What does a borderline abnormal EKG mean?

Borderline means it. Is just off by a little bit from normal, a variation of normal or early changes indicating possible problems.
EKG. Borderline abnormal is slightly below the low cutoff range or slightly above the high cutoff range of normal. For example, for example; if normal range is 10-20 , borderline abnormal would be something like 9 or 21 .