Absence of diurnal variation of C-reactive protein concentrations in healthy human subjects

Background: The concentration of C-reactive protein (CRP) in otherwise heal thy subjects has been shown to predict future risk of myocardial infarction and stroke. CRP is synthesized by the liver in response to interleukin-6, the serum concentration of which is subject to diurnal variation. Methods: To examine the existence of a time-of-day effect for baseline CRP values, we determined CRP concentrations in hourly blood samples drawn from healthy subjects (10 males, 3 females; age range, 21-35 years) during a ba seline day in a controlled environment (8 h of nighttime sleep). Results: Overall CRP concentrations were low, with only three subjects havi ng CRP concentrations >2 mg/L. Comparison of raw data showed stability of C RP concentrations throughout the 24 h studied. When compared with cutoff va lues of CRP quintile derived from population-based studies, misclassificati on of greater than one quintile did not occur as a result of diurnal variat ion in any of the subjects studied. Nonparametric ANOVA comparing different time points showed no significant differences for both raw and z-transform ed data. Analysis for rhythmic diurnal variation using a method fitting a c osine curve to the group data was negative. Conclusions: Our data show that baseline CRP concentrations are not subject to time-of-day variation and thus help to explain why CRP concentrations a re a better predictor of vascular risk than interleukin-6. Determination of CRP for cardiovascular risk prediction may be performed without concern fo r diurnal variation. (C) 2001 American Association for Clinical Chemistry.