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Questions About Blood Work Answered, Part 2: The Tests

The last article I wrote focused on many of the aspects of getting blood work done, and the reasons behind them. In this follow up, I am going to expand on this. As a phlebotomist, my patients often ask me why their doctors are ordering the tests that they are ordering, and what the test results mean. We always tell our patients that we cannot interpret these things for them; it would be dangerous and irresponsible to tell a patient what their results indicate when we are not trained to do so. Working in a medical laboratory, however, I can usually shed some light on what the most common tests are for. So, I thought that in case any of you have ever found yourself in a situation where you wondered why your tests were being done, I might be able to share what I know. Here I will list the most common tests that I see along with a brief scientific explanation and some reasons why they might be necessary.


Disclaimer: This article should in no way be used to interpret your own blood results. This is a guide but does not replace the opinion of your doctor. Also, the required tube for collection will often depend on the target lab, as they may have different blood analyzers.


The Blood Collection Tubes

There are many types of blood collection tubes — far too many to list here — but I touch on those that are most commonly used (especially those associated with the tests listed in this article). Provided there is enough blood, multiple tests can be run on one sample tube, and each color has its own use and purpose.

  • Yellow top serum separator tube (SST) – used in testing for most blood chemistry tests. It is necessary to wait for the blood to clot after it is drawn, at which time it will be placed in a centrifuge and spun down. The gel at the bottom will then separate the serum (which is what is of interest here) from the cells. A larger version of this tube is available that has a red and grey top, often called a “tiger top tube.”
  • Light green serum separator tube (GSST) – contains lithium heparin, an anticoagulant. Like the yellow SST, it is used in the testing of blood chemistry, and also like the SST has the separator gel. However, because it has the anticoagulant in it, there is no need to wait for the blood to clot before spinning. It is much quicker, therefore, to obtain results if tests are drawn on this. Not all blood chemistries can be run on this, however, so most nurses and phlebotomists will draw a yellow SST and a light green GSST together just in case. Some chemistries need the anticoagulant but cannot be done on a serum separator tube, so a dark green topped lithium heparin tube is also available.
  • Lavender top tube – Contains the anticoagulant called EDTA. Used in some special chemistry tests, but is especially used in hematology tests such as the CBC (complete blood count), it prevents cells and platelets from clumping together so they can be examined for morphology. A larger version of this tube exists and is used relatively frequently; it has a pink top.
  • Red top tube – contains no additive and no separator gel. It is used on some chemistries and can be substituted for the yellow top tube, although this is not preferred in most cases.
  • Grey top tube – contains sodium fluoride and/or potassium oxalate in a powdered form, used for special blood chemistry testing.

Because of the nature of each of the chemicals added to the tubes, there is an order in which they must be drawn to prevent errors in lab results: blood cultures, then blue top, yellow top/tiger top/red top, light green/dark green top, lavender top EDTA/pink top EDTA, grey top. Often, nurses and phlebotomists will draw more than what is ordered just in case; for example, I will often draw what I call The Trio, yellow, green, and lavender, or The Rainbow, which is blue, yellow, green, and lavender.


Complete Blood Count


The complete blood count, also known as a CBC, is one of the most common tests I see as a phlebotomist. The CBC is a hematology test that quantifies the different types of cells and other items in your blood, including red blood cells, white blood cells, hemoglobin, hematocrit, and platelets, among other things. Red blood cells are responsible for carrying oxygen to and from your lungs and throughout your body, with the help of the hemoglobin protein that exists within them, while the white cells (and all their various types) are responsible for your immune system function. Platelets are important in clot formation. Abnormalities in any of these can indicate a possible infection, a weakened immune system, anemia of various kinds, and other maladies. This test can also be ordered with or without a manual differential. If it’s ordered with a manual differential, the medical laboratory technician or technologist looks at the blood under a microscope and manual counts each type of cell in question. Otherwise, the automated analyzer does that automatically. Most of the components of the CBC, such as the hemoglobin and hematocrit (or H&H) can be ordered separately, but when many of these values are needed, it is often more efficient (and cheaper) to order the complete blood count. This test is drawn on a lavender top EDTA tube.


Comprehensive Metabolic Panel


Like the CBC, the comprehensive metabolic panel (or CMP) consists of many different tests in one, however unlike the CBC it concerns itself with blood chemistry. It concerns itself with the substances that naturally occur in the blood that are often tied to organ and organ system function. For example, the CMP includes the blood sugar, kidney function enzymes, liver enzymes, and electrolytes. It is a very common test, as it can give a pretty revealing snapshot of how the body is doing, and it is ordered for a wide variety of reasons, ranging from simple checkups to emergency situations. The basic metabolic panel, or BMP, is essentially the same thing only without the liver enzymes. This test is often ordered for people who otherwise appear to be healthy or in non-emergent situations, as it is cheaper and less involved. As with the CBC, all the components can be ordered separately if needed, and many other panels such as the hepatic panel and the renal panel are actually just specific parts of this test. The CMP and BMP are drawn on a light green or yellow tube.


Lipid Panel


The lipid panel is often used to assess risk factors for heart and blood vessel disease. Sometimes called the lipid profile or the coronary heart disease profile, it consists of the total cholesterol level, the triglyceride level, the HDL (the “good cholesterol”) and the calculated LDL (the “bad cholesterol”). This is often ordered as a fasting test; doctors often ask that patients not eat for 12-14 hours before hand so that a baseline can be established. Eating within this window can temporarily elevate these levels and give false results. The lipid panel is drawn on the light green or yellow tube.


C-Reactive Protein


The CRP is used as an indicator of inflammation and or tissue/cell death (necrosis). It is a great way to test for potential infection. It is drawn on a light green or yellow tube.


Brain Natriuretic Peptide


The BNP can help doctors diagnose congestive heart failure, and so is frequently ordered on cardiac patients. In the lab I work for, it is drawn on a light green tube, but the University of Michigan Health System’s own MLabs prefer a lavender top EDTA tube.




Otherwise known as the HgBA1C or just the A1C, this is a very important test for patients with diabetes, as it is used to assess how well controlled the condition is by evaluating glucose metabolism. More specifically, it evaluates what percentage of your blood cells are coated with sugar, which gives an estimation of the blood glucose control over the past two or three months. It is drawn on a lavender EDTA tube.


Type and Screen

Everybody has heard of people receiving blood when they are low, because of extensive blood loss through injury, GI bleeds, or other reasons. But few realize how much work actually goes into preparing these. A type and screen evaluates a patient’s ABO and Rh blood types, along with an antibody screen. This is one of the most important steps in determining a patient’s compatibility with units of blood that they may later receive. It is drawn on a pink EDTA tube.


Prothrombin Time with INR/Partial Thromboplastin Time


The PTINR and the PTT are complex tests used in the diagnosis of many different diseases that have to do with the blood’s clotting ability. Patients may receive one or both of these tests if they have a natural inability to clot normally or if they are on blood thinners such as Coumadin or Warfarin to prevent blood clots from forming. In the case of blood thinners, it is used to ensure that the medication maintains the clotting factors at just the right amount, as too little or too much can be deadly. It is drawn on a blue top tube.


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Obviously, these are only some of many different tests in a medical laboratory’s arsenal. As always, it is important to understand that this article is meant for you to use as a personal reference, and should not be used to interpret your own blood work. This must be done with your doctor, but the hope is that these explanations will give you a better understanding of your own situation. Patient involvement in their own care is of the utmost importance. If there is something not included on this list that you would like to see, write it in the comments and I will try my best to answer it!


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