Biochemical Assessment Of Skeletal Activity

Biochemical measurements in the prediction of histologic subtype of renal transplant bone disease in women

Renal transplant osteodystrophy encompasses several histologic subtypes. Bone histomorphometric examination reliably distinguishes these groups but is invasive, is time-consuming, and delays diagnosis. Establishing a noninvasive method of correctly predicting histologic subtype in an individual to direct management is an attractive proposition. We identified 19 female renal transplant recipients with histologic evidence of hyperparathyroid bone disease (HPTH) and 14 with adynamic bone (ADB). We evaluated serum osteocalcin and bone-specific alkaline phosphatase as bone formation markers and urinary hydroxyproline (Hypro) and deoxypyridinoline cross-links as bone resorption markers. Mean concentrations for all markers were higher in the HPTH group, reaching significance for Hypro (HPTH, 24.8 +/- 4.2 micromol/mmol creatinine; ADB, 13.2 +/- 5.0 micromol/mmol creatinine; P = 0.01). A cutoff of 16.4 micromol/mmol creatinine for Hypro (Youden's index, 0.65) gave a sensitivity of 93% and specificity and positive predictive value (PPV) of 72% in predicting HPTH. In combination, Hypro greater than 16.4 micromol/mmol creatinine and parathyroid hormone greater than 80 pg/mL gave a specificity of 100%, sensitivity of 32%, and PPV of 100%. Conversely, for predicting ADB, Hypro less than 15.1 micromol/mmol creatinine (Youden's index, 0.45) gave a specificity of 93%, sensitivity of 53%, and PPV of 91%. Hypro less than 15.1 micromol/mmol creatinine plus osteocalcin less than 6.8 microg/L gave a specificity of 84.2%, sensitivity of 64.3%, and PPV of 75%. Significant associations between markers and histomorphometry were evident only for Hypro and osteocalcin (with osteoblast surface) and all markers (except deoxypyridinoline cross-links) with cortical volume. Markers have limited utility in identifying histologic subtype (Hypro was most effective) and, with the exception of Hypro and osteocalcin, showed little association with cell surface markers of bone cell activity.

The role of bone turnover markers and risk factors in the assessment of osteoporosis and fracture risk

The clinical evaluation of osteoporosis in individual patients involves confirmation of the diagnosis, the investigation of secondary causes of osteoporosis and the evaluation of subsequent fracture risk. Optimum clinical assessment involves bone mineral densitometry with the treatment thresholds modified by clinical risk factors for individual patients. Bone turnover markers and clinical risk factors can be used to identify patients at risk of osteoporotic fracture and those who have secondary osteoporosis. Risk assessment should involve the evaluation of absolute rather than relative risk. Further work is required to improve the integration of clinical risk factors, bone turnover markers and bone densitometry into appropriate models to enable the assessment of the absolute risk of fracture for individual patients.

Serum Galactosyl Hydroxylysine as a Biochemical Marker of Bone Resorption

Background: Serum-based biochemical markers of bone resorption may provide better clinical information than urinary markers because direct comparison with serum markers of bone formation is possible and because the within-subject variability of serum markers may be lower. We describe a method for the measurement of free β-1-galactosyl-O-hydroxylysine (Gal-Hyl) in serum.

Methods: The assay used preliminary ultrafiltration of serum, dansylation, and separation by reversed-phase HPLC with fluorescence detection. Healthy subjects were recruited from population-based studies of bone turnover.

Results: The within-run (n = 15) and between-run (n = 15) CVs were 7% and 14%, respectively, at a mean value of 48 nmol/L. In women and pubertal girls, serum free Gal-Hyl correlated with urine free Gal-Hyl (r = 0.84; P <0.001). Serum Gal-Hyl was higher during puberty and increased after menopause. The fractional renal clearance of free Gal-Hyl relative to that of creatinine was 0.90 (95% confidence interval, 0.82–0.98). Serum free Gal-Hyl decreased by 36% (SE = 4%) in 14 patients with mild Paget disease treated with an oral bisphosphonate, and this decrease was significantly (P <0.001) greater than that seen for either serum tartrate-resistant acid phosphatase (9%; SE = 4%) or serum C-terminal telopeptide of collagen I (19%; SE = 8%).

Conclusion: Serum free Gal-Hyl may be useful as a serum marker of bone resorption.