Does creatinine adjustment of urinary pregnanediol glucuronide reduce or introduce measurement error?

This is the authors' abstract. We don't add key points for this paper.

Correlation between urinary pregnanediol and serum progesterone measurements and the influence of age, race, smoking and urinary creatinine adjustment was determined during the luteal phase of the menstrual cycle in 175 volunteers. A decline in serum progesterone was observed with increasing age. Mean baseline urinary creatinine declined with increasing age in non-smokers and was not affected by race or baseline weight. An excellent correlation between urinary pregnanediol glucuronide and serum progesterone levels existed except when urinary pregnanediol concentrations were adjusted using creatinine measurements in older individuals. Adjustment of urinary pregnanediol glucuronide concentration using creatinine measurement is therefore discouraged.