Impact of Storage Conditions on the Stability of Urinary Biomarkers

This study aimed to assess the influence of different storage conditions and pH levels on biomarkers in urine.

Urine samples were collected from 20 healthy individuals and 20 patients with kidney disease. The stability of nine proteins with normal and pathological concentrations was investigated under 4 types of storage conditions: 25°C for 8 hours, 4°C for 7 days, -20°C for 12 weeks, and -80°C for 12 months. Ten pH values (4.0 to 8.5) were also tested. Nine biomarkers included U-α1MG, U-Alb, U-Trf, U-IgG, U-NAG, U-NGAL, U-RBP, U-CysC, and U-TP, mostly measured in the laboratory.

U-Alb, U-α1MG, U-Trf, U-TP, and U-CysC were relatively stable across four storage conditions. However, the stability of other biomarkers may be impacted by the storage conditions. For instance, U-IgG was unstable at -20°C after eight weeks and -80°C after one month. U-NAG was unstable at 4°C after four days and -20°C for 12 weeks. U-RBP was unstable at -80°C after five months and showed a significant upward trend at -20°C within 12 weeks in the pathological levels. Regarding the impact of the pH levels, the bias of U-α1MG and U-CysC did not exceed ± 10% across the pH range of 5.0 to 7.5. The bias of another 5 biomarkers (U-Alb, U-Trf, U-IgG, U-NGAL, and U-TP) was less than -10% at pH 5.0 to 6.0. U-NAG and U-RBP were unstable, with bias exceeding -10%.

U-NAG and U-RBP should not be stored at -20°C. U-IgG could only be stable for a short period under -20°C and -80°C. Urinary pH should be monitored and adjusted if necessary.