Proper diabetes control means maintaining your blood glucose levels within the normal range, i.e. between 80 mg/dL and 120 mg/dL.
Why measure?

Before and after meals: This is when you should definitely check your blood glucose levels and how often it is necessary for you to check yourself on average.
Measuring frequency

You cannot dose your insulin correctly unless you know your current blood glucose level. The most important tips for measuring your blood glucose levels correctly.
Measure correctly

Is your measured blood glucose value out of step with the way you feel? Why is that? Did an error occur with your test strip, your meter or during measurement?
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If you switch from a blood glucose meter calibrated for whole blood to a meter calibrated for plasma or vice-versa, then you may need to adjust your target values.
Whole blood vs. plasma