Urine protein-to-creatinine ratio is a reliable measure of proteinuria in lupus nephritis.
OBJECTIVE
To evaluate the 24-hour urine protein-to-creatinine (U pr:cr) ratio compared to 24-h urine total protein excretion as a measure of proteinuria in patients with lupus nephritis.


METHODS
Proteinuria in 8 patients with lupus nephritis treated with cyclophosphamide was monitored by total protein excretion and U pr:cr ratio in 24-h urine collections. A median of 16 measurements per patient were collected over a median of 47 months. Adequacy of the 24-h collection was assessed by comparing total urine creatinine to the predicted creatinine. Collections in which the difference between the predicted 24-h urine creatinine and the measured 24-h urine creatinine was greater than or equal to 20% were defined as inadequate collections.


RESULTS
Timed 24-h urine collections were frequently inadequate (30.2% of total collections were under-collections, while 14.3% were over-collections). We found 87.5% of patients had at least one under-collection whereas 75% had at least one over-collection. Correlations between total protein and U pr:cr ratio for individual patients ranged from 0.87 to 0.99 (mean 0.95). For the entire sample, the correlation (R2 = 0.89) of total urine protein to Upr:cr ratio was excellent. Excluding the 38 under-collections led to improvement in the overall correlation (0.94). Excluding the 18 over-collections led to a correlation of 0.89. Excluding both under-collections and over-collections led to a correlation of 0.94.


CONCLUSIONS
In patients with lupus nephritis, the 24-h U pr:cr ratio is highly correlated with the 24-h urine protein excretion when the collections are adequate. The error of the estimate is higher when collections are poor.
Department of Medicine, The Johns Hopkins Bloomberg School of Public Health, The Johns Hopkins University, Baltimore, Maryland, USA. LChrist4@jhmi.edu
Lisa
L
Christopher-Stine
Michelle
M
Petri
Brad C
BC
Astor
Derek
D
Fine
D000328:Adult; D002940:Circadian Rhythm; D015331:Cohort Studies; D003404:Creatinine / Q000652:urine*; D003520:Cyclophosphamide / Q000008:administration & dosage; D005260:Female; D006801:Humans; D007166:Immunosuppressive Agents / Q000008:administration & dosage; D007275:Injections, Intravenous; D008181:Lupus Nephritis / Q000188:drug therapy / Q000652:urine*; D008297:Male; D011507:Proteinuria / Q000175:diagnosis* / Q000209:etiology / Q000652:urine*; D013048:Specimen Handling
D007166:Immunosuppressive Agents; D003520:Cyclophosphamide; D003404:Creatinine