An unexpected distribution of sodium concentration in serum specimens stored for more than 30 years

PURPOSE: Sera from over 50,000 pregnant women in the Collaborative Perinatal Project have been frozen at -20 degrees C since 1959 to 1966, and with the health data on their offspring constitute a resource that is still actively used. In two studies using these specimens, we measured sodium concentration to assess desiccation. METHODS: Sodium was measured in over 5,000 specimens by two different methods. For 10 specimens with unusually low sodium values, a substudy was done to investigate the cause. RESULTS: High sodium levels (>140 mmol/L) were present in more than 20% of specimens, and levels were unusually low (<130 mmol/L) in more than 40% of specimens. The substudy showed that filtering the specimens increased sodium levels and that about 14% of the sodium was trapped in particulate matter. CONCLUSIONS: High sodium levels in these specimens were probably due to desiccation and possibly to leaching of sodium from the glass containers. Low sodium levels were probably caused by the particulate matter, which clogged the analytical sampling devices and also trapped sodium. In serum specimens that have been stored for long periods, use of routine laboratory procedures for analysis can yield erroneous results. Furthermore, measured analyte levels can be affected by more than just degradation and desiccation.