Arterial blood sampling devices influence ionized calcium measurements

OBJECTIVE: To determine the effect of commercially available arterial blood sampling devices on ionized calcium measurements. DESIGN: Prospective study. SETTING: Neurosurgical and shock-trauma intensive care units (ICU) at a tertiary care teaching hospital. PATIENTS: Fourteen patients admitted to the ICU. Each patient had an indwelling arterial catheter. INTERVENTIONS: Arterial blood sampling. MEASUREMENTS AND MAIN RESULTS: In 14 ICU patients, measurements of arterial blood ionized calcium concentrations were performed, using 12 different commercially available arterial blood sampling devices. Significant underestimation of ionized calcium in blood samples compared with the reference test tube (Vacutainer 45) was seen in seven of the devices. Arterial blood ionized calcium concentrations measured, using one commercially available syringe, were significantly higher compared with the reference test tube. There was no correlation between either the amount or type of heparin in the arterial blood sampling devices and arterial blood ionized calcium measurement. CONCLUSIONS: This study demonstrates that various commercially available arterial blood sampling devices alter arterial blood ionized calcium measurements. These alterations are clinically important because ICU patients may be treated with inappropriate calcium supplementation.