Proper zinc evaluation in clinical practice: Effect of sample type and it's stability

Zinc (Zn) is a ubiquitous element that plays a vital role in the growth and development of human body. Traditionally, Zn has been measured in plasma samples using "trace element" dedicated sampling tubes. However, the recent increase in its assessment leads to the need to use others than plasma samples and the use of sera could be a justified and valid alternative. We evaluated the differences between plasma and serum, for Zn quantification. 307 blood paired samples from patients enrolled for the treatment of obesity-related pathology at our out-patient department were assessed. The quantification of Zn was performed by Flame atomic absorption spectroscopy. Using 123 serum samples randomly selected from our own biobank and stored at -80 °C for 5 years, we further investigated the long-term stability of serum Zn. The mean result for Zn was 77.8 ± 13.2 μg·dL^-1 and 77.4 ± 12.8 μg·dL^-1, respectively for plasma and for serum, (p = 0.43). Bland-Altman analyses demonstrated excellent concordance of the assay in the two different blood matrices. The mean difference ± SD between serum and plasma matrices was 0.32 ± 3.40 μg·dL^-1. The assessment of serum Zn long-term stability indicated a significant change after 5 years storage. The mean value was 74.2 ± 10.9 μg·dL^-1 for fresh samples and 83.3 ± 10.9 μg·dL^-1 after 5 years of storage at -80 °C, corresponding to a mean difference of +9.1 μg·dL^-1(+10,9%, p < 0.05). The increase in Zn values described after long-term storage has to be considered though that should not have a significant clinical value. In conclusion, the routine measurement of Zn can be made in an accurate way using a serum sample, without the need for a specific tube for trace elements assessment.