Reevaluation of the usefulness of serum free hydroxyproline as a parameter for assessing renal osteodystrophy

Hydroxyproline (HYP) is an amino acid which is highly specific for bone collagen. Measurement of HYP provides an index of bone resorption, although the usefulness in this respect of the serum free HYP, which is the easiest HYP fraction to determine, has not yet been established. In the present study, the serum free HYP was measured using the method of Dabev et al., and the data were compared against other parameters of renal osteodystrophy (ROD). In comparison with values for the normal control group (n = 10), the serum free HYP was significantly elevated in the patient group with conservatively treated end-stage renal failure (n = 14) and in the patient group on chronic hemodialysis (n = 107), with the latter group showing the highest value. Also, in the group with radiographic evidence of bone resorption, the free HYP exhibited a significant elevation. Significant positive correlations were noted between the free HYP and both the parathyroid hormone (PTH) and alkaline phosphatase (Al-P) levels. When the subjects undergoing bone biopsy were divided according to their histological findings into an increased osteoid group and an increased osteoid + resorption group, the latter displayed a significantly higher free HYP value. In addition, the free HYP values were low in the group administered 1 alpha-OH-D3 and showing only slight bone resorption. These results suggest that since the free HYP closely reflects accelerated bone resorption in ROD and is easier to measure than non-protein-bound HYP, it can serve as a clinically useful index of ROD.