Impact of renal failure on metabolic bone parameters in a vitamin D-deficient patient cohort

Chronic renal failure is associated with major changes in bone metabolism, but studies evaluating bone metabolism with mild or moderate renal failure are rare. Moreover, the study populations were often heterogenous and/or patients were pretreated with calcium and vitamin-D preparations. Therefore, we prospectively evaluated metabolic bone parameters in patients with renal insufficiency (Stage 1 - 4) on their first visit to outpatient nephrologists.

285 patients were prospectively evaluated regarding renal function, serum phosphorous, plasma parathyroid hormone (PTH), serum 25-OH-vitamin-D and serum bone specific alkaline phosphatase (BAP) concentrations. Patients were subdivided according to the stages of chronic kidney disease.

Hypocalcemia occurred in only 10% of patients in Stage 4, whereas serum phosphorous was elevated at the same stage in 40% of the patients. PTH increased from Stage 1 to 4 continually with a high prevalence of elevated PTH levels (> 65 pg/ml) in Stage 1: 44%; Stage 4: 84%. Serum 25-OH-vitamin-D levels were very low irrespective of renal function: < 15 ng/dl, i.e., 37.5 nmol/l in 70% of all patients. 25-OH-D was negatively correlated with PTH (r = 0.3, p < 0.0002). BAP was within the normal range in all stages but with a high prevalence of BAP values < 7.5 ng/ml in up to 25% in Stage 4. Only 6.5% of patients had features of classical renal hyperparathyroidism. Nearly 20% had low BAP levels in the presence of normal (9.5%) or increased (9.6%) PTH levels.

This study demonstrates a high prevalence of hyperphosphatemia in patients with moderate renal failure. Hyperparathyroidism was present even in earlier stages and was aggravated by a high prevalence of vitamin D deficiency. However, also in the presence of elevated PTH levels, there is indication of low bone turnover as evidenced by low BAP levels, suggesting adynamic bone disease.