A good friend has started telling her husband, “don’t let them take me.”

She’s not a drug dealer or a tax cheat terrified of the cops finding her, but someone who has seen both her parents enter hospitals for minor problems . . . and leave dead. The “they” she doesn’t want are EMTs who might load her into an ambulance and take her to the nearest ER (unless it’s after a car crash or other trauma; she’s not completely crazy).

So I’m not going to tell her that HealthGrades, a private company in Golden, Col., that rates hospitals, nursing homes and physicians found that from 2004 to 2006, medical errors at U.S. hospitals killed 270,491 people. And that’s just Medicare patients.

The company analyzed 41 million Medicare records for those years, from almost all of the nearly 5,000 non-federal hospitals. It looked for 16 kinds of medical errors, ranging from pulmonary embolism or deep vein thrombosis (clots that develop after surgery; the patient should have been monitored better and given blood thinners), post-operative sepsis (infection), and post-operative abdominal wound separation/splitting to bed sores (the nurses weren’t turning the patient enough and watching for the first signs of sores) and leaving a tool or anything else inside a surgical patient (enough said). It found 1.1 million mistakes in the three years studied. Do the math, and you’ll see that about one-fifth of the mistakes were fatal.

HealthGrades isn't unrealistic. Of course some mistakes can't be helped. Of the 270,491 deaths from these so-called “patient safety incidents,” finds HealthGrades, “238,337 were potentially preventable.” That's almost quarter-million people who might be alive today.

If you’re treated at a top hospital, your chance of falling victim to a medical error is 43 percent less than if you’re at a poor hospital, HealthGrades found. “While many U.S. hospitals have taken extensive action to prevent medical errors, the prevalence of likely preventable patient safety incidents is taking a costly toll,” Dr. Samantha Collier, HealthGrades' chief medical officer and the primary author of the study, said in a statement. “HealthGrades has documented in numerous studies the significant and largely unchanging gap between top- performing and poor-performing hospitals.”

The company has posted the results, so you can find the hospital where your friendly local EMT guys are likely to take you.

What can patients, or their families, do? Assume nothing. Ask the nurses if your bedridden father has been turned, so he doesn’t develop bed sores. Ask if your hospitalized mother has been checked for blood clots in her legs. Find the doctor who’s overseeing the patient’s care, and don’t be shy about asking lots of questions and pestering him or her. If the patient is basically immobile—a good way to develop clots—where’s the physical therapist to have him do exercises sitting or lying down? See that little dispenser of hand sanitizer by the door? Don’t let anyone touch the patient without first using it. As for what you can do about forceps left in mom’s abdomen during surgery, against some stupidity even the gods are helpless.