To freeze or not to freeze

To freeze or not to freeze

Drug testing usually involves 3 stages – urine specimen collection, followed by initial screening and lastly, a confirmatory test if the specimen is non-negative.

In the case of confirmatory testing, how should you be storing the urine specimen until it is tested?

Few studies have investigated the influence of storage conditions on urine samples and none of them used targeted mass spectrometry (MS).

We investigated the stability of metabolite profiles in urine samples under different storage conditions using targeted metabolomics.

Pooled, fasting urine samples were collected and stored at −80 °C (biobank standard), −20 °C (freezer), 4 °C (fridge), ~9 °C (cool pack), and ~20 °C (room temperature) for 0, 2, 8 and 24 h. Metabolite concentrations were quantified with MS using the AbsoluteIDQ™ p150 assay. We used the Welch-Satterthwaite-test to compare the concentrations of each metabolite. Mixed effects linear regression was used to assess the influence of the interaction of storage time and temperature.

The concentrations of 63 investigated metabolites were stable at −20 and 4 °C for up to 24 h when compared to samples immediately stored at −80 °C. When stored at ~9 °C for 24 h, few amino acids (Arg, Val and Leu/Ile) significantly decreased by 40% in concentration (P < 7.9E−04); for an additional three metabolites (Ser, Met, Hexose H1) when stored at ~20 °C reduced up to 60% in concentrations. The concentrations of four more metabolites (Glu, Phe, Pro, and Thr) were found to be significantly influenced when considering the interaction between exposure time and temperature.

Our findings indicate that 78% of quantified metabolites were stable for all examined storage conditions. Particularly, some amino acid concentrations were sensitive to changes after prolonged storage at room temperature. Shipping or storing urine samples on cool packs or at room temperature for more than 8 h and multiple numbers of freeze and thaw cycles should be avoided.

Rotter, M., Brandmaier, S., Prehn, C. et al. Stability of targeted metabolite profiles of urine samples under different storage conditions. Metabolomics 13, 4 (2017).


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