Before examining the blood, you first must get the blood. It’s not as easy as simply poking a hole in a patient.
Today Pete Farmer, continues his series on doing a CBC by examining the science behind blood collection. To read Part V-a of the series, click HERE.
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Blood cells are adapted to exist in solution within a specific solute concentration range. If red blood cells, for example, are placed in hypertonic solution, i.e., there is a higher solute or salt concentration outside the cell than in it, they shrivel and shrink in size. If placed in a hypotonic solution, i.e., there is a lower solute or salt concentration outside the cell than in it, they swell and even burst (lyse). Blood cells normally exist in a slightly hypotonic environment, which allows maintenance of cell volume (or turgor). An isotonic solution contains an equilibrium between the internal and external solutions; since their solute concentrations are equivalent, there is no net movement of fluid or solutes across the membrane.
[The image above this article shows what happens when a red blood cell is placed in a hypertonic, an isotonic, and a hypotonic solution. This explains why I.V. fluids always contain either salt, sugar, or both. Otherwise, the cells swell and may burst. – Doc Cindy]
The foregoing is important because when blood is drawn for analysis via venipuncture, it must be collected according to a protocol best-suited for the tests to be run on it. The most-commonly used tube for blood collection in offices and clinics is the vacuum tube first popularized by Becton Dickson Company, under the “Vacutainer” name. A vacuum tube set-up consists of a cannula (flexible tube inserted into the body to deliver or aspirate fluid) surrounding a trochar (a rigid and sharpened tube similar to a needle, used for introducing or removing fluids from the body via a blood vessel or other cavity); both are attached to a sleeve or collar, into which the vacuum tube is placed. The vacuum tube is placed into the sleeve/collar once the vein has been accessed; since the pressure in the vein is greater than that in the vacuum tube, blood flows into the tube. Once it is filled, another tube may be filled or the phlebotomist can finish the procedure. Vacuum tubes have a rubber stopper which seals closed once the tube is removed from the collar.
Venipuncture vacuum tube stoppers are color-coded by type of test to be performed on the collected blood. Often, the vacuum tube is pretreated with an anticoagulant such as heparin, EDTA or sodium citrate to prevent the blood from clotting. If several tubes of blood are to be drawn for laboratory analysis, the order in which they are drawn may be important – because cross-contamination is minimized by using the optimal order. Different hospitals, clinics and labs follow specific protocols, depending on the tube manufacturer, lab standards, and other criteria. A brief summary of different vacuum tube colors and contents may be seen at the following link, http://en.wikipedia.org/wiki/Vacutainer, or at the manufacturer’s website (Becton Dickson or other). For a CBC, purple or lavender top tubes are usually used; these contain the potassium salt of ethylenediaminetetraacetic acid (EDTA), a strong anticoagulant.
Image below: a vacuum-tube venipuncture set-up. Note the elastic rubber tourniquet above the collection site (source: Bobjgalindo at Wikipedia).
Image to the right: a fresh (right) and a centrifuged (left) specimen. [Note: the height of the blood column (dark red) compared to the height of the blood plus serum (red plus yellow-orange fluid, at top) allows an approximation of hematocrit, which should be about 37 to 50% of total blood volume. This patient is in good shape in that regard. – Doc Cindy] (source: JHeuser at Wikipedia).
Typically, in a clinical setting, collected blood or other tissue samples are analyzed by the med tech or pathologist as quickly as possible, to minimize the effects of autolysis or other degradation of the sample. Samples not slated for immediate testing and analysis are usually refrigerated or frozen, depending on tissue type. Blood samples are rarely frozen, because freezing lyses (ruptures) cells. It bears repeating that the manner in which blood is collected and the specific vacuum tube used, depend on the tests to be done. Do not assume that blood collected in a single type of tube is suitable for all analyses.
This concludes today’s installment. Next time around, we will delve more deeply into the clinical laboratory science of doing a complete blood count.
Copyright © 2011 Peter Farmer