Resumen
The use of urinary analytes to monitor physiological processes relies on making the correct measurement. Three alternatives are commonly contemplated: concentration, creatinine-corrected concentration and excretion rate. Of these, the latter is the most reliable, but is perceived by some to be difficult to measure. This has led to the more frequent reliance on concentration and one of the justifications for this is the reported linear relationship between the concentration and the creatinine-corrected concentration. We show that this correlation is spurious in that the magnitude of the correlation coefficient depends on the ratio of the standard deviations of the creatinine and analyte concentrations. As an example urinary analyte we use pregnanediol (Pd) which is an important tool for women wishing to monitor their own fertility. Urinary Pd concentration is not a reliable substitute for creatinine-corrected Pd concentration or the Pd excretion rate.