There is no easy answer to this question. There are two large dialysis organizations (Fresenius and DaVita) in the United States that operate most of the dialysis units. Nephrologists may admit patients to either of these units if they have medical staff privileges. Each dialysis unit has one Medical Director that is paid by the unit to supervise the dialysis that is provided by that unit. In some cases, he or she may only admit to one unit that is affiliated with either Fresenius or DaVita. In some instances, your observation may be true, but in others, if the nephrologist has privileges in both types of units, he or she may admit to one or the other. In some cases, the proximity to the unit may be the deciding point, in other cases, it may be that the nephrologist only has admitting privileges in one unit.