > 150 ng/ml, FAO > 40 ng/ml and iPTH > 200 pg/ml showed a positive predictive value of 1 (with a specificity of 1, but sensibility below 0.78 except for iPTH that is 0.95) in the diagnosis of high-bone remodelling. After an analysis with ROC curves the cut-off value to differentiate high from low-bone remodelling was obtained. iPTH level > 200 pg/ml combined with one of the other markers (FAT > 150 U/l, FAO > 30 ng/ml or OC > 100 ng/ml) are predictive of high-bone remodelling, while values below those figures are predictive of low-bone remodelling.