To answer this question, you need to be clear on where the patient responsibility originated.
If the patient responsibility or coinsurance is from Medicaid, such as a patient on a spend down or sliding scale, then you need to collect the balance from the patient. All the previous suggestions for a charity/financial aid assessment/process apply.
However, if the patient responsibility or coinsurance is from Medicare, the patient may not be responsible. You need to fully understand the Medicaid remittance.
Many states have implemented a Medicaid "cap" where the provider cannot receive more than Medicaid would have paid for the service overall. In these instances, you may not bill the patient for the difference. But if you are an institutional provider, you can claim this in your Medicare Cost Report as Medicare Bad Debt.
If the Medicaid payment was not capped, you should make sure that all the charge lines were paid and pay close attention to the reason code for any charge line that isn't paid in full by Medicaid.
Does this help?