Assessment of urinary N-telopeptide: point-of-care testing, sample types, and relationship to urinary helical peptide excretion

A point-of-care (POC) device would be useful in the space program for measuring N-telopeptide (NTX), a marker of bone resorption. This study was done to establish whether NTX measurements from a POC device compare favorably with standard (ELISA) techniques. We also compared results from fresh and frozen urine samples, samples collected on consecutive days, and second voids (of the day) and 24-h urine pools. Helical peptide (HP), another crosslink, was compared in second voids and 24-h urine pools.

Ten subjects collected urine for 96 h. NTX was measured with the POC device and by ELISA, and HP measured by ELISA. Seven subjects collected 24-h urine pools, and samples were analyzed fresh and after 27 days of freezing.

Excretion of NTX was greater (P<0.05) when measured by the POC device than when measured by ELISA, but was not different between second voids and 24-h urine pools, or among days. HP was similar in second voids and 24-h pools. Less NTX (P<0.05) was found in fresh 24-h pools [mean (S.D.) values, 38.4 (11.6) and 33.6 (9.2) nmol/mmol creatinine for the POC device and ELISA] than in previously frozen 24-h pools [42.7 (17.4) and 41.5 (12.5) nmol/mmol creatinine for the POC device and ELISA].

Comparisons between NTX measurements from frozen and fresh samples and those analyzed by POC and ELISA techniques should be made with caution.