Critically low sodium levels due to concentration gradients formed in patient samples after undergoing a freeze-thaw cycle

Background: Concentrations gradients that form in plasma as a result of freezing and thawing is a well-known phenomenon. As the water fraction converts into ice, plasma constituents diffuse from the freezing front by natural convection in the liquid phase. This process can lead to erroneous lab results, if the sample is not thoroughly mixed prior to testing. Methods: A series of patient samples received at the clinical chemistry core lab were found to have low sodium levels that normalized after tube inversion. We suspected that the samples may have frozen during shipping and therefore examined the effects of freezing and thawing on serum. Results: Our investigation revealed that prior to arriving at the core lab, samples from one of our satellite clinics were undergoing a freeze-thaw cycle during shipping, which resulted in the formation of concentration gradients and spurious lab results on arrival. Conclusions: Large hospitals that have a central core lab and receive patient samples from satellite clinics need to be aware of this phenomena, which can contribute to erroneous lab results being posted in a patient's electronic medical record, resulting in a misdiagnosis.