Death rates at 19 NHS hospital trusts in England were alarmingly high last year, according to an influential report.
Two of these trusts - Hull and East Yorkshire Hospitals NHS Trust and University Hospitals Birmingham NHS Foundation Trust - as well as two others also recorded a high number of deaths after surgery, the Dr Foster Hospital Guide found.
The measures are intended to act as a "smoke alarm" for where problems with care might exist.
Hospitals recorded 62,800 adverse medical events, with 30,500 patients developing avoidable blood clots, more than 13,000 mothers suffering obstetric tears during childbirth, nearly 10,000 accidental lacerations or puncture, about 6,000 patients with pressure sores, more than 2,000 post-operative haemorrhages and 1,300 cases of post-operative blood poisoning.
Trusts also reported 56 incidents of "wrong site" surgery and 150 "foreign objects" left inside patients after an operation.
Researchers said many trusts were not accurately recording incidents of harm to patients, making it harder to prevent similar occurrences in the future.
Roger Taylor, director of research at Dr Foster said: "It is concerning that no hospital in the country can accurately assess the level of adverse events compared with the best achievable rates.
"Blood clots kill more people than superbugs every year - and yet the reporting of blood clots is just not always sufficient to identify and address the problem.
"Dr Foster is asking the Department of Health to review the way this information is recorded and we hope to revisit this topic next year and be able to identify trusts and their rates."
The risk of patients developing a blood clot is increased by most surgical and some medical treatments and conditions. But hospitals can take measures to reduce the risk.
