Analysis of ionised calcium - full vials provide correct test results

Background: The analysis of free ionised calcium is measured in patients with critical illness, acid-base disorders, and during operations/surgeries that require blood transfusions when total calcium may not provide an accurate assessment. The purpose of this project was to investigate whether half-filled vials (50% filling level ±5%) can be used for free ionized calcium analysis while ensuring that the relevant quality requirements are met. Material and methods: Blood samples were collected from both in- and outpatients. From each patient two blood vials were sampled, one full, and one half full. The vials we handled were sampled for already requested analyses. To ensure standardisation of our procedure, only blue butterfly cannulas, and 5 mL serum blood vials were used for blood sampling. ABL800 FLEX was used to analyse the concentration of free ionised calcium. Our demands for correctness, precision and accuracy were based on Westgaard’s criteria. We conducted statistical tests, including Bland-Altman plot, scatter plot, and paired t-test, on the differences between the vials. Results and conclusions: There is a statistically significant difference in the measured concentration of free ionized calcium when comparing a half-full vial to a fully filled vial. Furthermore, the analysis conducted with half-full vials failed to meet the biological quality standards for both bias and permissible total error. This indicates that half-full vials are unsuitable for routine use in this context.