Simplified quantification of urinary protein excretion in children.

The protein/creatinine ratio (Up/Uc) measured in 71 early morning urine samples (EMU) correlated closely with timed overnight urine (ONU) protein excretion rates (r = 0.96). The relationship was linear throughout the entire range of normal and abnormal protein excretion, an ONU rate of 1 mg/h/m2 body surface area being proportional to an EMU Up/Uc of 5 mg/mmol. Using the Coomassie Blue dye-binding method the upper limit of Up/Uc in 377 apparently healthy children and adolescents aged 3-19 years was shown to be 20 mg/mmol. Albumin/creatinine ratios (Ua/Uc) were also determined in the 377 healthy subjects, yielding a normal working upper limit of 3 mg/mmol. Although in normal individuals studied longitudinally the day-to-day variation of both Up/Uc and Ua/Uc was appreciable, all measurements remained within the cross-sectional normal range. While the determination of Ua/Uc has a role in the study of "microproteinuria", it is comparatively costly for routine use. The measurement of the EMU Up/Uc avoids errors and difficulties associated with timed urine collection, simplifies sample handling by the laboratory and is inexpensive. In clinical practice this is the method of choice for the quantification of proteinuria in patients with renal disease.