How do I fix this? I was screwed over by the system.?

So my doctor told me some test for STD's would be free. (yes, i know ... STD's AAHHH. >_>)

I knew I didn't have sh$t but she said it was free so I was like why the hell not.

Turns out, they billed me like 300$ using the wrong insurance. I sent them the right info.

Didn't do shiizzz. >_> AND they didn't even send me the results.

Sent it into collections. Which I JUST found out yesterday that they put interest onto the bill. >_>

How should I fix this?
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If it was a doctor's recommendation then you need proof of that, so the insurance company doesn't think that you're just getting a bunch of unneeded work done.
Call the collections agency or the testing facility that charged you and ask to speak to a manager. Tell them what has happened, you gave them your correct insurance and they didn't apply it.

Your doctor should have NEVER advised you it was free! She doesn't know all the stipulations to insurance companies. Everything is so individualized.
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I would first call your doctor to ask why your "free" test cost $300. Also ask them why they sent the bill to the wrong insurance company. They may be able to fix it on their own if they're nice. If not, then call your insurance company. Explain the situation. They may pay the other insurance company or they may have you pay it then reimburse you. Just take care of it quickly before it affects your credit too much.
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Your doctor recommended tests for STDs that you knew you didn't need? Something doesn't sound right. Why would anyone undergo tests for STDs when they knew they were unnecessary?

Bottom line is, you had the tests and you owe the money. Get your insurance company involved. You may have to take care of paying the collector -- negotiate a pay for delete so that the derogatory gets removed from your credit report. Then get reimbursed from your insurance company.
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Call the insurance company & tell them what's going on... And/or get a lawyer... AND get a new doctor.

*glares at your annoying incompetant doctor's office*
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Take care of your own business. Never assume someone else is doing things for you.