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 taken in.

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.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.