Routine and diagnostic is different:

Routine is mainly for mamoscreening and preventive medicine. Diagnostic is not for screening. I am pretty sure your obgyn mistakenly used diagnostic. You need to talk to your obgyn office and request an evidence that they have resubmitted the code. It is called ICD code.

By any means, they should revoke their collection request. But I honestly do not know what will happen to your credit score.

Good luck!