Interpretation and clinical significance of bone alkaline phosphatase discriminating adynamic bone disease and high turnover in dialysis patients

Measurement of bone alkaline phophatase in combination with PTH-levels belo w a 2 - 3 fold normal range can increase sensitivity and specivity on discr imination sec. hyperparathyreoidism (sHPT) from adynamic bone disease (ABD) . Therefore we would expect a decreased bone turnover, indicated by low AKP -levels, for PTH-levels signalizing an ABD. However we often see elevated A KP-levels concommitant to PTH-levels below the 2 - 3 fold normal range. Qua ntification of this effect was successful by clustering 135 dialysis patien ts in 3 groups (K-Cluster-Means, SPSS 7.2) concerning different relationshi ps between iPTH to AKP. Group 2 shows the patients with dissociated relatio nsship of AKP and iPTH (n = 21) with a center of 87 U/l, 14,82 pmol/l for A KP and iPTH. Group 1 refers to a severe sHPT (n = 13, AKP 50 U/l, iPTH 52,6 2 pmol/l), group 3 to normal or adynamic bone disease (n = 98, AKP 34 U/l, PTH 25,16 pmol/l). The observed dissociation of AKP und PTH cannot be expla ined by factors typically inducing renal osteodystrophy (ROP) like Vit D, C a, Pi. This could be shown by an evaluation of all 135 patients for iPTH, A KP an Ca by separating patients in Pi > 7 mg/dl and Pi < 7 mg/dl: Patients with more elevated Pi-levels show a significant higher level. of iPTH (29,2 pmol/l to 20,0 pmol/l), whereas AKP does not differ within patients with d ifferent Pi (47,5 U/l and 46,5 U/l). In addition patients with different be havior of AKP and PTH have significant lower Pi-levels than all other group s. The following reasons may explain the observed HT in group 2 apart from factors influencing ROP. Aging, immobilisation and morbidity often contribu te to elevated bone turnover. Alkoholism and resections of the gastrointest inum are found more often in men. Postmenopausal metabolism additionally in duces HT in women on dialysis. Steroid-induced osteoporosis may also result in elevated bone turnover. Conclusion: Factors inducing HT in dialysis pat ients don't have to be related only to renal osteopathy. The shown influenc ing factors also should be of concern, when starting stimulation of bone co rrecting PTH-levels under a treshold indicating ABD by lowering dialysate-C a. Overstimulation of bone may then result in bone loss elevated fractur ri sk and last not least severe sHPT.