A comparison of various evacuated blood collection tubes for monitoring warfarin therapy with Innovin

Abstract number: P1607

Munro^* R., Hardingham^* S., Dorward^* N. R., Morris† K.

^*Swansea NHS Trust, Wales; †University of Wales Institute, Cardiff, Wales

Background  

The nature of the materials from which blood collection tubes are made, combinations of differing concentrations of additives and artifacts relating to the ratio of sample volume to tube size may influence INR measurement. We have undertaken a review of the effects of a variety of evacuated collection tubes on the measurement of INRs. Tube specifications are shown in Table 1. All tube sizes were 13 mm × 75 mm except tube D (10 mm × 47 mm). Samples were taken in random order from 50 stabilized patients (33 M, 17 F; mean age 66.0 ± 14.8 years) on warfarin therapy. INRs were calculated using geometric means of normals (n = 32) tested with each specific type of container. The vitamin K-dependent factors II, VII, IX and X were also measured. All tests were performed on a Sysmex CA1500 coagulometer. Data was subjected to a one-way analysis by anova and the various tube types tested for equivalence at 5 and 10% using tube A as reference since this is used by most UK laboratories.

Results  

All tubes were equivalent for vitamin K-dependent factors except tube C for factor VII. As an overall group, mean INR's were all equivalent at the 10% level, but at the 5% level, the short draw tube C and the CTAD tubes were nonequivalent. In a small subgroup of patients where the INR's exceeded 3.5, nonequivalence mean INRs occurred at the 10% level in both CTAD tubes.

Conclusion  

INR differences between citrate and CTAD-containing tubes with Innovin may be clinically significant at the higher end of the therapeutic range and above. ISIs may require reassessment when this combination is used. The use of CTAD tubes with more conventional thromboplastins needs investigating.

Tubes A–E were from Becton Dickinson; F-H were from Greiner; SG siliconized glass; PP polypropylene; PET polyethylene terephthalate; CTAD citric acid, theophylline, adenosine, dipyridamole.

Table 1