Medicare is the "Primary" insurer for people who already have Medicare - due to age - who then need dialysis. However, half the people who start dialysis each year (about 40,000 of the 80,000ish people who start) are under 63 years of age. For these people, who are not otherwise eligible for Medicare, other insurance is "Primary" for 33 months after starting dialysis: the MSP period (a thirty month clock starts after three months).
Other insurance can be Employer Group Health Plans, Medicaid or other state plans but for someone who is under the age threshold their dialysis is not funded by Medicare until well into the third year of treatment . (Under the so called MSP Extension in the President's 2009 budget the period before Medicare kicks in would be extended to 60 months.)
Medicare pays a set rate for three dialysis treatments a week (a clear dose restriction, which leads to unnecessary illness and death). This set rate, the Composite Rate (CR), is case mixed to an extent based on age, BMI and other clinical factors. This is why the reimbursement rate varies among patients at the same unit. There is also a geographic wage adjustment, which is why the CR also varies by location. That said Medicare's allowed CR for one hemodialysis treatment is about $155 (+/-$30) PDF link.
As with all allowed charges Medicare pays 80% of this CR. The 20% is picked up by secondary insurance, self-paid or written off. In my case the "Secondary" is the Carpenters Union but it could be Medicaid or a Medicare supplemental plan. Private payers who are "Primary" are charged and generally pay much more. This is the subject of lawsuits and state legislation in Indiana and Kentucky but it has been revealed that private payers are charged on the order of 20 time more than what Medicare allows.
Here then is the source of dialysis industry profits - those few diayzors in the first three years of treatment, who have private insurance. The provision of dialysis in the US depends disproportionately on these revenues from private payers. Medicare is not paying its own way (as required by law) in providing dialysis to Medicare beneficiaries.