CIRCADIAN VARIATION IN URINARY-EXCRETION OF BONE-COLLAGEN CROSS-LINKS

Bone resorption can be evaluated by measuring the urinary excretion of collagen type I cross-linked N telopeptides (NTx). Since it is diffic ult to obtain (and verify) 24 h urine collections from patients, untim ed spot urines are more practical, Such measurements, however, need co rrection for urine dilution and potentially may vary with collection t ime since a circadian rhythm in bone metabolism has been reported, Thi s study examined cross-link excretion in urine voids serially collecte d during a 24 h period from subjects living their normal daily routine (as opposed to a controlled hospital setting), This mimics the situat ion for walk-in patients visiting a clinician and providing a spot uri ne, A total of 35 dentists (20 males, 15 females) collected all urine voids separately over a 24 h period, Urines were analyzed for creatini ne and NTx. The effects of time of day on the excretion rates of these metabolites (in nmol/h) and on the cross-link:creatinine ratio were a ssessed, A circadian rhythm was evident in the excretion rate of creat inine with a peak in the late afternoon (18% higher than the 24 h mean , p = 0.0004), The NTx excretion rate peaked in the morning (9% higher than the 24 h mean) but this latter rhythm was not statistically sign ificant (p = 0.31). The NTx:creatinine ratio fell during the day from a high (122% of the 24 h mean) in the early morning to a low in the ea rly evening, This rhythm in the NTx:creatinine ratio in untimed spot u rines was statistically significant (p < 0.0001). In conclusion, the N Tx:creatinine ratio in spot urines from adult outpatient subjects show ed a significant circadian rhythm, Variations in creatinine excretion were the primary cause, Time of day should, therefore, be taken into a ccount when comparing test results of spot urines with normal ranges o r with other samples from the same subject.