Thank you for your response.
I was afraid of just what you replied. Unfortunately, the doctor's office can only speak to the charges from the doctor's office. The doctor's office cannot legally make any agreements for payments to outside vendors, including the lab. Pursuant to your agreement with the doctor, if the doctor is not charging you any more than the $900, then they are not in breach of their contract with you and you would be obligated to their contract.
As far as the lab, if they did not give you any price on their independent services, you would still be liable to pay them, but you certainly have grounds to negotiate those payments with them and you should have offered them a settlement of lesser amount than they were seeking. Generally, in these medical cases they will agree to reduce charges by upwards of 50% in most all cases. Unfortunately, from what you are stating it does not sound like you did this and you already paid them most everything you owe.
Sadly, you cannot sue the doctor's office for something a third party has charged you for independent services, especially since you did not have any written contract for any of this with the doctor or lab. I am afraid that your issue here is legally with the lab, who did not give you any price up front and they are the one you can negotiate with or could have negotiated with, but not the doctor as they did not have any control over what the lab charged you.
As far as the doctor stopping seeing you, legally a doctor can refuse to see any patient, but in your case you have grounds to argue that they are breaching their contract with you to see you for $900 and as they are in breach of contract
you could sue them to try to recover any portion of money that they owe you for stopping your treatment before you were completed with the treatment they promised and you can take that suit to small claims.
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