Everyone has gone for a blood test at least once in their lives. There might have been times when you've gotten 3 tubes taken at one time and you wonder why. Maybe, the phlebotomist/nurse had to come back to re-draw your blood and you don't understand the reason. There are many factors involved in blood collection that you may not realize.
Vacutainers:
A few of the available Vacutainers for blood collection
These evacuated tubes are either glass or plastic with a plastic cap that can only be utilized once the rubber stopper is punctured. They are typically used to collect venous blood samples but can also be used to collect urine in some occasions. The tubes can contain additives to help stabilized blood and help prepare it for testing on an analyzer or manual testing.
When performing a blood draw, the collector will first insert the bore of the needle into the patient's vein, then click on the Vacutainer. This will prevent the vein from collapsing and also prevent discomfort for the patient. The blood then will fill the container quite quickly; a matter of seconds. The collector will then remove the tube carefully before snapping on another tube or removing the needle. The tube is removed first to, again, prevent venous collapse once the needle is removed.
The tube must be filled within +/- 10% of the fill line to ensure accurate testing due to blood/additive ratios. If the ratio is off, a new blood draw needs to be ordered. The lab is not being lazy or demanding a new draw to create more work, rather they are needing to be extremely accurate because an under/overfill can skew tests results greatly. Ensuring a quality specimen is one of the most important jobs of the collector.
Selecting the correct tube is also essential when collecting a blood specimen. Plastic tubes are the most commonly used with glass tubes only being used when detecting trace elements such as Lithium or Zinc.
Additives:
A few of these tubes have chemical additives while some other have "serum separators" to allow for quick testing. The additives are present to prepare the blood for quick testing and to prevent the deterioration of sample quality.
The serum separator in the gold tubes or tiger top tubes will allow the serum and plasma to be separated during collection which cuts down the turn around time on results. There is no need to centrifuge and the tube can be placed directly on the machine for sampling.
Citrate is used when testing for coagulation times (PT/PTT) on a patient who may be bleeding or be on heparin therapy. This freezes the coagulation cascade or prevents coagulation in the tube and allows for Ca2+ to be added during testing to allow for coagulation to occur. [link]
Heparin is used as an anti-coagulant for chemistry testing due to its minimal chelating properties, meaning it will not interfere with the testing of various ions. There are 3 common "salts" used in Heparin tubes, Ammonium, Lithium, and Sodium. If testing for any of those specific ions, a different salt must be used. (Ex: if testing for lithium, either sodium heparin or ammonium heparin can be used.) Lithium heparin is the most commonly used heparin tube. [link]
EDTA or Ethylenediaminetetraacetic acidis an anti-coagulant used for collecting whole blood specimens for blood counts or cellular morphology. It is different from heparin as metal ions will remain in the blood following mixture which is why it is not used for chemistry testing as it could falsely elevate or decrease test results.
Fluoride/Potassium oxide is used for glucose, drug or blood alcohol testing. These tests are time sensitive as the molecules can break down very quickly and the fluoride present (anti-glycolytic agent) is able to stabilize the glucose for up to 4 hours to allow for accurate testing.
Order of Draw:
The order that the tubes are placed on the needle is extremely important when doing multiple draws. The prevention of carry over of additives is key to ensure accurate and reliable testing.
BD Vacutainer also has a more in-depth chart on the tubes and uses available on their website here.
Again, the fill volume of these tubes is extremely important. If under-filled or over-filled the ratio of additives can cause increases and decreases of testing components in the sample. This can cause incorrect test results and incorrect diagnosis. The range for some ions is extremely small and being 0.01 out due to an overdraw can be the difference between someone showing heart issues or not.
I hope this post was helpful in showing the hows and whys of blood testing and you can feel more knowledgeable when getting your next test!
-K
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