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Reliability of Urinary Albumin, Total Protein, and Creatinine Assays after Prolonged Storage: The Family Investigation of Nephropathy and Diabetes

Rulan S. Parekh^*,, W.H. Linda Kao, Lucy A. Meoni^,, Eli Ipp^||, Paul L. Kimmel^¶, Janine La Page^||, Carol Fondran^**, William C. Knowler, Michael J. Klag^,,; and the Family Investigation of Nephropathy and Diabetes Research Group

Departments of ^* Pediatrics, Medicine, Johns Hopkins University School of Medicine Epidemiology, Biostatistics, and Health Policy and Management, and Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland; ^|| Los Angeles Biomedical Research Institute, Harbor-UCLA Medical Center, Los Angeles, California; ^¶ Kidney Urologic and Hematologic Diseases, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland, and Department of Medicine, George Washington University Medical Center, Washington, DC; ^** Department of Epidemiology and Biostatistics, School of Medicine, Case Western Reserve University, Cleveland, Ohio; and Diabetes Epidemiology and Clinical Research Section, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Phoenix, Arizona

Correspondence: Dr. Rulan S. Parekh, Welch Center for Prevention, Epidemiology & Clinical Research, 2024 Monument Street, Suite 2-511, Baltimore, MD 21205. Phone: 410-502-5745; Fax: 410-502-5746; E-mail: rsparekh{at}jhmi.edu

Background and objectives: This study investigated the effect of long-term storage at –70°C on urinary albumin, protein, and creatinine measurements in the Family Investigation of Nephropathy and Diabetes, a multicenter study designed to identify genes for diabetic nephropathy.

Design, setting, participants, & measurements: Spot urine samples were collected at eight centers and shipped overnight on ice packs to a central laboratory. Samples were aliquotted and frozen at –70°C for a median of 8 d before initial assay. As part of quality control procedures to determine interassay variability, 351 replicate samples were retrieved from storage at –70°C after a median storage time between original and quality control analyses of 126 d (range 28 to 869 d). Freezer time was characterized as the difference in days between the initial assay and quality control assay. Percentage difference [(quality control – original/original) x 100%] between samples was regressed on storage time and adjusted for original value, age, race, gender, hypertension, and diabetes.

Results: After adjustment, freezer time per 30 d was associated with small decreases in percentage difference of urinary albumin (0.25%, P = 0.02), total protein (0.23%, P = 0.02), and albumin-to-creatinine ratio (0.34%, P = 0.001). Urinary creatinine levels were not affected by freezer time (P = 0.25).

Conclusions: Measurements of urinary albumin, total protein, and albumin-to-creatinine ratio are minimally affected by storage at –70°C for approximately 2.5 yr. Prolonged storage results in small decreases of urinary albumin and protein that do not substantially affect phenotype classification of overt renal disease.