Be careful, you can't just change a dx to get something paid. The dx code should reflect the reason(s) for the visit. If the patient came in for STD screening only, meaning she/he was having no symptoms whatsoever than the V74.5 would be accurate. However, if the patient came in for any symptoms that made her/him or the dr suspect there may be an STD then that dx would go primary, and the V74.5 would be secondary. For example if the patient had vaginal pain/discomfort, or discharge, or rash or something that would be the primary dx. But if the patient simply came in to be tested then there is no other dx that should be used. And Medicare does not allow 99203 with a primary dx of V74.5.
I hope that helps!