Clinical significance of free plasma hydroxyproline measurement in metabolic bone disease

Free hydroxyproline was measured in plasma of 67 normal subjects and in 70 patients with bone disease including primary hyperparathyroidism (n = 19), osteoporosis (n = 18), Paget's disease (n = 14), cancer involving bone (n = 8), chronic renal failure (n = 6), and osteomalacia (n = 6), and osteomalacia (n = 5). A good correlation was found between plasma and urinary values of the amino acid in normal subjects (r = 0.66; p less than 0.001). In patients with skeletal disorders a highly significant direct correlation was observed between free plasma hydroxyproline on the one hand and urinary hydroxyproline (r = 0.92; p less than 0.001) and serum alkaline phosphatase activity (r = 0.86; p less than 0.001) on the other, even though there were a few examples of dissociations among these parameters. Free plasma hydroxyproline decreased in the patients with Paget's disease following chronic administration of salmon calcitonin. Following successful parathyroidectomy, free plasma levels of hydroxyproline decreased in all the cases studied. Measurement of free plasma hydroxyproline thus appears to provide a specific index of bone metabolism that may be usefully employed as an alternative to the assay of other markers of bone turnover.