In your example given of an EKG in addition to the E/M, I would say you do not need modifier 25 because the test (service) was a necessary part of the exam and MDM...key components of the E/M code. Another example would be an in-office urine dip to test for UTI. This was necessary for the assessment and MDM of the CC, so no modifier is needed.
To contrast: If a patient presented to the office with pain in the elbow and after a detailed history and physical exam, was diagnosed with left lateral epicondylitis requiring an injection of Aristospan, then you would append the modifier 25 to the E/M code along with the proper injection code because a service was actually performed for the TREATMENT of a diagnosis in addition to the E/M service provided that day.
My simplified rationale to understanding modifier 25 in short is: If the service (CPT code) was DIAGNOSTIC (required for the physical exam and/or MDM of the CC), then no modifier 25 is needed on the E/M code. If the service (CPT code) was actually TREATING the condition diagnosed, then modifier 25 is needed on the E/M code.
To the experts out there, am I on the right track or way off?