There was, when I was training, a maxim that before action a test should be done three times if possible. Not always possible for some things, but knowing the variability of some of the tests we do, quite a Good Idea. A f’rinstance for us would be high pressure inside the eye, which may indicate glaucoma. It is really, really easy to have your fingers in the wrong place when taking the measurement, so if I get a high reading, I will repeat the test, at least once. Another f’rinstance of ours is field testing (again for glaucoma), which has a very definite learning effect…..you can even see it with the testing of the second eye on a patient who has never done the test before. That does require subjective input from the patient though, so it’s a bit different.
I personally would not accept treatment for the common stuff…..blood pressure, cholesterol, blood sugar, without at the very least two high readings on different days. I believe the metabolism of the body is rather variable.