Every year, an estimated 4,000 cases of “retained surgical items,” as they are known in the medical world, are reported in the United States. These are items left in the patient’s body after surgery, and the vast majority are gauzelike sponges used to soak up blood. During a long operation, doctors may stuff dozens of them inside a patient to control bleeding.

Though no two cases are the same, the core of the problem, experts say, is that surgical teams rely on an old-fashioned method to avoid leaving sponges in patients. In most operating rooms, a nurse keeps a manual count of the sponges a surgeon uses in a procedure. But in that busy and sometimes chaotic environment, miscounts occur, and every so often a sponge ends up on the wrong side of the stitches.