Introduction to Laboratory Testing

Before looking at any the information on specific tests, it is important to understand some basic limitations about laboratory tests in general and how to get the best answer possible from your laboratory.

Introduction

There are 3 different aspects to each laboratory test; preanalytical, analytical and post-analytical. Preanalytical issues involve what happens to you and your specimen prior to testing. Analytical issues involve the choice of instrumentation or testing protocol used, while post analytical issues involve assessing the appropriateness of the results prior to and the reporting itself.

Preanalytical aspects

Preanalytical aspects include

• fasting/non fasting state,
• time of day,
• medications,
• mood,
• in short anything that is or has been altered since the previous specimen was drawn.

Most compounds (example: glucose) should vary from time to time, depending upon the patient’s diet and activity. So, rule #1 always try to have your specimens taken at approximately the same time of day and in the same state (fasting or non-fasting). Even if the test does not require a fasting or non fasting state, if you started out in one state, try and keep to that condition in the future.

If you minimize the variability inherent in your life, then your specimens will reflect that consistency and be more valuable.

Specimen collection, storage, and transport

Also in this category are the proper collection, transport, and storage of the sample. As a patient you have little control over this part of the preanalytical testing concerns but explaining some might make it easier to understand why some things are done the way they are. It is obvious that the larger the size of the blood drawing needle, the more discomfort. So, why do we not use the smallest needles possible? Because small needles cause pressure changes in the collecting tubes and may cause hemolysis or damage to red cells. This hemolysis can cause a specimen to be unacceptable or worse yet, when it is small enough to get by, will cause incorrect answers. So, the standard size needles became standard because they cause the least amount of trauma to the patient while giving the laboratory an acceptable specimen.

When blood is drawn from a vein in your arm, the cells are still alive and that cause changes that must be dealt with. Prolonged time in transit can cause false result. For example, collection of a blood glucose in a red stoppered tube allows the red cells to continue to use the blood glucose for energy and thus a specimen can "experience" a drop in blood glucose while waiting to be tested. Some test tubes have chemicals that separate cells from plasma to prevent contamination. Others have compounds that interfere with the cells’ function. Some tests must have their specimens chilled on ice immediately while others requires heat. An arm that has been rubbed or "slapped" to locate a vein has already begun a clotting process that has invalidated any coagulation testing. Jiggling the needle or other such maneuvers may also increase contamination invalidating the result. This is a poignant problem when trying to draw blood from a struggling and terrified child.

Labeling

This is probably the most frustrating preanalytical issue for the laboratory. Common sense and federal regulations require that incompletely labeled specimens must be rejected. Yet, many clinics, hospitals, and physicians’ offices are over-busy and specimens or the requisition sheets that accompany them are occasionally submitted with a last name or no name only or a last name and an initial. These must be rejected and no one wins.

Analytical Aspects;

Analytical aspects include the preparation of the sample to be tested and the method used. There are literally dozens of methods and instruments out there. And, yes, with different methods come different reference ranges.

Accuracy

Accuracy Now, don’t you think this is silly - of course, everyone wants a test result to be accurate. The problem is that no test is perfectly accurate all the time. So, really the question is how inaccurate is still okay? If you can’t get 100% accuracy from a test, is 95% accuracy good enough? What about 85% accuracy? Most laboratories prefer to use tests that are 95% or higher in accuracy but what does that mean? If a test of 99% accurate and you test 1,000 people, then 1% or 10 people will get an incorrect answer. Consequently,most laboratories will retest any results that look suspicious when compared to other test results from the same patient. This may take a little longer but it is well worth the added time.

Precision

Precision If accuracy is hitting the bulls-eye, then precision is how often you can repeat the throw. Most tests have a small variation of precision. For example, if the accurate concentration of glucose in a sample is 100 mg/dL, the variation might be plus or minus 5 or 95 to 105. This variability is insignificant to the physician. Typically, precision is determined by testing one sample multiple times and then calculating the range of variation. This range is sometimes called the range of reproducibility and is used to separate clinically significant versus insignificant changes. For example, if a diabetic’s blood sugars for several days are 147, 150, 145, 157, and 160, you could say that there has been no change. If, on the other hand, the values were 147, 150, 165, 187, and 190, you could say that there had been upward trend signifying trouble.

Reference ranges

These are also called normal values by some people. When setting up a test, it is customary to run many samples multiple times. Those results then give you an average or mean. From that mean and the original values, a standard deviation is calculated. This reflect 95% of all of the values reported on the population of samples. This is the reference range and since we tend to think that 95% of us probably are normal, we use this range as a guide to normalcy. One problem is fairly obviously: just because a value is outside of the 95% range does not mandate that it is abnormal. For example, if I were to find the reference range for height and used the first 500 people I meet, would the 2 year old or the basketball player who is 6’6" be considered "abnormal" because they didn’t fit in the ranges?

Another issue with these ranges is the fact that each laboratory should do their own ranges based on their own instruments and on their specific patient population which might mean that the ranges from one lab might not be the same as another. For example, there are many different procedures for blood glucose. One has a range of 80 - 120 mg (it measures glucose, several other sugars and some vitamins) while another has a range of 60 - 90 (it only measures glucose). Having your tests in different laboratories is a big problem because you just might be trying to compare apples and oranges. Some tests will have ranges segregated by age or sex or location (altitude). For example, hemoglobin values are different in men, women, children and infants. They tend to be higher if you live at a high altitude so comparing some of your tests with someone else’s is bound to cause trouble.

Post analytical Aspects:

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