Stability of urinary albumin and creatinine after 12 months storage at -20 °C and -80 °C

To determine the stability of urinary albumin and creatinine over 12 months in samples stored at -20 °C and -80 °C using an assay which enables assessment of previously undetectable levels of albumin and to investigate if additives can be used to prevent urinary albumin degradation. Method: ACR was measured in 30 urine samples from healthy subjects on the day of collection. Each sample was divided into 5 portions, each receiving a different treatment; alkalisation, protease inhibiter, boric acid, low protein binding tubes and no treatment (control). Samples were stored at -20 °C and -80 °C and ACR was analysed again after 12 months.

Increasing albumin to creatinine ratio (ACR) within the normal range is a risk factor for cardiovascular disease in the general population. Clinical and epidemiological studies often store urine samples for long durations prior to ACR assessment. The stability of ACR at the lowest urinary albumin concentrations during prolonged storage has not been previously studied because routine clinical assays can't quantify very low concentrations of albumin.

The loss in urinary albumin concentration which occurs during storage at -20 °C for 12 months is not prevented by pre-treating samples prior to storage. For accurate determination of albumin concentration or ACR, samples should be stored at -80 °C on day of collection.

Mean (95% CI) percent change in ACR was -34.3% (-47.2 to -21.4; p < 0.0001) and -1.8% (-9.4 to 5.8; p = 0.91) in samples stored at -20 °C and -80 °C respectively. Treating samples did not prevent the reduction in albumin at -20 °C (p < 0.001).