You're right. Having Medicare Part B with an employer group during the first 30 months eliminates the out-of-pocket costs the employer plan requires people to pay since the employer plan is billed more and pays more than Medicare. Although DaVita billed your plan weekly, I believe dialysis clinics bill Medicare claims monthly after the month of service is ended. Medicare is required to wait a certain amount of time before paying any claim. For "clean" electronic claims (those with nothing unusual about them), Medicare waits 13 days (26 days for paper claims). Medicare is required to pay "clean" claims within 30 days. Medicare routinely covers 3 HD treatments a week. If you're doing daily dialysis and receiving more than 3 treatments a week, Medicare requires your physician to submit medical justification with the claim for additional treatments. That may be delaying claim filing and/or payment. Here's the manual chapter that describes Medicare's claim payment process (not ESRD specific).
Here's the Medicare Claims Processing Manual that discusses dialysis billing:
The Medicare allowable for HD is around $250/treatment and is a consolidated or bundled payment that includes several drugs, supplies, and labs that may have been billed separately to your employer plan. The 20% should be around $50/treatment. I have heard that DaVita and other dialysis providers bill a commercial rate to employer plans that is significantly higher than $250/treatment. Some Medicare Administrative Contractors (insurance companies that pay Medicare claims) pay for more than the 3 treatments a week Medicare routinely allows when medically justified. If Medicare doesn't cover and pay for all the treatments you get, DaVita may bill your employer plan at its commercial rate for treatments Medicare doesn't cover. This may take longer for DaVita to get paid as much as it will by Medicare and your employer plan. Here's information on "consolidated billing" (what's included in the Medicare bundle of services for dialysis):
Review your employer plan policy and talk with your plan about how it coordinates with Medicare as a secondary payer and if the deductible is part of the out-of-pocket, which I suspect it is. If your plan has a $2000 deductible before claims are paid, DaVita would bill you the $50/treatment until your deductible is met (plus any amount owed for extra treatments). After you have met the deductible, you would owe the 15% coinsurance/copay and your plan would pay the remaining 5% until you have met the $4000 out-of-pocket. Other medical bills would be applied to your out-of-pocket maximum too. Therefore, you should meet your out-of-pocket maximum this year. Once you do, your employer plan will pay 100%.
The best people to advise you about the status of Medicare and insurance claims and what you owe are DaVita's billing personnel. They should be able to tell you information about your claim specifically, including when they filed the Medicare claim and how long it typically takes for Medicare to pay claims, if they've been paid by Medicare for January, if they've filed a claim with your employer plan for January. Ask if they expect to get payment from Medicare for all the treatments you do or if they expect to have to bill your employer plan for any treatments over 3 a week. This should help you know what to expect and whether the amount of the bills they expect you to get will present a financial hardship to you. If so, ask if you can make payments over time. DaVita and other dialysis providers have a form patients can complete to determine if any balances can be written off based on family income.
One thing that might make this easier to take is that your employer plan will pay significantly less this year than it did in previous years. This may help keep your employer's premium costs lower, a factor that your employer may consider in raises, promotions, and other benefits.