DEAR DR. ROACH: I’m a 74-year-old male in good health. My lab work has been pretty consistent over several years. My cholesterol level is always below 200 mg/dL with a high HDL (80 mg/dL). All of these results over the years have been done at the same commercial lab company.

I recently had blood drawn, but this time, I used a different lab. In three months, my cholesterol went from 178 mg/dL to 211 mg/dL. There were no changes in my diet or exercise. Some other test results showed higher numbers than my usual ones as well. Is there a difference in how they calculate lab results from one lab to another?

• L.E.

ANSWER: Cholesterol levels from the lab are highly reliable, and I doubt that the lab made a mistake — or that one lab was standardized to a different level from the other. Rather, I suspect it was a normal variation of the cholesterol level in your body that caused the high results.

With my own patients who use our excellent hospital lab, I’ve certainly seen changes of 10% or more. Part of this may be related to the time of day; cholesterol numbers are reliably higher in the morning. The numbers also fluctuate based on when you last ate and how much fluid you’ve had.

The lab test itself has a range of potential error, which is a reflection of the way that cholesterol is measured — not an error by the laboratory personnel. It’s possible that you had a “perfect storm” of personal daily variations, subtle diet/fluid status changes, time-of-day variations and lab variations.

This is so common a phenomenon that it has a name. When a person has an unusually high reading (for them), the next reading is likely to be closer to their usual value. It’s called “regression toward the mean,” and it’s a big reason as to why we seldom make changes to a person’s medical care based on a single value.

DEAR DR. ROACH: I’m an 82-year-old man in overall good health. I’m a registered organ donor. At this age, is there anything still worth donating?

• J.M.N

ANSWER: Absolutely. Liver and kidney organ transplants are common from donors in their 80s. Corneas can be taken at any age. Hearts and lungs aren’t used as often, but registering to be an organ donor gives you the possibility of helping many people. The organ donor team will do an evaluation to see whether you can be a donor after you pass.

ToYourGoodHealth@med.cornell.edu