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Things You Need to Know About Lab Tests

Analyzing Lab Tests
Various aspects of health care have made significant advances in the last few decades . Laboratory testing has also improved, not only in the range of tests available, but also in the quick turnaround-time of results.

Unfortunately though, no lab test is perfect. Tests may either inaccurately suggest that a patient has a particular disease they don't have, or instead miss the presence of the disease in someone who actually has it. Either of these situations can be very problematic.

MRIs, for example, are a helpful tool that reveal more information than ever before. But not every suspicious shadow on an MRI means something. It takes time and experience to accurately interpret MRI images. It is not unusual for new technology to become available well in advance of our ability to accurately interpret the results.

Lab tests need to be interpreted cautiously. In order to have an accurate diagnosis, we need to also have supporting evidence in the form of patient history and physical exam findings, in addition to the lab test results.

You're in your doctor's office because you noticed some unusual symptoms and are afraid something might be wrong. Even though you know some lab tests are probably necessary, that doesn't prevent you from feeling anxious. We may want to know the results, but at the same time we probably are very concerned about what the outcome might be. Hearing the doctor casually mention that "we need to do some blood work," can be enough to trigger a strong bout of anxiety in many people.

Then when you get the results, it can be difficult to decipher the medical jargon, especially when the tests are "positive". Most of us have had the experience of our mind tuning out after hearing the initial news that "your test results are positive".  In this situation, our minds begin racing ahead, imagining all sorts of awful possibilities.

Occasionally, a compassionate physician will endeavor to put the lab results in a less-worrisome perspective for the patient. For patients, it helps to know some important facts  that will help to empower you whenever you have lab tests done.

First of all, statistically one out of every 20 tests performed will be reported as "abnormal", even though the result is in fact "normal" for the individual being tested. If you are perfectly healthy and have 20 tests done on your blood sample, one of those 20 test results will be "out of the normal range" based on statistics alone.1

This anomaly is a result of the way the normal test range is calculated, which involves testing a large group of "normal" people and eliminating the top and bottom 2.5% of the results, leaving the middle 95% as the normal range. So if 20 tests are done, statistically one will be considered "abnormal", only because its results will fall in the top or bottom 2.5%.  It may be perfectly normal for you, but it will still be reported as "abnormal".

Other things to consider are the specificity and the sensitivity of the lab test.2  Specificity has to do with whether or not a particular positive test result actually indicates the presence of the disease for which you are being tested.  If a test has a specificity rate of 90%, then in ten out of 100 positive results, the patient does not actually have the disease. Sensitivity has to do with how accurate the test is in detecting the disease when the disease is truly present. For example, if a test for colon cancer is 90% sensitive, it will miss ten cases of the disease out of every 100 cases.

To summarize, if a test has low specificity, test results reporting the presence of the disease may be inaccurate, and if a test has low sensitivity, actual cases of the disease may be missed. All these factors need to be taken into consideration when interpreting lab test results. Things are not always as they seem.3

The important thing to realize from all of this, is that lab test results should always be evaluated in the specific context of the patient. An isolated lab test result - or a set of lab test results - needs to be related to the patient's circumstances and condition.  Making a diagnosis based on lab results alone can be a big mistake.

Similarly, x-ray, MRI, and ultrasound results must also be interpreted in context. Attempting to interpret an x-ray without any clinical information will likely result in an inaccurate conclusion. Context is everything. Your doctor should be willing and able to explain why certain tests are necessary, and to help you understand what the results indicate, when taken into consideration along with your exam and other findings.

1Thomas SL, et al: How accurate are diagnoses for rheumatoid arthritis and juvenile idiopathic arthritis in the general practice research database? Arthritis Rheum  59(9):1314-1321, 2008
2Friston KJ, et al: Classical and Bayesian inference in neuroimaging: theory. Neuroimage 16(2):465-483, 2002
3Kobayashi M, et al: Intraindividual variation in total and percent free prostate-specific antigen levels in prostate cancer suspects. Urol Int 74(3):198-202, 2005

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