The relationship between intact PTH and biointact PTH (1-84) with bone and mineral metabolism in pre-dialysis chronic kidney disease (CKD).

OBJECTIVES: Abnormalities in PTH are implicated in the pathogenesis of bone abnormalities in chronic kidney disease (CKD)-mineral bone disorder (CKD-MBD). PTH concentrations are important in clinical decision and management. This emphasises the importance of providing an assay which measures biologically active PTH. We compared concentrations of intact PTH with biointact PTH (1-84) in CKD and end stage renal disease (ESRD) and investigated the relationship between the 2 PTH assays with bone and mineral laboratory parameters and bone mineral density (BMD) in CKD.

DESIGN AND METHODS: We assessed 140 patients (61 in ESRD and 79 with CKD stages 1-4) in this cross-sectional study. We measured biointact PTH (1-84) as well as routine biochemical parameters on all subjects. In the CKD cohort, bone turnover markers; bone alkaline phosphatase (BAP) and tartrate resistant acid phosphatase (TRACP)-5b and bone mineral density (BMD) were also determined.

RESULTS: In ESRD, intact PTH concentration was significantly higher compared to biointact PTH (1-84) (422 [443] v/s 266 [251] pg/mL, (p<0.001) with an average bias of 60%. In CKD, intact PTH concentration was also higher compared to biointact PTH (1-84) (79[55] v/s 68[49] pg/mL p<0.001) with an average bias of 18%. Only the biointact PTH (1-84) assay showed any significant correlation with serum calcium concentrations (r=-0.26, p<0.05) and phosphate (r=0.25, p<0.05) in CKD. Following multilinear regression analysis and adjustment for all significant co-variables, only eGFR, BAP and 25 (OH)vitamin remained significantly associated with intact PTH and biointact PTH (1-84). The strength of association was stronger between BAP and biointact PTH (1-84) (biointact PTH (1-84): p=0.007, intact PTH: p=0.01). In adjusted analyses, only biointact PTH (1-84) was significantly associated with BMD at the fore-arm (FARM) (p=0.049).

CONCLUSIONS: The study confirms the differences between intact PTH and biointact PTH (1-84) in ESRD. Whilst there may be similarities in the diagnostic ability of both intact and biointact PTH (1-84), our data suggest that biointact PTH (1-84) assay may better reflect bone metabolism and BMD in CKD. Further longitudinal studies are needed.