Medical Tests: What Does a "Normal Range" Mean?

Written by Gary Cordingley

We have a marvelous array of medical tests available to us. Many of them—typically blood-tests—even come with results expressed in numbers signifyingrepparttar exact quantity of something that was measured. On laboratory reports these measured quantities are often accompanied by a "normal range" for whatrepparttar 148659 laboratory apparently thinksrepparttar 148660 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 onrepparttar 148661 particulars. First of all, it matters what is being measured and why it was measured inrepparttar 148662 first place. In theory, a laboratory test is ordered whenrepparttar 148663 doctor poses a question for whichrepparttar 148664 laboratory test is supposed to provide an answer. (If there was no question, how couldrepparttar 148665 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. Sorepparttar 148666 doctor's question is: Does this person have too much thyroid-hormone inrepparttar 148667 bloodstream? A test measuringrepparttar 148668 thyroid-hormone would provide a clean answer if it was either too high (yes) or withinrepparttar 148669 normal range (no). Because thyroid-hormone levels that are too low do not produce tremors, a measurement that was too low would be irrelevant torepparttar 148670 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 answeringrepparttar 148671 original question.

How aboutrepparttar 148672 same blood-test, but with a different question? Supposerepparttar 148673 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. Sorepparttar 148674 doctor's question is: Does this person have too little thyroid-hormone inrepparttar 148675 bloodstream? This time, measuringrepparttar 148676 thyroid-hormone would provide a clean answer torepparttar 148677 question if it was lower thanrepparttar 148678 normal range (yes) or withinrepparttar 148679 normal range (no). Because elevated thyroid-hormone levels do not usually cause weight-gain, a number higher thanrepparttar 148680 normal range would produce an answer torepparttar 148681 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-testsrepparttar 148682 only meaningful result is in one direction. For example, a blood urea nitrogen (BUN) measurement assesses kidney function. Ifrepparttar 148683 BUN measurement is too high, it could signify thatrepparttar 148684 kidney is impaired. But what does it mean if your BUN measurement is lower thanrepparttar 148685 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 torepparttar 148686 question of howrepparttar 148687 normal ranges are created inrepparttar 148688 first place. They are produced by statistics generated by measurements obtained in healthy volunteers. Inrepparttar 148689 case ofrepparttar 148690 BUN measurement, for example, this substance might get measured inrepparttar 148691 blood of, say, 100 people without kidney disease. An average number would be calculated by addingrepparttar 148692 numbers produced by all 100 people, and then dividing by 100. This average would berepparttar 148693 center ofrepparttar 148694 normal range. Butrepparttar 148695 upper and lower numbers are produced by another method looking at how widely spread apartrepparttar 148696 BUN measurements are in these 100 people. After all, it would be highly unlikely that all 100 people would producerepparttar 148697 exact same number-value. So how far fromrepparttar 148698 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 widelyrepparttar 148699 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.

Lumbar Puncture: This (Really) Is Spinal Tap

Written by Gary Cordingley

I couldn't resistrepparttar title's corny riff onrepparttar 148658 name ofrepparttar 148659 rock band and their movie, butrepparttar 148660 kind of spinal tap featured in this article was a spinal tap before Spinal Tap was Spinal Tap. (Does that make any sense?)

Known more formally as a lumbar puncture, this kind of spinal tap is a valuable medical test with an interesting history. In 1891 Heinrich Quincke, of Kiel, Germany, introduced this procedure as we know it today. His original intent was to help babies suffering from hydrocephalus (water onrepparttar 148661 brain) by draining away excess fluid, but fromrepparttar 148662 outset he was also interested in lumbar puncture's use as a diagnostic tool.

To understandrepparttar 148663 usefulness of this test and why you might someday need to have one, a little background is helpful. The brain and spinal cord are wrapped in a membrane calledrepparttar 148664 meninges. Withinrepparttar 148665 meninges, a watery fluid calledrepparttar 148666 cerebrospinal fluid (CSF) bathesrepparttar 148667 inside and outside ofrepparttar 148668 brain andrepparttar 148669 outside ofrepparttar 148670 spinal cord. Withinrepparttar 148671 brain's fluid chambers (ventricles),repparttar 148672 body perpetually manufactures new CSF from constituents ofrepparttar 148673 bloodstream. Oncerepparttar 148674 CSF has percolated through openings to get outsiderepparttar 148675 brain, it is reabsorbed and recycled intorepparttar 148676 bloodstream. The entire volume of CSF—about 150 milliliters or five ounces—is made and reabsorbed several times per day.

Dr. Quincke understood that analyzingrepparttar 148677 CSF's makeup could be useful in diagnosing infections and other diseases affectingrepparttar 148678 central nervous system (brain plus spinal cord). Measuringrepparttar 148679 CSF's protein and glucose (sugar) content along with inspecting a sample of CSF under a microscope to count red and white blood-corpuscles soon became standard practices.

The premier use of lumbar puncture in both Quincke's time and ours has been to diagnose meningitis. The suffix "-itis" signifies inflammation, so meningitis means inflammation ofrepparttar 148680 meninges. Most, but not all, instances of meningitis are due to infections, butrepparttar 148681 kinds of infections seen have evolved overrepparttar 148682 years. In Quincke's lifetime tuberculosis and syphilis germs were common causes of meningitis, but presently, in developed countries these are uncommon. Nowadays,repparttar 148683 usual causes of meningitis are other bacteria, viruses or even funguses. In cases of suspected infection, CSF protein, glucose and blood-corpuscle measurements are supplemented by other tests onrepparttar 148684 fluid that can track downrepparttar 148685 specific, infecting organisms.

Another important use of lumbar puncture is to diagnose subarachnoid hemorrhage, an abrupt, devastating, and potentially lethal bleed intorepparttar 148686 CSF space caused by rupture of an aneurysm or other abnormal blood vessel. In suspected cases—classically presenting with "the worst headache of my life"—a computed tomographic (CT) scan is usually performed first. While very sensitive in detecting subarachnoid hemorrhages, CT scans can still miss cases. So ifrepparttar 148687 doctor is still suspicious that a bleed occurred,repparttar 148688 next step is to do a lumbar puncture which is 100% sensitive in detecting this condition. That is, it never misses.

Lumbar puncture with CSF analysis can also help inrepparttar 148689 diagnosis of multiple sclerosis, a disease in whichrepparttar 148690 patient's own immune system attacksrepparttar 148691 central nervous system. In this conditionrepparttar 148692 immune reaction produces abnormal proteins that can be detected and measured inrepparttar 148693 CSF.

How isrepparttar 148694 test performed? Well,repparttar 148695 first step, of course, isrepparttar 148696 informed consent process in which your doctor explainsrepparttar 148697 risks and benefits ofrepparttar 148698 test and you sign a permission form. In this author's opinion, lumbar puncture isrepparttar 148699 most benign test for which written permission is traditionally required and is less risky than some other procedures—like drawing blood from a high-pressure artery—for which written permission is traditionally omitted.

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