Biochemical markers of bone turnover in seronegative spondylarthropathy: relationship to disease activity.

To investigate bone turnover in patients with seronegative spondylarthropathy, a bone formation marker, type I procollagen carboxy-terminal propeptide (PICP), and resorption markers, the pyridinium cross-links of collagen [urinary free (f) PYR and DPYR], were measured. The median f-PYR, f-DPYR and PICP (±interquartile range) were 15.8 (6.00) nmol/mmol creatinine, 3.8 (2.2) nmol/mmol creatinine and 101.5 (38) μg/l respectively. There was a positive correlation between resorption markers and acute-phase reactants such as C-reactive protein (r = 0.4 for PYR, r = 0.42 for DPYR, P < 0.05), and a negative correlation observed between PICP and the erythrocyte sedimentation rate (r = -0.64, P < 0.05). In the subgroup of patients with an elevated CRP concentration, the concentration of PYR and DPYR was significantly increased (f-PYR 25.7 vs 15.8 and f-DPYR 6.6 vs 3.8, P < 0.01 for f-PYR, P < 0.05 for f-DPYR). This study suggests than an elevation in acute-phase response in patients with seronegative spondylarthropathy is associated with increased concentration of bone resorption markers with a tendency for reduction in bone formation markers. This may represent uncoupling of bone formation and resorption, leading to bone loss in such patients.