Victoria Richardson, Quinnita Reid and William A. Anong
Background: Blood ionized calcium (iCa) concentration has been shown to be pH dependent. In the clinical laboratory, plasma specimen for iCa measurement is routinely rejected when exposed to air prior to analysis. A preanalytical variable such as specimen exposure to air is believed to alter the pH and consequently the iCa concentration. Specimen exposure leads to the loss of carbon dioxide resulting in an increase pH and decrease iCa concentration. The purpose of this study is to investigate the rate at which these changes affect iCa concentration. We hypothesize that the changes are slow and insignificant to warrant the rejection of plasma specimen.
Methods: Appropriately collected whole blood specimen were centrifuged and analyzed for iCa concentration per laboratory procedure on AVL by Roche. The first, at time zero and subsequent measurements were made at various time intervals of sample exposure to air. The pH of plasma and whole blood samples exposed to air at various intervals were also measured to evaluate for changes.
Results: The iCa concentrations remained stable in plasma samples exposed to air for a considerable period of time. Exposed plasma pH measurements were equally stable (for up to ninety minutes), compared to that of whole blood specimen. The average changes seen in whole blood pH (~0.5 units) were ten times greater than that of plasma pH (~0.05 units) for the same length of time.
Conclusion: Plasma samples for iCa that are accidentally exposed to air should not be summarily rejected because both the pH and iCa level are stable for almost eighty minutes. The changes in whole blood pH are due to the respiring red cells. Red cells contain carbonic anhydrase (not found in plasma) that catalyzes the reversible interconversion of carbon dioxide and bicarbonate ion. Unnecessary specimen rejection/redraw request is costly. Besides, repeat injection is painful, exposes the patient to infections and hematoma at the injection site.