Imagine that your last hemoglobin A1c result was 5.9. Now it's time for a new test, and the results show that your A1c is 6.1. Does this mean your control has gotten a lot worse?

Not necessarily.

We have to remember that all these lab tests we get are subject to all kinds of errors. First, they're performed by human beings, and all human beings occasionally make mistakes.

When I was first diagnosed, my hemoglobin A1 (the old test that didn't break down into A1c) was 16. This was equivalent to an A1c of about 13. Obviously not good! I was put on metformin and given the standard dietary advice.

I drastically reduced my food intake, especially starches and sugars, went back for another test in several weeks, and then saw my doctor. He said my A1 hadn't changed. I told him that was impossible. "If it had gone from 16 to 14, I'd believe you, but it's impossible that it hasn't changed when I've changed my diet so much (read more about dietary changes for diabetes here). My fastings have come way down." He gave me the patronizing look that doctors can give and obviously didn't believe me.

I went home and ate a candy bar. Why bother to diet if your A1 doesn't change. Then the telephone rang. It was the doctor. He said he'd been looking at the wrong A1 test sheet. My new A1 was actually 8 (equivalent to an A1c of about 7).

So it's always possible that, like my doctor, someone working in the lab read the wrong paper or the wrong line on the paper giving the results. [I once got a letter from my college congratulating me on my recent marriage. I wrote back to say I had a policy never to get married before the first date. It turned out someone typing up the lists had skipped a line.]

Many tests are actually performed by machines. Machines also make errors or get out of calibration. I used to use a OneTouch Profile meter that measured blood glucose (BG) levels in terms of whole blood, not plasma, and the results were always higher than the lab results, ranging from about the expected 12% up to about 25%.

One time I was in a research study at the Joslin Diabetes Center that involved testing my BG level every 5 minutes. To do this, they took the blood, spun it down in a centrifuge to remove red blood cells, and then measured the BG level of the resulting plasma in a Beckman spectrophotometer that was supposed to be very accurate.

The results they were getting were almost exactly the same as what I was getting on my Profile meter, which should have been reading higher. I mentioned it to the researchers, and they said perhaps their Beckman spectrophotometer needed recalibration.

So it's always possible that the machine used to do some test is out of calibration. Or perhaps the chemical solutions used for the test were made up improperly, or had gotten too old. Or perhaps some test tube was contaminated.

I once decided to test my meter by taking 5 readings at the doctor's office just before they drew blood for a blood test. I did this several times, and every time, both my Ultra and my Profile gave results that were much higher than what the lab reported.