But Ms. Brecher and several workers at other nonprofit or independent clinics told similar stories.

An administrator at an independent clinic in a Midwestern city said he had helped a handful of patients maintain their coverage through the fund after they transferred to his clinic from a large chain. He declined to be identified because, he said, he did not want to anger DaVita and Fresenius, who sometimes send him patients.

Each time, he said, the charity’s workers later demanded that the clinic make a donation that at a minimum covered the amount it had paid for the patient’s premium. If he did not pay, he said he had been told, the patient risked losing the financial help from the charity for his insurance.

The administrator said he had refused to donate to the charity. The Kidney Fund continued to help pay for the patients’ insurance, he said, but the aggressive approach angered him.

Ms. Burton said the charity never declined a patient because a clinic did not donate. But she said the Kidney Fund did not hesitate to ask clinics for donations.

“We are a charitable organization,” she said. “We fund-raise for everything that we do.”

She said nearly 40 percent of the 213 dialysis companies whose clinics had successfully helped patients apply to the fund had never donated. She would not say, though, what percentage of the 80,000 patients the fund helps annually comes from clinics that do not donate, or how many of those patients come from the biggest companies, which donate most of their revenue.

Still, some social workers say the assumption at many clinics where they work is that the aid decisions are not always based on financial need.

Jennifer Bruns, now a social worker at the St. John Transplant Specialty Center in Detroit, worked for years in dialysis clinics and said she had many clients who received assistance from the American Kidney Fund. She said sometimes patients would tell her that their insurance premiums — which the Kidney Fund had agreed to pay — had not been paid that month.

Ms. Bruns called the fund to find out why, she said in an interview, “and they would say, ‘Well you haven’t made your contribution this month.’”