Comments on plasma parathyroid hormone levels and their relationship to bone histopathology among patients undergoing dialysis

A new approach for evaluating bone turnover in chronic kidney disease

BACKGROUND

The validity of serum parathyroid hormone (PTH) as a surrogate marker of bone turnover in chronic kidney disease (CKD) is limited by several factors such as relative resistance of bone to PTH, hyperphosphatemia, diabetes, gender, age, race and vitamin D analog action on the PTH-bone axis. Urinary collagen N-terminal telopeptide X (NTx), a bone collagen degradation product, expressed as bone collagen equivalents (BCE) per mM of creatinine (NTx/Cr ratio), is routinely used to estimate bone turnover in osteoporosis. The purpose of this study is to evaluate NTx as a marker of bone turnover in CKD.

METHODS

We studied the relationship between bone-specific alkaline phosphatase (BSAP), PTH and urine NTx/Cr in 37 CKD out-patients.

RESULTS

In a multivariate model, PTH had a positive correlation with BSAP (r=0.44, P<0.19) and U-NTx/Cr (r=0.55, P<0.30), after adjusting for age, gender, estimated glomerular filtration rate (GFR), serum phosphorus, corrected calcium, and race. However, the strongest correlation was found between the two direct markers of bone resorption and formation (U-NTx vs. BSAP; r=0.80; P<0.0001), suggesting a tight coupling of bone resorption and formation in CKD. The effect of gender on U-NTx/Cr was studied in a multivariate model after adjusting for age, race, GFR, serum calcium, phosphorus and PTH. Females had a higher U-NTx/Cr than males.

CONCLUSION

Our findings indicate that urinary NTx, a promising marker of bone resorption in CKD patients, exhibits a strong positive correlation with other markers used to assess renal osteodystrophy i.e. PTH and BSAP. Unlike PTH and BSAP, urine NTx also measures bone loss secondary to osteoporosis.

Association of serum alkaline phosphatase and bone mineral density in maintenance hemodialysis patients

Recent studies indicate that serum alkaline phosphatase (AlkPhos), a surrogate of high turnover bone disease, is associated with coronary artery calcification and death risk in maintenance hemodialysis (MHD) patients. The association between AlkPhos and bone mineral density (BMD) is not well studied. We studied the association between AlkPhos and dual-energy X-ray absorptiometry-assessed BMD in a group of MHD patients in Southern California. In 154 MHD patients, aged 55.3 +/- 13.6 years, including 42% women, 38% Hispanics, 42% African Americans, and 55% diabetics, the mean serum AlkPhos was 121 +/- 63 U/L (median: 101, Q(25-75): 81-141); 36% had AlkPhos>/=120 U/L and 50% had a total T-score< or =-1. Whereas the total BMD did not correlate with age (r=0.01, P=0.99) or body mass index (r=0.10, P=0.22), it correlated negatively with AlkPhos (r=-0.25, P=0.002), including after multivariate adjustment (r=-0.24, P=0.003). The proportion of patients with a high coronary artery calcification score>400 was incrementally higher across worsening BMD tertiles (P trend=0.04). The BMD was significantly worse in MHD patients with serum AlkPhos> or =120 U/L compared with <120 U/L (1.01 +/- 0.016 vs. 1.08 +/- 0.013 g/cm(2), respectively, P<0.001). The multivariate adjusted odds ratio of AlkPhos> or =120 U/L for having a total T-score<-1.0 was 2.3 (1.1-4.8, P=0.037). Among routine clinical and biochemical markers, serum AlkPhos> or =120 U/L was a better predictor of total T-score< or =-1 in MHD patients. An association exists between higher serum AlkPhos and worse dual-energy X-ray absorptiometry-assessed BMD in MHD patients. Given these findings, studies are indicated to examine whether interventions that lower serum AlkPhos improve BMD in MHD patients.