Objectives: To investigate the inter- and intra-individual variation in the excretion of pyridinium crosslinks of collagen in first morning void urine samples in healthy young adults.
Design: An observational study in which urinary pyridinium crosslinks of collagen were measured in fasting first void urine samples over five consecutive days.
Setting: The study was conducted in the Department of Nutrition, University College, Cork, Ireland.
Subjects: A total of 17 healthy young adults (11 females and 6 males) aged 22-26 y were recruited from University College, Cork. The subjects were without any history of bone or articular disease, and with no intake of medicine that could affect bone or cartilage metabolism.
Results: There were large intra-individual variations in the excretion of both pyridinoline (Pyr) and deoxypyridinoline (Dpyr) in first morning void urine samples and the ratios of intra- to inter-individual variation were high. In comparison, inter-individual variation in excretion of crosslinks was relatively low. No significant day-to-day variation was observed either in Pyr or Dpyr excretion when either five, four or three consecutive days or four or three non-consecutive days of urine sampling were used. However, there were significant differences between any two consecutive or non-consecutive days in both Pyr and Dpyr excretion.
Conclusions: The relatively large day-to-day variation in urinary excretion of the pyridinium crosslinks emphasises the need to use multiple samples to characterise the crosslink excretion of an individual. The present study demonstrates that urine sampling for these crosslinks necessitates at least a 3 d collection to ensure that day-to-day effects are not a significant source of variation included in the data analysis.