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Vacutainer |
Vacutainer is a registered brand of test tube specifically designed for venipuncture. It was developed in 1947 by Joseph Kleiner,1 and is currently marketed by Becton, Dickinson and company.2
Contents |
The vein is first punctured with the hypodermic needle, which is connected to a translucent plastic holder. The needle actually has a second, smaller needle, and when a Vacutainer test tube is pushed down into the holder, its rubber cap is pierced. The vacuum in the tube causes blood to move through the needle and into the tube. The tube is then removed and another can be inserted and filled the same way. It is important to remove the tube before withdrawing the needle, as there may still be some suction left, causing pain upon withdrawal.
The test tubes are covered with a color-coded plastic cap. They often include additives that mix with the blood when collected (see below), and the colour of the tube's plastic cap indicates which additives that tube contains.
The plastic caps are opaque for tubes with a normal vacuum. Translucent-topped tubes contain a weaker vacuum in the same sized tube, and will obtain less blood. The weaker suction makes them more suitable for smaller sized veins. Because of the standardised suction of the tubes, they may cause the veins of elderly people, or those with delicate veins, to collapse. In this case a syringe should be used instead.
The tubes may contain additional substances that preserve the blood for processing in the medical laboratory. Using the wrong tube may therefore make the blood sample unusable.
The substances may include anticoagulants (EDTA, sodium citrate, heparin) or a gel with intermediate density between blood cells and serum. Additionally, some tubes contain substances that preserve certain chemicals or substances within the blood, such as glucose. When the tube is centrifuged, the blood cells sink to the bottom of the tube, are covered by a layer of the gel, and the serum is left on top. The gel enables the tube to be tipped upside-down and transported without the blood cells remixing with the serum. When a tube that does not contain gel or a clotting agent is centrifuged, the clear liquid is plasma, which contains platelets.
The meaning of the different colors are standardized across manufacturers.345
The order of draw refers to the sequence in which these tubes should be filled. The needle which pierces the tubes can carry additives from one tube into the next, and so the sequence is standardized so that any cross-contamination of additives will not affect laboratory results 5.
The purple top tube became an issue in the O.J. Simpson murder case when the defense alleged that small droplets of blood found at the crime scene contained the preservative EDTA; had this been true, it would have meant that the droplet might have been taken from the purple top tube used to collect Simpson's blood and planted at the crime scene.6
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