QUESTION: I am a Canadian reader and notice that the numbers you use for blood sugar levels are much higher than the ones my doctor uses for comparable levels. I inquired and find that a newer system is in use here than in the U.S. My level is in single-digit decimals, while the ones you give are up in the 100s. It's confusing. I have diabetes, so blood sugar is important to me. - Mrs. I.H.

ANSWER: Your blood sugar is being expressed in Systeme International numbers (SI for short). SI units are, in fact, used in most countries as well as in most U.S. medical journals. But the system has not come into everyday usage in U.S. medical practice.Example: Normal fasting blood sugar in SI units ranges from 4.2 to 6.4 mmol/l - millimols per liter. Under the older system, that same range is expressed as 75 to 115 mg/dl - milligrams per deciliter. To change old sugar values to SI, simply multiply by .05551. To avoid confusion, we sometimes speak of blood sugar values in both systems, for example 75 (4.6 SI). It's a bit awkward, but helpful until we all get on the same wavelength in this matter. The SI system is being used for all lab values. For example, for cholesterol the SI conversion multiplier factor is 0.02586. See next item.

QUESTION: My cholesterol is 220. My doctor wants to test again. Is this necessary? I'm not far above normal, am I? - P.L.

ANSWER: You are far enough above the normal of 200 (5.17 SI) to prompt a retest. When the doctor gets the second test results he can average the two. Then if you are on the high side you will need a low-fat, low-cholesterol diet, and will also need a recheck in about a year. But read on.

If a person's cholesterol is over 200 (5.17 SI) and if he has at least two heart disease risk factors, like smoking, family history, high blood pressure or diabetes, he needs further tests. They will break down the kinds of cholesterol included in the total figure. You need to know how much of the total is LDL, the bad kind, and HDL, the good kind. Depending on those results, you may actually need cholesterol-lowering drugs to bring you into a more favorable range. I am sending on the cholesterol report. Other readers may order, by writing: Dr. Donohue/No. 5, P.O. Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped (52 cents), self-addressed envelope and $2.

QUESTION: I had my tubes cut and tied. What are my chances of having an ectopic pregnancy? - Mrs. J.A.

ANSWER: The main consideration is of conception occurring at all with tubes tied. The odds of that happening are about 2 in 1,000. The chances of ectopic (tubal) pregnancy occurring from such conception are about 2 in 10. With that statistical background, I would say chances of tubal pregnancy are minuscule.

QUESTION: Can you discuss double vision? When looking at a distance through both eyes, I have double vision. With one eye, the vision is single. Can you offer any suggestion? - Mrs. E.E.D.

ANSWER: The muscles that move our two eyes must work in unison to align their lenses properly and blend the two images to form a single picture. If an eye wanders, even a bit, you get double vision.

You have to find what is causing the malalignment - whether nerves, muscles or the brain itself, which produces the melding of images for sight to occur. Diabetes, myasthenia gravis, even an overactive thyroid cannot be dismissed as potential causes of diplopia (double vision). I think you need a checkup.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.