Preanalytical variables in measurement of free (ionized) calcium in lithium heparin-containing blood collection tubes.
BACKGROUND
Measurements of free (ionized) calcium (iCa) are increasingly requested in patient care locations where immediate analysis is unavailable. Evacuated blood collection tubes containing lithium heparin and gel separator material are widely used in clinical laboratories, but little information is available on the effects of these tubes or of delay prior to analysis on the concentration or stability of iCa.


METHODS
We collected blood from volunteers into lithium-heparin tubes (PST, Vacutainer PST, BD Pre-Analytic Systems) of multiple lots and into electrolyte-balanced heparin syringes (Portex Dry Heparin, Smiths Medical). iCa was measured (Siemens 1265 blood gas analyzers) immediately and, in PST, at 0-7 h with or without transportation of the tubes from remote sites.


RESULTS
The mean difference of free calcium results in the PST tubes and electrolyte-balanced syringes was -0.08 (95% confidence interval -0.17 to 0.012) mmol/l, and the SD of the residuals (Sy, x) of the regression was 0.03 mmol/l. There was no detectable lot-to-lot variation in results. Free calcium was stable in tubes at room temperature and at 4 degrees C for at least 7 h with or without transportation.


CONCLUSIONS
iCa measured in the examined blood collection tubes is stable and unaffected by lot-to-lot variation of tubes, but results are slightly lower than with special blood gas syringes.
Department of Pathology, University of Virginia, Charlottesville, VA, United States. dmh2t@virginia.edu
Doris M
DM
Haverstick
Louis B
LB
Brill
Mitchell G
MG
Scott
David E
DE
Bruns
D001800:Blood Specimen Collection / Q000379:methods*; D002118:Calcium / Q000097:blood*; D006493:Heparin; D006801:Humans; D007753:Laboratories; D008094:Lithium; D013696:Temperature; D013997:Time Factors
D006493:Heparin; D008094:Lithium; D002118:Calcium