Urinary element concentrations in kidney stone formers and normal controls: the week-end effect.

Early morning urines were collected on each of three days (Monday-Wednesday) from 19 male stone-formers and 20 healthy male controls. Concentrations of 10 elements were determined using inductively coupled plasma atomic emission spectroscopy, graphite furnace atomic absorption spectroscopy and particle induced x-ray emission spectrometry. Data were treated using multivariate statistical methods. The results showed that the concentrations of several elements in the control urines were significantly raised on Mondays and that controls and stone formers differed with respect to certain elements, also on Mondays. The observed elevations were termed the "weekend effect". It is suggested that while stone-formers may regulate their diets throughout the entire week, normals may indulge in dietary excesses over the weekend. The weekend effect highlights the danger of placing too much emphasis on a single measurement of a urinary parameter and alerts investigators to take cognizance of the day on which urine collections are effected, especially when attempting to identify abnormal renal excretion patterns in stone-formers by comparison of their urinary variables with those of controls.