Instability and variability of urinary telopeptides and free crosslinks

The use of the urinary bone degradation markers, NTx and free Dpd, in the diagnosis and monitoring of pathological bone resorption has been studied intensely. Urinary NTx and Dpd are normalized by creatinine to account for differences in diuresis.

We investigated the effects of storage (-20 degrees C for 119 days) on concentrations of NTx, Dpd and creatinine in the urine of 40 subjects. Precipitates (visible upon thawing) were resuspended, but specimens were not recentrifuged (in contrast to previous experiments).

After storage at -20 degrees C, the NTx and creatinine concentrations in urine specimens were significantly decreased by about 18% and 22% (p < 0.01 and p < 0.0001), respectively. Thus, the quotient of NTx/creatinine was slightly increased after storage (p < 0.05). Free Dpd and free Dpd/creatinine were both significantly increased after storage (p < 0.0001)--by about 20% and 60%, respectively.

We suggest that storage at -20 degrees C frees Dpd and denatures the epitope used in NTx quantification, and that these changes are usually masked by the removal of free Dpd via recentrifugation and by the decrease in creatinine in NTx/creatinine. Additionally, we suggest that the quotient Dpd/NTx should be evaluated as an alternative to normalizing via creatinine excretion.