Urine protein-to-creatinine ratio in an untimed urine collection is a reliable measure of proteinuria in lupus nephritis.
OBJECTIVE
To evaluate the accuracy of urine protein-to-creatinine (P/C) ratio in an untimed urine specimen as compared with 24 h total protein excretion for measurement of proteinuria in patients with lupus nephritis.


METHODS
Proteinuria in patients with lupus nephritis was assessed by 24 h total protein excretion and spot urine P/C ratio. Correlation and limits of agreement between the two methods were evaluated. The discriminant cutoff values for spot urine P/C ratio in predicting 24 h protein 'threshold' excretion of > or =0.3, > or =0.5, > or =1.0 and > or =3.5 g/day were determined using receiver operating characteristic curves.


RESULTS
A total of 165 samples were available for assessment with 21.8% excluded due to inadequate collection. A strong correlation (r = 0.91, P < 0.0001) was found between spot urine P/C ratio and 24 h urine protein excretion. Bland-Altman plot showed the two tests had acceptable limits of agreement in low level of protein excretion (-0.86 to +0.92 g/day when protein excretion was <2.0 g/day). The limits became wider as the protein excretion increased. The spot urine P/C ratios of 0.45 (sensitivity 0.92; specificity 0.88), 0.7 (0.92; 0.89) and 1.84 (1.0; 0.86) mg/mg reliably predicted 24 h urine total protein equivalent 'thresholds' at > or =0.5, 1.0 and 3.5 g/day.


CONCLUSIONS
This study supports the recommendation of using spot urine P/C ratio in screening and monitoring proteinuria in patients with lupus nephritis. However, in assessing the exact amount of proteinuria, the urine P/C ratio may have unacceptably wide limits of agreement in high protein excretion range.
Leung
Y Y
YY
Department of Medicine and Geriatrics, Tai Po Hospital, 9 Chuen On Road, Taipo, N.T., Hong Kong SAR. katyccc@hotmail.com
Szeto
C C
CC
Tam
L S
LS
Lam
C W K
CW
Li
E K
EK
Wong
K C
KC
Yu
S W
SW
Kun
E W
EW
D000328:Adult; D015415:Biomarkers / Q000097:blood; D003404:Creatinine / Q000652:urine*; D005260:Female; D006801:Humans; D008181:Lupus Nephritis / Q000150:complications*; D008297:Male; D008875:Middle Aged; D011507:Proteinuria / Q000175:diagnosis* / Q000209:etiology; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D013048:Specimen Handling / Q000379:methods
D015415:Biomarkers; D003404:Creatinine