[Bone biomarkers in haemodialysis patients: bone alkaline phosphatase or ß-Crosslaps?]

Bone turnover (BT) abnormalities are frequently observed in patients with chronic kidney disease. Bone biopsy remains the gold standard for diagnosis; however, its invasive nature has led to its decreased utilisation. The serum parathyroid hormone (PTH) level is not a reliable bone marker (BM) for BT assessment. The latest international recommendations suggest the use of total alkaline phosphatase (t-ALP) or bone-specific alkaline phosphatase (b-ALP), but not ß-CrossLaps (CTX). We compared b-ALP, t-ALP, and CTX levels in patients on haemodialysis (HD).

All HD patients at a single institution following a standard 3×4 to 3×5 hours schedule were included in the study, provided they were free from liver disease. Serum intact PTH, t-ALP, b-ALP, and CTX values were compared at baseline and after 18 months of treatment. A kinetic study was performed for pre- and postdialysis CTX values over a 2-week period. We described the longitudinal evolution of these BMs in two typical patients.

A total of 98 patients on HD (46% female) were evaluated. The mean age was 69.8±11 years and the mean duration of dialysis was 54.4±61 months. At baseline, CTX (2.1±1 μg/L) correlated well with b-ALP (18±11 μg/L; r=0.64; P<0.001) and PTH (221±165 pg/mL; r=0.62; P<0.001). The changes in these values at 18 months were also correlated (ΔCTX compared with Δb-ALP: r=0.51; P<0.001; Δb-ALP compared with ΔPTH: r=0.37, P<0.01). b-ALP and t-ALP (245±132 U/L) were closely correlated (r=0.78), as was their variation over 18 months (r=0.67), but t-ALP did not correlate with PTH, and correlated poorly with CTX (r=0.38). The CTX reduction ratio during standard dialysis was approximately 70 to 75% over each session, although predialysis values remained stable.

In HD patients, mean CTX values are five times higher than the normal range. CTX appears to be an alternative to b-ALP for assessing BT. b-ALP remains the standard BM, despite being expensive, infrequently available in many laboratories, and not useful for patients with liver disease.