A sieve is health care professional who admits every single patient, thereby screwing over his or her colleagues.
From Chapter 10, The House of God
I began to work through the multiglomerate experienced ill. At one point, as I was buffing a gomer, I felt a tap on the back of my leg, low down. I turned and saw Chuck and the Runt, kneeling on the tile floor, looking up at me like cocker spaniel pups in the window of a pet shop. The Fat Man stool behind them.
"Don't tell me," I said, "let me guess what you're on."
They told me anyway. They were on their knees.
"Man, do you know why?" asked Chuck.
"Because the last twelve weeks," said the Runt, "Howard has been in the E.W. and he's so scared of missing something by sending the patient back home that he admits them all. He's a sieve."
"A sieve?" I asked.
"Right," said Fats, "he lets everyone through. At Bellevue half the ones Howie admitted would have been turfed out by the receptionist. Or they would have been too embarrassed to come in. New Yorkers have some pride, especially when it comes to degradation. Howie's been letting through six admissions per tern per day. These poor boys are on their knees. They were your friends, remember?"
"They still are," I said. "What can I do?"
"Man," said Chuck, "be a wall. Don't let anyone in."
"In New York once," said Fats, "we had a contest to see how long the medical service could go without an admission. Thirty-seven hours. You shoulda seen what we sent outta here. Roy, help them. Be a wall."
"You can count on me," I said, and watched them leave.