Albumin determination in frozen urines--underestimated results

Albumin determination by radioimmunoassay in fresh and frozen urine collections from 73 patients were performed. The values for albumin in fresh urines were 1-200 mg/24 h and were significantly higher (p less than 0.001) than the corresponding values in urines frozen for seven days (40.7 mg/24 h +/- 5.0 vs. 32.0 mg/24 h +/- 4.3). Similar results were obtained for protein determination, using turbidimetry, in urine collections from 45 proteinuric patients. Iodinated human albumin added to urine specimens was higher (p less than 0.001) in the pellets from frozen urines compared to urines kept at 4 degrees C for 1 and/or 7 days. By contrast, the radioactivity in the pellet of fresh urines kept at 4 degrees C for 1 or 7 days did not show any significant change. We suggest that freezing results in a partial albumin and protein sedimentation. Thus, determination of albumin in frozen urine specimens underestimates the real value by about 20%. This underestimation will limit our ability to diagnose borderline cases of microalbuminuria.