Loss of Stability for Blood Urea Nitrogen in Cattle Serum Samples Stored for 3 and 6 Months at -20°C and -80°C

Biological samples are stored for delayed or add-on analyses. However, storage conditions, including time and temperature, can affect the stability of measurands in the samples.

To assess the stability of blood urea nitrogen (BUN) in cattle serum samples.

Blood samples collected from 72 heifers were immediately analyzed for BUN by a commercial laboratory and our in-house laboratory (optimal conditions, t₀). In-house, samples were also analyzed after storage at -20°C and -80°C for 3 and 6 months (test samples, t_x). Agreement between laboratories was assessed using Passing-Bablok and Bland-Altman analyses. The maximum permissible instability (MPI = 4%) was calculated as the sum of in-house intra- and inter-assay analytical variation. Percentage deviation (PD% = [(t_x-t₀)/t₀] × 100) was estimated for each frozen sample. Stability was assessed by comparing individual PD% against the MPI.

Our in-house analytical methodology performed within the ASVCP recommended guidelines. BUN (mmol/L) ranged from 1.9 to 5.4 in the commercial laboratory and 1.6 to 6.1 in our in-house laboratory. There was poor agreement with proportional bias in which the higher the BUN concentration, the greater the difference between laboratories. Storage at -20°C and -80°C yielded PD% between -18.6% and 27.1% after 3 months and -17.5% and 71.6% after 6 months. Regardless of freezing temperature, 54% and 80% of PD% values were greater than the MPI after 3- and 6-month's storage, respectively.

Although cattle serum sample degradation begins by three months storage time, further studies are needed to establish the actual stability limit for BUN.