Medical Tests: What Does a Normal Range Mean?
We have a marvelous array of medical tests available to us. Many of them-typically blood-tests-even come with results expressed in numbers signifying the exact quantity of something that was measured. On laboratory reports these measured quantities are often accompanied by a "normal range" for what the laboratory apparently thinks the value should have been, showing a lower number and a higher number. So if you had a laboratory test, what does it mean when your measurement falls inside or outside this range? It all depends on the particulars. First of all, it matters what is being measured and why it was measured in the first place. In theory, a laboratory test is ordered when the doctor poses a question for which the laboratory test is supposed to provide an answer. (If there was no question, how could the lab-test be an answer?) For example, suppose a doctor is wondering if your hand tremors are due to too much thyroid-hormone in your bloodstream. So the doctor's question is: Does this person have too much thyroid-hormone in the bloodstream? A test measuring the thyroid-hormone would provide a clean answer if it was either too high (yes) or within the normal range (no). Because thyroid-hormone levels that are too low do not produce tremors, a measurement that was too low would be irrelevant to the question posed. It might still warrant consideration in its own right as an "incidental finding," but is no different than a within-range outcome in answering the original question. How about the same blood-test, but with a different question? Suppose the doctor is trying to figure out why you gained weight. The doctor knows that some people gain weight when their thyroid glands produce too little thyroid-hormone. So the doctor's question is: Does this person have too little thyroid-hormone in the bloodstream? This time, measuring the thyroid-hormone would provide a clean answer to the question if it was lower than the normal range (yes) or within the normal range (no). Because elevated thyroid-hormone levels do not usually cause weight-gain, a number higher than the normal range would produce an answer to the original question no different than one that was within-range. (But even if this outcome was unexpected, it might still be followed up.) For some blood-tests the only meaningful result is in one direction. For example, a blood urea nitrogen (BUN) measurement assesses kidney function. If the BUN measurement is too high, it could signify that the kidney is impaired. But what does it mean if your BUN measurement is lower than the normal range? It means absolutely nothing. It's a non-event. So then it's curious that a normal range for BUN even includes a lower number. How did it get there? These examples lead up to the question of how the normal ranges are created in the first place. They are produced by statistics generated by measurements obtained in healthy volunteers. In the case of the BUN measurement, for example, this substance might get measured in the blood of, say, 100 people without kidney disease. An average number would be calculated by adding the numbers produced by all 100 people, and then dividing by 100. This average would be the center of the normal range. But the upper and lower numbers are produced by another method looking at how widely spread apart the BUN measurements are in these 100 people. After all, it would be highly unlikely that all 100 people would produce the exact same number-value. So how far from the average is still okay? The 100 measurements are plugged into a mathematical formula to compute a "standard deviation," a widely-used statistic related to how widely the numbers are spread apart. Numbers that are farther apart produce a larger standard deviation, while numbers that are closer together produce a smaller standard deviation. The next step is to decide how many standard deviations above and below average should be accepted as normal. A typical choice for a blood-test is two standard deviations in either direction. It is known that measurements falling within the range of two standard deviations above and below the average will include, on average, 95% of the healthy people. It will also exclude or label as apparently abnormal the other 5% of healthy volunteers. So if the "normal range" is generated in this fashion, one thing we already know is that it will be wrong 5% of the time. Another outcome of this statistical system is that if your doctor measures twenty different things in your bloodstream, then, on average, one of them will be outside its normal range-even if you are totally healthy! Moreover, the statistical method automatically generates a lower and an upper value. This is done without consideration of whether or not it means anything to be too high or too low. The normal range merely expresses typical values obtained in healthy volunteers. It is up to your doctor to determine whether or not a result outside the normal range is clinically meaningful. The "normal range" is not a judgment; it is merely a statistical statement. You want the judgment to come from a trained clinician. Your doctor has special words that are useful in navigating this intersection between statistics and clinical meaning. One favorite word is "unremarkable." You might not like the idea of being described as unremarkable. But this is a handy term that your doctor uses to lump together your laboratory results that fell within their normal ranges with those that fell outside their normal ranges but were still considered benign. So in this case, "unremarkable" is a good thing to be. (C) 2005 by Gary Cordingley Gary Cordingley, MD, PhD, is a clinical neurologist, teacher and researcher who works in Athens, Ohio. For more health-related articles see his website at: http://www.cordingleyneurology.com
Related News
|
 |
|
|
RELATED ARTICLES
Melatonin, Sleep Enhancement, and ADHD
Overview
Melatonin is a natural hormone, which is produced and secreted by the pineal gland. Melatonin plays an important role in the regulation of many hormones in the body. Among its key roles, melatonin controls the body's circadian rhythm, an internal 24-hour time-keeping system that controls when we fall asleep and when we wake up.
Phentermine Diet Pills: When Diet and Exercise Arent Enough To Help You Lose Weight
Have you been struggling to lose weight, but diet and exercise aren't giving you the results you need to be healthy? Are you sick of people telling you that you could lose weight if only you had the will-power? It may be time to ask a physician about a phentermine prescription.
Medical Billing PDA: 21st Century Convenience for Medical Practices
Medical Billing PDA
Paxil Recall Lawyer Discusses Paxil Addiction
Paxil went on the market in 1992 at the height of antidepressant drug popularity. It is a member of the class of antidepressants called selective serotonin reuptake inhibitors, or SSRIs. The SSRI drugs revolutionized antidepressants because they have very few side effects and it is practically impossible to overdose on them. It makes sense not to prescribe a depressed person with pills they can easily overdose with. Though a latecomer to the SSRI market, Paxil grew successful very quickly by gaining FDA approval for very specific conditions like "social anxiety disorder" and "general anxiety disorder", which were very rare conditions at the time. In running add campaigns, Paxil encouraged people to try their drug if they'd experienced any general anxiety and, as can be expected, sales of Paxil soared.
Blood Pressure Treatment: High and Low Blood Pressure Tips and Medication!
High blood pressure is on the rise worldwide. But, doctors believe that small lifestyle changes can go a long way in preventing this silent killer.
Curing Lou Gehrigs Disease
ALS is the disease that took the life of the famed baseball player. The disease benched Lou Gehrig from the game that he loved and later took his life. Now, ALS is referred to as the Lou Gehrig's disease. Today more than fifty years later there still is no cure for ALS. ALS is a motor neuron disease. ALS stands for Amyotrophic Lateral Sclerosis. The disease attacks nerve cells in the brain and spinal cord. It attacks a persons muscles one at a time. Not unlike being tortured. Sadly there is no cure and no viable treatment.
Powerful Arthritis Pain Relief For All Of Us
When you ask what arthritis is, professinals will tell you it's inflammation of one or more joints. But you know it better as pain, swelling, stiffness, deformity, and/or a diminished range of motion of those joints! It's estimated that over 50 million Americans suffer from osteoarthritis, rheumatoid arthritis and other related conditions.
Bextra and Vioxx -- Tips for Arthritis Relief Without Them
The recent withdrawal of Vioxx and Bextra from the marketplace, due to concerns over unwanted side effects, has many arthritis sufferers concerned. These drugs, which belong to a powerful class of non-steroidal anti-inflammatory drugs known as COX-2 inhibitors, are quite effective at fighting the pain and inflammation of arthritis. Unfortunately, they have been linked to increased risk of strokes and heart attacks. Their manufacturers have voluntarily withdrawn them from the market.
Joint Pain Relief
If you joints are constantly killing you, here are three suggestions:
At Last! Good News For Gout Sufferers!
Gout, an arthritis affliction with its roots in antiquity, has not always been easy to treat. The traditional medicines employed for gout have been non-steroidal anti-inflammatory drugs (NSAIDS), colchicines, probenicid (Benemid), and allopurinol (Zyloprim).
Clinton on South Beach Diet
Clinton's recent quadruple bypass surgery has been largely debated. Being on the South Beach Diet made everybody wonder how the former president's illness advanced so much as to need to be surgically corrected. Some even blame it on the diet. Some wonder why the diet didn't help avoiding surgery.
Toenail Fungus: Tips For Treatment
The medical term for toenail fungus is "onychomycosis," pronounced on * EE * ko * my * ko * sis. Despite the commonly used term "fungal toenails", onychomycosis describes both fungus and yeast infections in the nail. The prevalence in America is about 2-3%, but some have reported it as high as 13%. Even at a low estimate of 2%, this accounts for 6 million Americans with toenail fungus. Toenail fungus affects men twice as often as it affects women.
Integrative Medicine And Its Future
The NBC National News, quoted the Washington Post in March 2000 about the concerns that the FDA had over the mixing of supplements and conventional medications. There was concerns that millions of people are taking supplements (considered herbs, homeopathic, vitamins, minerals, amino acids, enzymes and some diet products) and having reactions when they mix these things with common and prescribed medications.
Is Anxiety Ruling Your Life?
Anxiety is the most frequently occurring mental health
disorder in the United States, according to the US Surgeon
General. There are millions of people who are diagnosed
with it every year. A small amount of anxiety helps to get
us going, but when it becomes more severe, it can be
overwhelming. If you start feeling like it's more than it
should be, you may want to visit a doctor for a clinical
diagnosis. The National Institute of Mental Health (NIMH)
has estimated that 19 million Americans aged 18 to 54
suffer from anxiety disorder every year.
VIOXX Lawsuits
Vioxx is a prescription medication that has been used primarily in the treatment of signs and symptoms of arthritis, but has also been prescribed to treat acute pain in adults and menstrual pain in women. It is a Cox-2 selective nonsteroidal anti-inflammatory drug or NSAID. Other Cox -2 selective NSAIDs on the market are Celebrex and Bextra. Vioxx is also related to nonselective NSAIDs such as ibuprofen and naproxen.
Emollients and Lubricant Drugstore Laxatives
Stool Softener Laxatives - Emollients
Lipitor Side Effects
Lipitor is a drug intended to help lower cholesterol. The generic name for this brand-name medication is atorvastatin.
Polysomnography: One Tool in Helping in the Diagnosis and Treatment of Fibromyalgia
Stop! and imagine for one moment that your body is being savagely and brutally attacked by chronic pain. This pain is so intense that you become less active. As you become less active you start to develop muscles weakness. Just trying to do normal daily activities such as, working, housekeeping, cooking, playing with the kids, shopping, walking the dog and sleeping has become an extreme ordeal. All is not peaceful in the Land of Nod. In fact, you as a fibromyalgia (FM) sufferer are downright restless.
Orphan Drugs: Hope Where There Is Little or No Hope
NEW YORK, N.Y., February 18, 2004 ? On a visit to his doctor, Gary Jacob received distressing news ? not about himself, but a friend of the doctor's.
Kids and Medication Part I
We have all talked a lot about the problems in our educational system, about latchkey kids, about teacher turnover (50 percent of teachers quitted within the first five years on the job), psychologists over prescribing medications such as Prozac and Ritalin with our friends and families. We can all foresee a terrible problem in the future because we are feeding these drugs to kids under the age of six, their brains have not been fully formed yet and these drugs and their predecessors and competing drugs all have adverse side effects.
|
