Predicting kidney transplantation outcomes using proteinuria ascertained from spot urine samples versus timed urine collections

Abstract

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 Jaffé 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.

Keywords: 24-hour urine collection; Proteinuria; albumin excretion rate (AER); albumin-creatinine ratio (ACR); albuminuria; end-stage renal disease (ESRD); kidney graft survival; outcomes; protein excretion rate (PER); protein-creatinine ratio (PCR); spot urine sample.