Research on Stable Ionized Calcium Concentration in Uncapped Plasma Samples

The concentration of ionized calcium (iCa) in the blood has been shown to be pH dependent. When a plasma specimen for iCa measurement is exposed to air prior to examination, it is routinely rejected in the clinical laboratory. The pH of the specimen, and thus the iCa concentration, is thought to be affected by a preanalytical variable such as specimen exposure to air. The loss of carbon dioxide caused by specimen exposure results in a rise in pH and a decrease in iCa concentration. iCa assessment is the best predictor of a patient’s calcium status since it is the most active form physiologically. The aim of this research is to see how quickly these changes affect iCa concentration. We believe that the modifications are too sluggish and insignificant to justify the plasma specimen being rejected.

Methods: Appropriately collected whole blood samples were centrifuged and tested for iCa concentration on AVL by Roche, as per laboratory procedure. The first measurement was taken at time zero, and subsequent measurements were taken at different time intervals after the sample had been exposed to air. To assess for shifts, the pH of plasma and whole blood samples exposed to air at varying intervals was assessed.

The concentrations of iCa in plasma samples exposed to air for an extended period of time remained constant. When compared to whole blood samples, exposed plasma pH measurements were similarly stable (for up to ninety minutes). For the same period of time, the average increases in whole blood pH (0.5 units) were ten times greater than those in plasma pH (0.05 units).

Conclusion: Plasma samples for iCa that are mistakenly exposed to air should not be refused outright since the pH and iCa level remain stable for nearly eighty minutes. The respiring red cells are responsible for the changes in whole blood pH. Carbonic anhydrase (which is not present in plasma) catalyzes the reversible interconversion of carbon dioxide and bicarbonate ion in red cells. It costs money to reject or redraw specimens that aren’t required. Furthermore, repeated injections are painful and can lead to infections and hematoma at the injection site.

Author (s) Details

Victoria Richardson
Department of Clinical Laboratory Science, School of Health Sciences, Winston Salem State University, Winston Salem, North Carolina, USA.

Quinnita Reid
Piedmont Atlanta Hospital Laboratory, Atlanta, Georgia, USA.

William A. Anong
Department of Clinical Laboratory Science, School of Health Sciences, Winston Salem State University, Winston Salem, North Carolina, USA.

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