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.
The researchers found clots varied widely between trusts, with the highest rates over 3.5 times greater than the lowest.
The guide found 19 of the 147 hospital trusts in England had mortality rates which were significantly higher than expected, down from 27 last year, while 26 trusts have mortality rates that are significantly low, down from 32 a year ago.
The deaths after surgery, or "failure to rescue", looked at patients who had a secondary diagnosis such as internal bleeding, pneumonia or a blood clot, and later died.
In many cases patients will have developed the condition as a result of surgery.
In good news, the number of England's 147 trusts reported to have high hospital standardised mortality rates (HSMRs) fell from 27 to 19, with the gaps between hospitals with the highest and lowest rates narrowing.
The number of people dying in hospital fell by 7% between 2008-09 and 2009-10 and the reporting of errors seems to have improved.
Of the 19 trusts with high HSMRs Royal Bolton Hospital and Pennine Acute Hospitals have been high for six years.
While University College London Hospitals and Royal Free Hampstead had 28% lower than expected mortality ratios, Buckinghamshire Hospitals was 18% higher than expected.
Newcastle upon Tyne Hospitals, University Hospital of North Staffordshire, University Hospitals Birmingham (UHB) and Hull and East Yorkshire, had significantly high "death after surgery" rates, with the latter two trusts suffering high rates of both HSMRs and deaths after surgery.
But Dr Dave Rosser, executive medical director of University Hospitals Birmingham NHS Foundation Trust, said: "The figures do not accurately reflect the quality of care given at the Queen Elizabeth Hospital Birmingham.
"We believe there is a risk that figures like these could cause unnecessary confusion and distress amongst patients and are a distraction to concentrating on providing the best in care for our patients."
He described the methodology as "fundamentally flawed and misleading" as well as "destructive and unhelpful" and said the figures may have been skewed by the high number of patients with liver disease.
He said: "The Care Quality Commission, which regulates clinical standards across NHS hospitals, is also satisfied that there are no causes for concern regarding our mortality rates or deaths after surgery."
Health Secretary Andrew Lansley welcomed the report, saying: "I have been clear that unsafe care will not be tolerated. Patients have a right to expect the very best care from the NHS and when something goes wrong, hospitals have a duty to report it and make sure that others can learn from their mistake.
"We have already taken action to improve safety and openness in the NHS - publishing more information and statistics for all to see, extending the list of mistakes the NHS will not be paid for and strengthening rights for whistleblowers."
Nigel Edwards, acting chief executive of the NHS Confederation, said: "The concerns Dr Foster raises over the way information is recorded and interpreted in the health service are very important.
"If we are going to manage and measure our health service using data on the outcomes of procedures and the success of treatments then we need to ensure that the collection of usable data is a priority and embedded in the culture of the health service."
Patients Association chief executive Katherine Murphy said: "Safety is still clearly not the top priority of every NHS organisation and it absolutely should be.
"We cannot continue with a situation where patients are stuck with poor healthcare because of where they live."Reuse content