Evaluation of small-volume tubes for venous and capillary PT (INR) samples

Frequent PT (INR) testing may represent a problem for patients on warfarin treatment, and capillary or small-volume tubes may be more appropriate for such patients. A demand for small-volume tubes also comes from pediatric wards. Yet, while various small-volume tubes are available, they have not been properly evaluated.

Three small-volume tubes were tested (MiniCollect 3.8% citrate, MiniCollect 3.2% citrate and Microvette EDTA) and compared with a standard 4.5-mL 3.2% citrated tube. Samples were taken by venipuncture from the back of the hand and by capillary sampling from the tip of the finger. The measures were compared with those after standard venipuncture of the arm fold. A total of 180 samples, using different combinations of tubes and sampling sites, were collected from 30 volunteers.

There were no differences in the results obtained using citrate tubes for venous samples in comparison with those obtained by standard sampling, while the results when using EDTA tubes were not comparable to those obtained by standard sampling (P < 0.001), expressing systematically lower values (by about 10%). The results observed after capillary sampling were significantly different to those obtained after standard sampling.

The MiniCollect 3.2% tube may be used for PT (INR) venipuncture samples when withdrawal of a small amount of blood is preferable, while EDTA tubes should not be used for PT (INR) testing.