A copay is a flat fee which is paid to the doctor's office, usually upon arrival.  Frequently, patients have one copay for the primary care doctor, and a higher copay for the specialist's office.  The copay offsets the reimbursement which the physician's office receives.  For example, if the insurance company allows $100 for the particular visit, and the patient's copay is $20, then the physician will receive $80 from the insurance company.

A co-insurance is a percentage of the total allowed amount.  Someone who has 90%/10% co-insurance will be responsible for 10% of the total allowed amount.  If the allowed amount is $100, then the patient is responsible for 10% of this, $10, and the insurance company will reimburse the physician's office $90.00.

A deductible is the amount for which the patient is responsible PRIOR to the health insurance company reimbursing.  If a patient has a $250.00 deductible, then the first $250.00 in the calendar year of doctor's visits will be the responsibility of the patient to pay.  So if the allowed fee for the doctor's visit is $200, the doctor's office will submit the claim to the insurnace company, so that the insurance company will know about the visit and the fee.  Then the insurance company will apply that amount to the deductible and the patient will be responsible for remitting the $200 to the doctor's office.

If a patient has a $200 deductible, a $20 copay, and a 20% co-insurance, the situation will look like the following example:

The allowed fee for the visit is $300.  The patient will be responsible for the first $200 of the $300, leaving a $100 balance.  They will also be responsible for the $20 copay, leaving an $80 balance.  The coinsurance of 20% is on the total allowed amount, so 20% of $300 is $60.  The patient will owe $60 of the $80, and that leaves a $20 balance for the insurance company to pay.  The patient has paid a total of $280.

Once the deductible is met however, this same patient on their next visit would only pay the $20 copay, and the 20% co-insurance.