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Spring 2005 Volume 6, Issue 4:
Supportive Care: Blood Tests
By Susan J. Leclair, PhD, CLS(NCA)
There are a number of factors that are likely to increase your comfort with a medical test. Susan J. Leclair, PhD, CLS (NCA)helps you understand why a test has been ordered for you, how it will be conducted, and what the reference ranges are.
06.09.05

Chancellor Professor
Department of Medical Laboratory Science
University of Massachusetts Dartmouth
Dartmouth, Massachusetts




There are a number of factors that are likely to increase your comfort with a medical test. Understanding why a test has been ordered for you can improve your attitude toward the procedure. Knowing how it will be conducted can help you feel more in control of the situation during the test. Understanding the concept of reference ranges can help you process what is garnered by the test.

Before Your Blood Draw

  • Blood drawing is called phlebotomy.

  • Ask your health care provider to explain the reasons for your test and how the test will be conducted.

  • On the day of the phlebotomy, prior to the test, drink lots of fluid.

  • Some peole have veins that are quite prominent and large. Others, unfortunately, do not. If you are one of the people who seems to always have problems with phlebotmy (drawing of blood from a vein), then one way to increase the chances of success is to increase the size of the veins. And that is esier than it sounds. Prior to the phlebotmy, wet a paper or cloth towel with warm water and place it on the arm site. This is more convenient than keeping your arm under the water faucet but the the running water is actually a little better since it also applies some pressure.

  • Tell the phlebotomist (the person drawing the blood) that you will allow one attempt and that if that attempt is not successful, you will require a more experienced person to perform the blood draw.

  • Tell each phlebotomist where the last successful blood draw attempt was and point to that site.

Technique for Blood Draws

  • The tourniquet can be drawn as tightly as you can stand it but cannot be left on for a prolonged period of time. Usually, 1 to 2 minutes is tops. After that, there are both chemical and cellular changes that alter test results.

  • Don’t let the phlebotomist slap the site. This increases inflammatory responses and interferes with coagulation testing.

  • Again, if you know that you have difficulty with the collection of blood, ask that the smallest needle possible be used. The smallest needle that can be used to collect a blood specimen that is not hemolyzed is a 23-gauge butterfly needle. However, when myeloma protein levels are high, a larger gage needle may be required.

  • When you make a fist, don't do it too tightly. A firm fist rather than an overtight one is best. One of the more frustrating things for phlebotomists is that sometimes veins will “roll” because there is not enough supportive tissue to keep them in place. They can put the needle in at the exact place they saw the vein but it “rolls” an inch or more to the side. This occurs more frequently when fists are overtight than when they are a little more relaxed.

  • Don't keep flexing your arm muscles. It too alters chemical balances which could skew your test results.

  • If your internal venous pressure is too low to withstand the force of the vacuum in the collection tube, the vein will collapse on itself only to re-establish itself after the vacuum is withdrawn. In that case, ask that the specimen be drawn either through a syringe or in a pediatric tube.

  • If you find yourself getting overly anxious, tensing your muscles, or becoming faint, soothe yourself with the following techniques:
    1. Take slow breaths, counting to 3 as you inhale through your nose and to 6 as you exhale through your mouth. If you start to feel lightheaded, slow down the count.
    2. Consciously make your muscles relax and feel loose.
    3. Fix your gaze on a focal point or close your eyes and envision a pleasing image.

Reference Ranges

The interpretation of any clinical laboratory test involves comparing the patient’s results to the test’s “reference range.” Reference ranges are highly complex because they reflect a highly complex world. There can be variations in test results due to collection, storage, transport and preparation techniques, types of instruments used, etc.

Just as no two automobiles run exactly the same way with exactly with same gas consumption, no two instruments run identically. Instruments age differently depending on use rate, maintenance, stability of electrical source, etc. In addition, there are dozens of reagent manufacturers; each with subtle differences in the composition of the reagents, ranging from minute variation in water quality to different sources of material. There are again dozens of manufacturers which make supporting supplies such as pipets, weigh boats, etc. There may be differences in the plastic composition, the amount of water trapped during processing, the accuracy of their volume makers, etc. Altitude, humidity, and temperature variation all play a part in the pre-analytical phase of testing as well. Particle counters in hematology are quite sensitive to changes in humidity while many chemical analyzers are sensitive to temperature changes. Just as your method of cooking changes at high altitude, people respond similarly to the changes at high levels. Hemoglobin increases as the oxygen percentage in the atmosphere decreases.

There are multiple methods to measure the same analyte. For example, there are a minimum of four different methods for analyzing blood glucose. One method measures glucose and a number of other substances called reducing agents, which include vitamin C. Its reference range is 80 - 120 mg/dL. Another method measures three sugars, one of which is glucose. This is a much narrower range and so the reference range of this method is 70 - 110 mg/dL. A third method analyzes only for glucose and uses an enzymatic method. A fourth method, also used for glucose alone, uses an electrical conductivity measurement. These two glucose-only methods have different reference ranges.

Then you need to look at specimen collection. To continue with the example of blood glucose, capillary blood gives a higher level of glucose than serum. Serum collected in a red-stoppered tube will gradually decrease its glucose level as the cells in the blood use the glucose prior to testing, while a blood specimen collected in a gray stoppered tube will not. If a specimen will be tested within a short amount of time, either collection process is adequate. If the specimen is to be transported or stored, then either the collection tube or a method of stabilizing the glucose is required.

You want the most accurate results possible but is there any difference between a glucose of 110 and 110.0001? There are instruments which will give you extraordinarily accurate results but sometimes that level of accuracy is not warranted. Other methods might lessen a tad on the accuracy without giving up the quality of the information. You want the earliest possible information but increased sensitivity comes at the price of increased false positives, which give everyone extra stress and increased testing to prove if the result was really clinically important. Besides, the more sensitive the test, the more expensive it is. I might choose the most expensive glucose test if I were in the laboratory at the Joslin Diabetes Clinic, where world class research into glucose management might justify it. I might choose an over-the-counter glucose machine if I were running a cardiac rehab clinic where diabetics might come. Community hospitals tend to have instruments with the broadest level of reliabillity and consistency. Tertiary care centers tend to have instruments with a higher level of sensitivity that might be a little more finicky.

Finally, we get to the important part — the patient population. You are part of a greater population. You do not expect the same results from a group of pediatric patients as from a group of geriatric patients. Reference ranges are developed to reflect the patient population that the laboratory serves. And, since the ranges that are developed are averages, not a definition of “normal,” the best way to look at those values is as a reference. Statistics work only on populations, not on individuals, so the best comparison for you is to compare your current results against your own previous reports. Physicians look to these comparisons to give them a sense of change over time. Typically, a change seen in three consecutive tests, or a significantly large change between two tests, is considered reason to re-evaluate the situation. For example, a hemoglobin dropping from 15.0 to 14.8 to 14.4 to 14.0 would be considered significant as would a drop from 15.0 to 13.0 So, a good practice would be to keep track of all of your results on a chart so that you (and your physician) can see the movement on a graph instead of trying to compare individual numbers.


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