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 range of two standard deviations above and below average will include, on average, 95% of healthy people. It will also exclude or label as apparently abnormal other 5% of healthy volunteers. So if "normal range" is generated in this fashion, one thing we already know is that it will be wrong 5% of 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, 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 normal range is clinically meaningful. The "normal range" is not a judgment; it is merely a statistical statement. You want 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 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

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 and tissue beneath skin are then numbed with local anesthetic, and then everything is ready to insert spinal needle.

The reason lower back (lumbar spine) is chosen is because here sac of meninges can be entered without risk of poking a hole in spinal cord. This is because spinal cord ends several inches higher within spinal canal. The composition of CSF is nearly same throughout its system. Thus, CSF from lumbar region is as good for diagnosis as from anywhere else, yet safer to obtain.

Once spinal needle enters lumbar sac of fluid, correct positioning of needle is confirmed by emergence of clear, colorless drops of fluid from back of needle. (When a similar procedure is performed for purpose of epidural anesthesia, tip of needle stops just short of entering meninges, and drug is infused outside sac.) A thin plastic tube is then attached to back of needle so CSF's pressure can be measured. Subsequently, CSF is allowed to drip into each of several sealable test-tubes suitable for sending to laboratory.

Once adequate fluid has been obtained, needle is withdrawn and small puncture site in 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 in vicinity, so even an off-course needle is unlikely to cause trouble. Theoretically, a needle-insertion could also bring germs into body and cause infection, but this almost never occurs because needle is sterile and because lumbar region had been surgically prepped.

About one-in-five patients experiences a headache from procedure. When a spinal-tap headache occurs, it always has following characteristics: it is present while patient is sitting or standing, and is promptly relieved by lying down. Spinal-tap headaches are due to persistent leaking of CSF through hole that needle made in meninges. (The leaking occurs within spinal column and doesn't leave body.) Until hole seals up again and full volume of CSF is restored, CSF cannot provide its usual cushioning effect with changes in head position, and a headache ensues. In such cases patient remains horizontal until 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. If test caused unforeseen problems, they should have turned up by now.

(C) 2005 by Gary Cordingley