BACKGROUND The study of novel urinary biomarkers of acute kidney injury has expanded exponentially. Effective interpretation of data and meaningful comparisons between studies require awareness of factors that can adversely affect measurement. We examined how variations in short-term storage and processing might affect measurement of urine biomarkers. STUDY DESIGN Cross-sectional, prospective. SETTING & PARTICIPANTS Hospitalized patients from two sites: Yale New Haven Hospital (n= 52) and University of California, San Francisco Medical Center (n=36) PREDICTORS We tested the impact of 3 urine processing conditions on these biomarkers: a) centrifugation and storage at 4°C for 48 hours before freezing at −80°C, b) centrifugation and storage at 25°C for 48 hours before freezing at −80°C, and c) uncentrifuged samples immediately frozen at −80°C. OUTCOMES Urine concentration of five biomarkers: neutrophil gelatinase-associated lipocalin (NGAL), interleukin 18 (IL-18), kidney injury molecule 1 (KIM-1), liver-type fatty acid–binding protein (L-FABP) and cystatin C MEASUREMENTS We measured urine biomarkers by an established ELISA method. Biomarker values were log-transformed, and agreement with a reference standard of immediate centrifugation and storage at −80°C was compared using concordance correlation coefficients (CCCs). RESULTS Neither storing samples at 4°C for 48 hours nor centrifugation had a significant effect on measured levels, with CCCs above 0.9 for all biomarkers tested. For samples stored at 25°C for 48 hours, excellent CCC values (>0.9) were also noted between the test sample and the reference standard for NGAL, cystatin C, L-FABP and KIM-1. However, the CCC for IL-18 between samples stored at 25°C for 48 hours and the reference standard was 0.81 (95% CI, 0.66–0.96). Limitations No comparisons to fresh “unfrozen” samples, no evaluation of the effect of protease inhibitors. CONCLUSIONS All candidate markers tested using the specified assays showed high stability with both short-term storage at 4°C and without centrifugation prior to freezing. For optimal fidelity, urine for IL-18 measurement should not be stored at 25°C before long-term storage or analysis.
SummaryBackground and objectives Preoperative proteinuria is associated with a higher incidence of postoperative AKI. Whether the same is true for postoperative proteinuria is uncertain. This study tested the hypothesis that increased proteinuria after cardiac surgery is associated with an increased risk for AKI.Design, setting, participants, & measurements This prospective cohort study included 1198 adults undergoing cardiac surgery at six hospitals between July 2007 and December 2009. Albuminuria, urine albumin-to-creatinine ratio (ACR), and dipstick proteinuria were measured 0-6 hours after surgery. The primary outcome was AKI, defined as a doubling in serum creatinine or receipt of acute dialysis during the hospital stay. Analyses were adjusted for patient characteristics, including preoperative albuminuria.Results Compared with the lowest quintile, the highest quintile of albuminuria and highest grouping of dipstick proteinuria were associated with greatest risk for AKI (adjusted relative risks Conclusions Higher levels of proteinuria after cardiac surgery identify patients at increased risk for AKI during their hospital stay.
Background Acute kidney injury (AKI) is common after cardiac surgery and is associated with post-operative mortality. Perioperative cardiac biomarkers may predict AKI and mortality. Objective We evaluated whether cardiac biomarkers were associated with severe AKI, defined as a doubling in serum creatinine or requiring renal replacement therapy during hospital stay after surgery, and mortality. Methods In a prospective multicenter cohort of adults undergoing cardiac surgery, we measured the following biomarkers in pre- and post-operative banked plasma: high-sensitivity troponin T (hs-cTnT), troponin I (cTnI), CK-MB and NT-proBNP. Results In the patients who were discharged alive, severe AKI occurred in 37/960 (3.9%) and 43/960 (4.5%) died within 1 year of follow-up. NT-proBNP was the only pre-operative biomarker that was independently associated with severe AKI (with log transformation, adjusted OR=1.4, 95% CI (1.0, 1.9)). Biomarkers measured within 6 hours of surgery (Day 1) were all associated with severe AKI. Pre-operative NT-proBNP was also independently associated with 1-year mortality (with log transformation, adjusted OR=1.7, 95% CI (1.2–2.2)). Patients in the highest tertile for NT-proBNP pre-operatively (>1006.4 ng/L) had marked increases in their risk for 1-year mortality (adjusted OR=27.2, 95%CI (3.5–213.5)). Day 1 NT-proBNP was associated with mortality independently of change in serum creatinine from pre-operative baseline. Conclusion Of the studied biomarkers, NT-proBNP was the only pre-operative biomarker independently associated with severe AKI and mortality. Early increases in post-operative cardiac biomarkers were associated with severe AKI after cardiac surgery. Future research should focus on whether interventions that lower NT-proBNP can impact upon post-operative outcomes.
Background and objectives Data reported to the Organ Procurement and Transplantation Network (OPTN) are used in kidney transplant research, policy development, and assessment of center quality, but the accuracy of early post-transplant outcome measures is unknown.Design, setting, participants, & measurements The Deceased Donor Study (DDS) is a prospective cohort study at five transplant centers. Research coordinators manually abstracted data from electronic records for 557 adults who underwent deceased donor kidney transplantation between April of 2010 and November of 2013. We compared the post-transplant outcomes of delayed graft function (DGF; defined as dialysis in the first posttransplant week), acute rejection, and post-transplant serum creatinine reported to the OPTN with data collected for the DDS.Results Median kidney donor risk index was 1.22 (interquartile range [IQR], 0.97-1.53). Median recipient age was 55 (IQR, 46-63) years old, 63% were men, and 47% were black; 93% had received dialysis before transplant. Using DDS data as the gold standard, we found that pretransplant dialysis was not reported to the OPTN in only 11 (2%) instances. DGF in OPTN data had a sensitivity of 89% (95% confidence interval [95% CI], 84% to 93%) and specificity of 98% (95% CI, 96% to 99%). Surprisingly, the OPTN data accurately identified acute allograft rejection in only 20 of 47 instances (n=488; sensitivity of 43%; 95% CI, 17% to 73%). Across participating centers, sensitivity of acute rejection varied widely from 23% to 100%, whereas specificity was uniformly high (92%-100%). Six-month serum creatinine values in DDS and OPTN data had high concordance (n=490; Lin concordance correlation =0.90; 95% CI, 0.88 to 0.92).Conclusions OPTN outcomes for recipients of deceased donor kidney transplants have high validity for DGF and 6-month allograft function but lack sensitivity in detecting rejection. Future studies using OPTN data may consider focusing on allograft function at 6 months as a useful outcome.
Background The interaction between baseline kidney function and the performance of biomarkers of acute kidney injury (AKI) on the development of AKI is unclear. Study Design Post-hoc analysis of prospective cohort study. Setting & Participants The 1,219 TRIBE-AKI Consortium adult cardiac surgery cohort participants. Predictor Unadjusted post-operative urinary biomarkers of AKI measured within 6 hours of surgery. Outcome AKI was defined as greater than or equal to AKI Network stage 1 (any AKI) as well as a doubling of serum creatinine from the pre-operative value or the need for emergent dialysis (severe AKI). Measurements Stratified analyses by a pre-operative eGFR ≤ 60 ml/min/1.73 m2 vs. > 60 ml/min/1.73 m2. Results 180 (42%) patients with a pre-operative eGFR ≤ 60 ml/min/1.73m2 developed clinical AKI compared to 246 (31%) in those with an eGFR >60 ml/min//1.73m2 (p<0.001). For log2-transformed biomarker concentrations there was a significant interaction between any AKI and baseline eGFR for interleukin 18 (IL-18; p=0.007) and borderline significance for liver-type fatty acid binding protein (p=0.06). For all biomarkers, the adjusted relative risk (RR) point estimates for the risk of any AKI were higher in those with elevated baseline eGFRs compared to those with an eGFR ≤ 60 ml/min/1.73m2. However the difference in magnitude of these risks were quite low (adjusted RRs were 1.04 [95% CI, 0.99–1.09] and 1.11 [95% CI, 1.07–1.15] for those with a pre-operative eGFR ≤ 60 ml/min/1.73 m2 and those with higher eGFRs, respectively). Although no biomarker displayed an interaction for baseline eGFR and severe AKI, log2-transformed IL-18 and kidney injury molecule 1 (KIM-1) had significant adjusted RRs for severe AKI in those with and without baseline eGFR ≤ 60 ml/min/1.73 m2. Limitations Limited numbers of patients with severe AKI and emergent dialysis. Conclusions The association between early post-operative AKI urinary biomarkers and AKI is modified by preoperative eGFR. The degree of this modification and its impact on the biomarker-AKI association is small across biomarkers. Our findings suggest that distinct biomarker cut-offs for those with and without a pre-operative eGFR ≤ 60 ml/min/1.73 m2 is not necessary.
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