Predicting Kidney Transplantation Outcomes Using Proteinuria Ascertained From Spot Urine Samples Versus Timed Urine Collections

Background Proteinuria has been associated with transplant loss and mortality in kidney transplant recipients. Both spot samples (albumin-creatinine ratio [ACR] and protein-creatinine ratio [PCR]) and 24-hour collections (albumin excretion rate [AER] and protein excretion rate [PER]) have been used to quantify protein excretion, but which measurement is a better predictor of outcomes in kidney transplantation remains uncertain. Study Design Observational cohort study. Setting & Participants Tertiary care center, 207 kidney transplant recipients who were enrolled in a prospective study to measure glomerular filtration rate. Consecutive patients who met inclusion criteria were approached. Predictors ACR and PCR in spot urine samples, AER and PER in 24-hour urine collections. Outcomes Primary outcome included transplant loss, doubling of serum creatinine level, or death. Measurements Urine and serum creatinine were measured using a modified Jaffe reaction that had not been standardized by isotope-dilution mass spectrometry. Urine albumin was measured by immunoturbidimetry. Urine protein was measured by pyrogallol red molybdate complex formation using a timed end point method. Results Mean follow-up was 6.4 years and 22% developed the primary end point. Multivariable-adjusted areas under the receiver operating characteristic curves were similar for the different protein measurements: ACR (0.85; 95% CI, 0.79-0.89), PCR (0.84; 95% CI, 0.79-0.89), PER (0.86; 95% CI, 0.80-0.90), and AER (0.83; 95% CI, 0.78-0.88). C Index values also were similar for the different proteinuria measurements: 0.87 (95% CI, 0.79-0.95), 0.86 (95% CI, 0.79-0.94), 0.88 (95% CI, 0.82-0.94), and 0.86 (95% CI, 0.77-0.95) for log(ACR), log(PCR), log(PER), and log(AER), respectively. Limitations Single-center study. Measurement of proteinuria was at variable times posttransplantation. Conclusions Spot and 24-hour measurements of albumin and protein excretion are similar predictors of doubling of serum creatinine level, transplant loss, and death. Thus, spot urine samples are a suitable alternative to 24-hour urine collection for measuring protein excretion in this population.