Proteinuria is still useful for the screening and diagnosis of overt diabetic nephropathy

Abstract

OBJECTIVE: To assess the performance of urinary total protein measurements in
timed 24-h urine collection and in a diurnal random urine specimen for the screening
and diagnosis of overt diabetic nephropathy.
PATIENTS AND METHODS: A total of 167 diabetic patients (20 type 1 and 147 type
2 diabetic patients; 78 women and 89 men), aged 20-84 years, collected 217 timed
24-h urine specimens. Albumin was measured by immunoturbidimetry, total protein
by the sulfosalicylic acid technique, and creatinine by Jaffé’s method. According to
the timed 24-h urinary albumin excretion rate, samples were divided into three groups:
normoalbuminuric (urinary albumin excretion rate < 20 mg/min; n = 84),
microalbuminuric (urinary albumin excretion rate 20-200 mg/min; n = 78), and
macroalbuminuric (urinary albumin excretion rate > 200 mg/min; n = 55). Eight-six
patients also collected 105 random urine specimens (normoalbuminuric, n = 47;
microalbuminuric, n = 37; macroalbuminuric, n = 21), and urinary protein concentration
and urinary protein-to-creatinine ratio were measured. The receiver operating
characteristics curve approach was used to analyze the performance of the diagnostic
tests.
RESULTS: Spearman’s coefficient of correlation of 24-h urinary albumin excretion
rate versus 24-h urinary protein was 0.95 ( P < 0.001), and of 24-h urinary albumin
excretion rate versus urinary protein concentration and urinary protein-to-creatinine
ratio were 0.77 and 0.72, respectively (P < 0.001). The calculated areas (±SEM)
under the receiver operating characteristics curve for the diagnosis of overt diabetic
nephropathy were 0.9987 ± 0.001 for 24-h urinary protein, 0.9926 ± 0.006 for urinary
protein concentration, and 0.9751 ± 0.014 for urinary protein-to-creatinine ratio. In
the receiver operating characteristics curves, the first points with 100% sensivity
were 541 mg (95.7% specificity) for 24-h urinary protein, 431 mg/l (92.9% specificity)
for urinary protein concentration, and 0.2 (76.2% specificity) for urinary protein-tocreatinine ratio.
CONCLUSIONS: Measurements of proteinuria presented almost perfect accuracy
for the screening and diagnosis of overt diabetic nephropathy. Protein measurement
in spot urine is a reliable and simple method for the screening and diagnosis of overt
diabetic nephropathy.