Objectives: To establish the threshold value for calcium at which coagulation commenced and to investigate the range over which changes in ionized calcium influenced coagulation.
Design: Controlled, nonblinded, in vitro observational study.
Setting: The study was conducted in a university-based laboratory.
Participants: Blood was obtained from healthy volunteers.
Interventions: One hundred samples of citrated blood were recalcified using varying quantities of 10% calcium chloride solution. Heparinized samples from the recalcified blood were obtained for measurement of ionized calcium concentration (Ca(2+)).
Measurements and results: Coagulation of the recalcified samples was analyzed using thrombelastography. The ionized calcium concentration in recalcified blood was measured using a calcium electrode in a blood gas analyzer. No sample with a Ca(2+) < 0.33 mmol/L showed any clot formation. Normal coagulation measures were obtained in almost all samples in which the Ca(2+) was >0.56 mmol/L. Final clot strength appeared to be independent of Ca(2+) once the threshold value of 0.33 mmol/L was exceeded.
Conclusions: Ca(2+) of <0.33 mmol/L is necessary to prevent coagulation. Ca(2+) > 0.56 is unlikely to be the cause of coagulation abnormalities during surgical procedures in which calcium metabolism is deranged. Between these values, Ca(2+) may exert an effect on the rate of clot formation, but final clot strength should be unaffected.