The pre-analytical stability of 25-hydroxyvitamin D: Storage and mixing effects

There is an increasing demand for serum 25-OH VitD testing globally, and this has led to the greater use of automated immunoassays. These may be more prone to non-specific interference, that is thought to be related to pre-analytical stability of biological samples. We have investigated the changes in serum 25-OH VitD concentrations that are caused by storage and mixing conditions, and if such changes are statistical, or clinically important.

Blood samples were collected into plain tubes from 31 healthy donors. After separation, serum samples were stored at -20°C and analysis was carried out with and without mixing (vortexing) at different time intervals of days (0, 1, 2, 3, 4, 5, 15, and 30). All samples were analyzed using a chemiluminescent immunoassay.

Mean serum 25-OH VitD concentrations for subsequent days of storage compared with day 0 showed a significant time effect (P < .05) except for the samples on day 1 (P = .654) in non-vortexed samples and day 2 (P = .087), 5 (P = .118) and 30 (P = .118) in vortexed samples. Comparing values for vortexed and non-vortexed samples on the same day, serum 25-OH VitD showed a significant difference on days 1 (P = .003), 4 (P = .037), 5 (P = .002), and 30 (P = .025). However, the maximum change value was 8.85% which was less than the known total allowable error (TEa) and reference change value (RCV) for serum 25-OH VitD.

25-OH VitD is pre-analytically stable after long-term sample storage at -20°C and can be analyzed without vortexing. This may be beneficial for both research and diagnostic laboratories.