The assay of the hypocalcemic PTH fragment inhibitor with PTH provides a more accurate assessment of renal osteodystrophy compared to the intact PTH assay.

As the chronic kidney disease patient is being managed for PTH, calcium, phosphate, vitamin D, calcium x phosphate product and bone quality an accurate PTH measurement is essential. Over and under PTH suppressive therapies pose significant risks of mineral metabolism disturbances, osteodystrophies and soft tissue calcifications. Until recently it was thought that there was only one hormone secreted by the parathyroid gland, 1-84 PTH (or CAP). It is now known that there is another hormone secreted by the parathyroid gland (CIP) which is most likely 7-84 PTH. 7-84 PTH has been demonstrated to be an antagonist of 1-84 PTH with inverse biological activities. 7-84 PTH has been demonstrated to be hypocalcemic and able to lower bone turnover through an inhibition of osteoclast formation resulting in an overall inhibition of bone resorption. Whereas, 1-84 PTH operates through the PTH/PTHrp receptor the 7-84 PTH appears to operate through a C terminal PTH receptor. The CAP/CIP ratio decreases in the dialysis patient when calcium increases and vice versa. The 2nd generation "intact" PTH assays measure the sum of CAP plus CIP which render them ineffective at predicting bone turnover (72% predictive) and monitoring PTH suppressive treatments. By contrast the CAP/CIP ratio predicts bone turnover in the dialysis patient with a histologically determined 93% predictability. An elevated CAP/CIP ratio indicates high bone turnover and a decreased CAP/CIP ratio indicates adynamic low bone turnover.