If you neglect to take your medication, the level in your blood will change, and that is what is detected through the test. Missing your medication only once in a long while will not be a problem, but if your forget to take it on a reular basis the level will be up and down and that can produce instability. It might help you to use a pill organizer if you don't already do so,
there are therapeutic drug levels for depakote, so potentially if you fall below that level it will show on a blood test. I would not suggest forgetting to take such an important med/.