an in-network doctor sent my sample to a non-network lab and insurance company does not pay the bill

I went to an in-network doctor and did my women's well exam. The doctor office sent my pap smear sample to a non-network lab (Quest Diagnostic). Now the insurance company (United Healthcare) does not pay the lab bills.  The lab bills should be covered 100%  if they were done in-network lab.  I already  appealed twice, both appeals were denied by the insurance company.  What should I do next?

Thanks