How acurrate are ekg?

They are extremely accurate are what the measure - the electrical activity of the heart. How likely they are to provide the right answer depends on the question.

If you want to know what heart rhythm the person is in they are extremely accurate.

If a person is having chest pains and you want to know if it is a heart attack they are moderately accurate.

If a person is at rest with no pain and you want to know if the pains they get upon exertion are from a blockage in a coronary artery (angina) they are not at all accurate.