The Influence of TheTourniquet on blood draw

[1] Various methods are used to obtain specimens for hematocrit determination in neonates. We evaluated arterial sampling and found this method to be acceptable. Arterial values correlated closely with simultaneously obtained venous samples. Venous blood obtained with a tourniquet had a significantly higher hematocrit than that obtained without a tourniquet.

[2] Influence of tourniquet application on venous blood sampling for serum chemistry, hematological parameters, leukocyte activation and erythrocyte mechanical properties

Background: Venous blood sampling is usually performed using a tourniquet to help locate and define peripheral veins to achieve successful and safe venipuncture. Despite widespread usage of tourniquets for venipuncture by medical and laboratory staff, very few are aware of the effects of tourniquet application on laboratory parameters. In addition, definitive guidelines regarding when and how to use a tourniquet for blood sampling are lacking. The aim of the present study was to define the optimal sampling time after tourniquet removal to avoid adverse impact on laboratory analytes.

Methods: Blood oxygen and carbon dioxide partial pressure, pH, oxyhemoglobin saturation (satO₂), hematological parameters, serum electrolyte concentrations, erythrocyte, deformability and aggregation, leukocyte activation and nitrite/nitrate concentrations obtained 180 s after tourniquet release were compared with baseline values for 10 healthy subjects.

Results: Blood gases, hematological parameters and serum electrolyte levels were not affected by the application and removal of a tourniquet. However, there were significant decreases in erythrocyte deformability at 90, 120, 180 s, and increases in erythrocyte aggregation at 5 and 30 s following removal of the tourniquet. A significant increase in granulocyte respiratory burst at 60 s was observed, confirming leukocyte activation due to application of the tourniquet. There were no significant alterations of blood nitrite/nitrate levels.

Conclusions: Our blood sampling technique which mimicked the application and release of a tourniquet indicated unaltered values for routine blood gases, hematological testing and serum electrolyte levels. Conversely, hemorheological measurements can be affected. Therefore, it is strongly recommended that tourniquet application should be avoided during blood sampling or, if this is not possible, the procedure should be well standardized and details of the sampling method should be reported.

[3] Additives and order of draw

The tubes in which blood is transported back to the laboratory contain a variety of additives or none at all. It is important to know which the laboratory requires for which test. In general whole blood needs to be mixed with EDTA which chelates calcium to prevent it clotting, unless the clotting time is the test to be measured in which citrates is used. The majority of biochemistry tests are performed on serum and so either a plain tube or a clotting accelerator is used. This clotting accelerator can interfere with some assays and so a plain tube is recommended in these cases but will obviously delay the result. Some assays also want whole blood but the EDTA can interfere and in this case Lithium Heparin is an alternative.

With the vacuum tube system, the needle pierces the top of the sample tube and will potentially come into contact with the additives in the tube. As it is a hollow needle some of this can be carried into the next tube and contaminate it. The most likely additive to cause trouble is EDTA which will affect the coagulation time assays and by chelating some of the metal ions may interfere with some of the biochemistry results(especially potassium). Thus EDTA samples should be drawn last in most cases and plain tubes drawn first.

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 Therefore : the serum calcium level will not get affected by using the tourniquet.