Nadege Neim won a $1.4 million verdict last year after suing her Ellicott City obstetrician for removing a healthy ovary and fallopian tube from her right side when she went into the hospital for surgery to have a cyst excised from her left.

A few years earlier, an unnamed man in his 50s sought treatment for pneumonia at a Maryland hospital and ended up losing both legs. No one properly assessed him, and scans that might have found the blood vessel blockage were delayed for nearly two days in a "cascade of poor decisions," state regulators said in an investigative report obtained by The Baltimore Sun.

Such preventable medical errors kill more than 400,000 Americans each year and seriously harm at least 10 times that number, according to a recent study published in the Journal of Patient Safety.

But it's impossible to know the full scope of the problem in Maryland.

While hospitals are supposed to report serious medical errors to state regulators, the mostly confidential system still doesn't capture all of those happening in the Maryland facilities, patient safety experts and regulators acknowledge. Confusion over reporting rules and fear of legal or financial repercussions can thwart disclosure, they say.

Details about even the most severe and deadly mistakes, called "adverse events," only become public if someone sues, or if regulators catch a hospital failing to report and launch an inquiry, the results of which are subject to open records laws.

"Are they grossly underreported? Shamefully, no one knows," said Dr. Peter J. Pronovost, a leading patient safety expert at Johns Hopkins Hospital. "If you added up all the adverse events in hospitals, they would probably be about the third leading cause of death. The public should be screaming that we deserve better."

As patients increasingly research their options for health care, consumer advocates are pushing for more transparency so patients can use information about medical errors when picking providers. Only Minnesota, New York and five other states have passed laws requiring that hospitals publicly report mistakes.

What information patients can find on medical errors at hospitals "is sorely lacking, unvalidated and without much meaning to the general public," said Michael Bennett, who became a patient safety advocate after his 88-year-old father's death. Six different bacteria infected his father's blood and organs and destroyed a leg while he was being treated for a respiratory virus in 2004.

According to court records, Bennett, of Pikesville, unsuccessfully sued Sinai and Northwest hospitals and two doctors alleging wrongful death. The hospitals said at the time that they wouldn't comment on litigation, but that they were taking various steps to prevent infections.

About the same time, the state began requiring its 66 acute care and specialty hospitals to report the most serious adverse events, such as wrong-site surgeries, falls, infections and advanced bedsores, to investigators at the Office of Health Care Quality, which licenses medical facilities.

The latest report from the office — which has drawn criticism after a disabled 10-year-old died at a group home it oversees — includes some information about egregious cases, including four patients oversedated to death, six who died when they fell and hit their heads and three premature infants fatally infected by catheters in fiscal 2013.

But it does not say where those cases occurred; that information is considered confidential. Regulators say they use the information to privately suggest improvements to hospitals and make overarching recommendations for all medical institutions.

And even the data that is collected may be flawed, because some kinds of mistakes appear to be under-reported.

For instance, Maryland hospitals reported that none of the millions of patients treated from 2004 to 2006 suffered from advanced pressure ulcers, or bedsores, an astonishing statistic given that bedsores are a common ailment caused by lying down too long.

After a push by state and hospital officials to improve reporting, the number increased to 144 in fiscal 2011 before falling to 52 in fiscal 2013.

But national statistics suggest that even those figures are low. Federal data show that, on average, about 0.6 percent of patients get bedsores that progress to the advanced state — a rate that would translate to more than 4,000 cases a year in Maryland.

Similarly, the number of bloodstream infections from central lines, or catheters, as reported to the Office of Health Care Quality appears low. Such infections have drawn special scrutiny in Maryland and nationally because they are preventable and deadly in up to half of cases.

The office received only nine reports in the last fiscal year of serious or deadly hospital-acquired infections of all kinds, a category that would include the bloodstream infections.

But other agencies found higher numbers. The Maryland Health Care Commission, another regulator, found 206 such bloodstream infections in hospitals in fiscal 2012, the last year for which data is available. And the Health Services Cost Review Commission, which sets hospital rates and penalizes those where complication rates are high, found 400 such infections in fiscal 2012 and 287 in fiscal 2013.