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

Written by Gary Cordingley

Continued from page 1

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 withinrepparttar range of two standard deviations above and belowrepparttar 148659 average will include, on average, 95% ofrepparttar 148660 healthy people. It will also exclude or label as apparently abnormalrepparttar 148661 other 5% of healthy volunteers. So ifrepparttar 148662 "normal range" is generated in this fashion, one thing we already know is that it will be wrong 5% ofrepparttar 148663 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,repparttar 148664 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 outsiderepparttar 148665 normal range is clinically meaningful. The "normal range" is not a judgment; it is merely a statistical statement. You wantrepparttar 148666 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 likerepparttar 148667 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:

Lumbar Puncture: This (Really) Is Spinal Tap

Written by Gary Cordingley

Continued from page 1

The next step is to lie on your side on a bed or procedure table with your knees tucked up to your chest. The skin of your lower back is painted with an iodine-based solution to produce a sterile field. If you have an allergy to iodine, an alcohol-based solution is substituted. The surrounding area is then covered with sterile paper or cloth. The skin andrepparttar tissue beneathrepparttar 148658 skin are then numbed with local anesthetic, and then everything is ready to insertrepparttar 148659 spinal needle.

The reasonrepparttar 148660 lower back (lumbar spine) is chosen is because hererepparttar 148661 sac of meninges can be entered without risk of poking a hole inrepparttar 148662 spinal cord. This is becauserepparttar 148663 spinal cord ends several inches higher withinrepparttar 148664 spinal canal. The composition ofrepparttar 148665 CSF is nearlyrepparttar 148666 same throughout its system. Thus, CSF fromrepparttar 148667 lumbar region is as good for diagnosis as from anywhere else, yet safer to obtain.

Oncerepparttar 148668 spinal needle entersrepparttar 148669 lumbar sac of fluid, correct positioning ofrepparttar 148670 needle is confirmed byrepparttar 148671 emergence of clear, colorless drops of fluid fromrepparttar 148672 back ofrepparttar 148673 needle. (When a similar procedure is performed forrepparttar 148674 purpose of epidural anesthesia,repparttar 148675 tip ofrepparttar 148676 needle stops just short of enteringrepparttar 148677 meninges, andrepparttar 148678 drug is infused outsiderepparttar 148679 sac.) A thin plastic tube is then attached torepparttar 148680 back ofrepparttar 148681 needle sorepparttar 148682 CSF's pressure can be measured. Subsequently, CSF is allowed to drip into each of several sealable test-tubes suitable for sending torepparttar 148683 laboratory.

Once adequate fluid has been obtained,repparttar 148684 needle is withdrawn andrepparttar 148685 small puncture site inrepparttar 148686 skin is covered with an adhesive bandage. Typically, there are no more than a few drops of blood-loss from this test.

How about risks? Fortunately, they are minimal. As with any other test in which a needle is inserted somewhere that Mother Nature never intended, bleeding is a possibility. Luckily, there are no major blood-vessels inrepparttar 148687 vicinity, so even an off-course needle is unlikely to cause trouble. Theoretically, a needle-insertion could also bring germs intorepparttar 148688 body and cause infection, but this almost never occurs becauserepparttar 148689 needle is sterile and becauserepparttar 148690 lumbar region had been surgically prepped.

About one-in-five patients experiences a headache fromrepparttar 148691 procedure. When a spinal-tap headache occurs, it always hasrepparttar 148692 following characteristics: it is present whilerepparttar 148693 patient is sitting or standing, and is promptly relieved by lying down. Spinal-tap headaches are due to persistent leaking of CSF throughrepparttar 148694 hole thatrepparttar 148695 needle made inrepparttar 148696 meninges. (The leaking occurs withinrepparttar 148697 spinal column and doesn't leaverepparttar 148698 body.) Untilrepparttar 148699 hole seals up again andrepparttar 148700 full volume of CSF is restored,repparttar 148701 CSF cannot provide its usual cushioning effect with changes in head position, and a headache ensues. In such casesrepparttar 148702 patient remains horizontal untilrepparttar 148703 leak has sealed over.

Reviewing a list of potential complications can have a discouraging effect on people who need a test. But it is reassuring to know that millions of people have had Dr. Quincke's test since he devised it over a century ago. Ifrepparttar 148704 test caused unforeseen problems, they should have turned up by now.

(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:

    <Back to Page 1 © 2005
Terms of Use