title: Reliability of urinary albumin, total protein, and creatinine assays after prolonged storage: the Family Investigation of Nephropathy and Diabetes.
abstract: OBJECTIVE
  This study investigated the effect of long-term storage at -70 degrees 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.
  
  
  METHODS
  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 degrees 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 degrees 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 degrees 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.
authors.affiliation: Department of Pediatrics, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. rsparekh@jhmi.edu
authors.forename: Rulan S
authors.initials: RS
authors.lastname: Parekh
authors.forename: W H Linda
authors.initials: WH
authors.lastname: Kao
authors.forename: Lucy A
authors.initials: LA
authors.lastname: Meoni
authors.forename: Eli
authors.initials: E
authors.lastname: Ipp
authors.forename: Paul L
authors.initials: PL
authors.lastname: Kimmel
authors.forename: Janine
authors.initials: J
authors.lastname: La Page
authors.forename: Carol
authors.initials: C
authors.lastname: Fondran
authors.forename: William C
authors.initials: WC
authors.lastname: Knowler
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Klag
mesh_terms: D000328:Adult; D000368:Aged; D000419:Albuminuria / Q000652:urine*; D016022:Case-Control Studies; D003404:Creatinine / Q000652:urine*; D005260:Female; D005615:Freezing; D006801:Humans; D008297:Male; D008875:Middle Aged; D011507:Proteinuria / Q000652:urine*; D011786:Quality Control; D013048:Specimen Handling
chemical_list: D003404:Creatinine