• Michael F M James and Anthony M Roche.
• Department of Anaesthesia, Groote Schuur Hospital and University of Cape Town, Western Cape, South Africa. james@cormack.uct.ac.za
• J. Cardiothorac. Vasc. Anesth. 2004 Oct 1; 18 (5): 581-6.

ObjectivesTo establish the threshold value for calcium at which coagulation commenced and to investigate the range over which changes in ionized calcium influenced coagulation.DesignControlled, nonblinded, in vitro observational study.SettingThe study was conducted in a university-based laboratory.ParticipantsBlood was obtained from healthy volunteers.InterventionsOne 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 ResultsCoagulation 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.ConclusionsCa(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.

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