Long-term Stability of Urinary Biomarkers of Acute Kidney Injury in Children

Background Recent meta-analyses support the utility of urinary biomarkers for the diagnosis and prognosis of acute kidney injury. It is critical to establish optimal sample handling conditions for short-term processing and long-term urinary storage prior to widespread clinical deployment and meaningful use in prospective clinical trials. Study Design Prospective study. Setting & Participants 80 children (median age, 1.1 [IQR, 0.5-4.2] years) undergoing cardiac surgery with cardiopulmonary bypass at our center. 50% of patients had acute kidney injury (defined as ≥50% increase in serum creatinine from baseline). Predictors We tested the effect on biomarker concentrations of short-term urine storage in ambient, refrigerator, and freezer conditions. We also tested the effects of multiple freeze-thaw cycles, as well as prolonged storage for 5 years. Outcomes Urine concentrations of neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule 1 (KIM-1), and interleukin 18 (IL-18). Measurements All biomarkers were measured using commercially available kits. Results All 3 biomarkers were stable in urine stored at 4°C for 24 hours, but showed significant degradation (5.6%-10.1% from baseline) when stored at 25°C. All 3 biomarkers showed only a small although significant decrease in concentration (0.77%-2.9% from baseline) after 3 freeze-thaw cycles. Similarly, all 3 biomarkers displayed only a small but significant decrease in concentration (0.84%-3.2%) after storage for 5 years. Limitations Only the 3 most widely studied biomarkers were tested. Protease inhibitors were not evaluated. Conclusions Short-term storage of urine samples for measurement of NGAL, KIM-1, and IL-18 may be performed at 4°C for up to 24 hours, but not at room temperature. These urinary biomarkers are stable at −80°C for up to 5 years of storage. Our results are reassuring for the deployment of these assays as biomarkers in clinical practice, as well as in prospective clinical studies requiring long-term urine storage.