Vacutainer

The test tubes cap a colour-coded plastic cap with a rubber centre. They often include additives that mix with the blood when collected.

To collect blood using a vacutainer, a needle is placed on a transulcent plastic holder. The needle is double-ended, one end designed to pierce the skin for venipuncture, the other end is coated in rubber. This end will pierce the rubber area on the vacutainer.

The vein is first punctured with the needle. Then the vacutainer is pushed down into the holder, and the rubber cap is pierced. The vacuum in the tube sucks blood though the needle and fills itself. The vacutainer is then removed and another can be inserted and filled the same way. It is important to remove the vacutainer before withdrawing the needle, as their may still be some suction left.

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.

Contents of vacutainer tubes As well as anticoagulants, the tubes can contain a gel with intermediate density between blood cells and blood plasma. When the tube is centrifuged, the blood cells sink to the bottom of the tube, then there is a layer of the gel, and the plasma (or serum) is left on top. The gel enables the tube to be tipped upside-down, and transported without the blood cells remixing with the plasma.

The colour of the tube's top indicates the additives in the tube.

• Red - Contains no additives. Tests for antibodies and drugs often require these.
• Yellow - A serum-separating tube[?] (SST). These contain particles that cause blood to clot quickly, as well as a gel to separate blood cells from serum. (Because the blood has clotted before it has been centrifuged, the liquid part is is called serum not plasma.)
• Green - Contains the lithium salt of heparin, an anticoagulant.
• Purple or lavender - contains EDTA. This is a strong anticoagulant and these tubes are usually used for full blood counts and blood films[?].
• Grey - These tubes contain fluoride and oxalate[?]. Fluoride prevents enzymes in the blood from working, so a substrate such as glucose will not gradually used up during storage. Oxalate is an anticoagulant.
• Light Blue - Contain a measured amount of citrate. Citrate is a reversible anticoagulant, and these tubes are used for coagulation assays. Because the liquid citrate dilutes the blood, it is important the tube is full so the dilution is properly accounted for.
• Dark Blue - Contains the sodium salt of heparin, an anticoagulant.

The purple top tube became an issue in the O.J. Simpson murder trial when the defense alleged that small droplets of blood found at the crime scene contained the preservative EDTA; had this actually 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.