Measurement of heparin concentration in whole blood with the Hepcon/HMS device does not agree with laboratory determination of plasma heparin concentration using a chromogenic substrate for activated factor X.
Measurement of circulating heparin concentration has been suggested to optimize anticoagulation during cardiopulmonary bypass. The Hepcon/HMS device (Medtronic HemoTec, Inc., Parker, Colo.) uses heparin/protamine titration to quantitatively determine heparin concentration. Extensive validation of this instrument is still lacking.


METHODS
Agreement between heparin concentrations measured by the Hepcon/HMS system and by laboratory determination was evaluated in 16 patients undergoing cardiac operations. For laboratory determinations, plasma heparin concentration was derived from the measure of anti-Xa activity by means of chromogenic substrate technique. The Hepcon/HMS instrument and cartridges measured whole blood heparin concentration. Samples were analyzed 5 minutes after administration of heparin, 15 and 30 minutes after the start of cardiopulmonary bypass, 5 minutes after aortic unclamping, at the end of cardiopulmonary bypass, and after administration of protamine. Data were plotted and interpreted according to the method of Bland and Altman: First, a difference less than 1.4 U/ml (i.e., +/- 0.7 U/ml) was chosen as acceptable, because it would not cause major difficulties in clinical interpretation; second, the difference between the two measurement techniques was plotted against the mean of the two measures.


RESULTS
The mean difference (bias) between heparin concentrations derived by the Hepcon/HMS device and those obtained by laboratory determination was as expected for measures performed on whole blood versus plasma (1.45 U/ml). Nevertheless, heparin concentrations derived by the Hepcon/HMS device may be as much as 2.76 U/ml above or 6.17 U/ml below the concentrations measured in the laboratory, differences well outside the predetermined limits of agreement and clearly unacceptable for clinical purposes.


CONCLUSIONS
We conclude that heparin concentrations determined with the Hepcon/HMS instrument do not agree with laboratory determination of heparin concentration. Monitoring of heparin concentrations during bypass with the Hepcon/HMS device cannot be recommended.
Hardy
J F
JF
Department of Anesthesia, Montreal Heart Institute, Quebec, Canada.
Bélisle
S
S
Robitaille
D
D
Perrault
J
J
Roy
M
M
Gagnon
L
L
D001774:Blood Chemical Analysis / Q000295:instrumentation / Q000379:methods; D001780:Blood Coagulation Tests / Q000295:instrumentation / Q000379:methods; D005069:Evaluation Studies as Topic; D015951:Factor Xa; D006400:Hematocrit; D006493:Heparin / Q000097:blood*; D006801:Humans; D008991:Monitoring, Physiologic
D006493:Heparin; D015951:Factor Xa