Turbidimetric measurement of total urinary proteins: a revised method.

A turbidimetric method for the determination of total urinary proteins is proposed that uses a specimen blank for each urine. The precipitant, trichloroacetic acid (TCA), is introduced into the blank and the sample in the same amount. The blank is the supernatant of the TCA-treated urine for each specimen. Turbidity is measured by reading the 420-nm absorbance 35 minutes after the TCA addition when the turbidity curves for most urine samples plateau. This improved method was compared with an established procedure that uses water to prepare the blanks and that measures turbidity 5 minutes after the TCA addition. Of the 69 urine specimens that were analyzed by the two methods, the revised procedure yielded higher values (greater than or equal to 150%) for 5 samples, while 21 samples had lower values (less than or equal to 50%). In the latter group, four cases would have been classified erroneously as having significant proteinuria by the established method.