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.
Christopher-Stine
Lisa
L
Department of Medicine, The Johns Hopkins Bloomberg School of Public Health, The Johns Hopkins University, Baltimore, Maryland, USA. LChrist4@jhmi.edu
Petri
Michelle
M
Astor
Brad C
BC
Fine
Derek
D
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