Because the protein in urine is generally quite low (< 1 g/dL), it cannot be measured using the Biuret procedure that is used for measuring total protein in serum or plasma. A more sensitive technique is required. On the Hitachi, we measure protein in urine (and CSF) with benzethonium chloride, which precipitates the protein and increases the turbidity of the sample. The turbidity is proportional to the protein concentration. This technique is very sensitive and can yield accurate results in samples with very low protein concentrations (< 20 mg/dL). Creatinine in urine is measured using our standard rate-blanked creatinine procedure.

The protein-creatinine ratio is used to provide an estimate of the amount of protein lost in the urine. The urine protein to creatinine ratio on random mid-day urine samples correlates well to 24-hour urine collection, for quantitating urinary protein loss. The degree of proteinuria yields useful information on the source of protein loss; namely losses of large amounts of protein (with high urine protein to creatinine ratios) is due to glomerular, and not tubular, disease. Urine protein to creatinine ratios should only be performed on urine samples with evidence of excess protein (with consideration of the USG) and no evidence of cystitis. For example, there is no point in performing a urine protein to creatinine ratio in a patient with a USG of 1.035 and trace protein on the dipstick or SSA reaction). Inflammatory conditions in the urinary tract will increase protein and negate the usefulness of the ratio for determining the source of protein loss.

For more information on interpretation of ratios, please refer to the Routine Urinalysis Module.