[Pitfalls in measuring urinary proteins: age-related changes in urinary creatinine excretion that affect the urine protein/creatinine ratio]

Abstract

Knowing the amount of protein excreted in the urine is important in determining the severity and activity of renal diseases. In general, screening tests have been carried out using the urine dipstick. However, there are limitations in determining the amount of urinary protein excretion using qualitative tests for protein in spot urine samples due to the concentration and dilution of urine. Therefore, when using spot urine samples, it is helpful to calculate the urine protein/creatinine ratio (P/C) by simultaneous measurement of urinary creatinine for determining daily protein excretion. We examined P/C measurements using the dipstick method in 22,718 subjects who visited our hospital for health examinations. The results showed positive rates for qualitative urinary protein (1 + and more) of 4.2% for males and 2.7% for females. Also positive rates for P/C (150 mg/g.cre and more) were found of 7.7% for males and 10.2% for females. The results showed a reversal of positive rates for males and females compared with the results of qualitative urinary protein. In addition, P/C showed a higher positive rate in 70 years old or older both for males and females. The distribution of urinary creatinine levels simultaneously measured by dipstick method showed that the percentage of diluted urine with urinary creatinine level less than 50 mg/dL was 6.8% for males and 18.3% for females overall. Females showed a higher rate and the percentage tended to increase with age both for males and females. From these results, it was suggested that changes in urinary creatinine excretion with age that affect the P/C ratio are large. We then measured the albumin excretion rate in the 24-hour urine as well as examined the correlation between the urinary creatinine concentration and albumin index with regard to age and sex in 1,280 diabetic patients. The results showed that daily urinary creatinine excretion overall in males, overall in females, in males over 80 years old and in females over 80 years old were 1.063 +/- 0.428 g/day (mean +/- S.D.), 0.714 +/- 0.270 g/day, 0.666 +/- 0.201 g/day, and 0.531 +/- 0.017 g/day, respectively, which showed large variations. From these results, creatinine correction, in which uniformity of daily urinary creatinine excretion is the principle, is useful for assessment of short-term fluctuations in the individuals. However, it is indicated that there is a problem when used for screening, from young people to the elderly.