The cost of the extra days in hospital needed is running to at least 1.5 billion dollars a year, said the research team from University College London.

A leading doctor said that the profession was failing to "learn from its mistakes".

The study looked at more than 1,000 medical and nursing records at two hospitals in London, then expanded these results to produce national predictions.

Researchers found that almost 11% of patients had suffered some sort of adverse event - some more than one.

In approximately 50%, the researchers judged that hospital staff should have prevented them happening.

Contributing to death

A third of the events led to "moderate or greater impairment" - in 19% of cases, the decline in their health as a result was permanent, and in 6%, the adverse event contributed to death.

The main causes of adverse events relate to operative errors, drugs, medical procedures and diagnosis. Each of these is amenable to prevention.

However, he also admitted that mistakes were inevitable because there were too few medical staff, who were forced to work too quickly to cope with intense demand.

An earlier Australian study estimated that adverse events accounted for 8% of hospital bed days and cost the Australian healthcare system $4.7 billion a year.

The researchers found that 10.8% of patients admitted to hospital experience an adverse event, with an overall 11.7% rate of adverse events when multiple adverse events are included.

About half of these events were judged preventable. A third of adverse events led to moderate or greater disability or death. Some adverse events are serious and are traumatic for both staff and patients. Others are frequent, minor events that go unnoticed in routine clinical care and yet together have massive economic consequences.

This study is primarily a pilot and has certain limitations. The study was small and based on only two hospitals.

In addition, the case mix does not accurately reflect hospital practice. The specialties included in the review could have higher rates of adverse events than other specialties.

Although we cannot extrapolate with any precision, our findings strongly suggest that adverse events are a serious problem in the NHS, as they are in the United States and Australia. We estimate that around 5% of the 8.5 million patients admitted to hospitals in England and Wales each year experience preventable adverse events, leading to an additional three million bed days.

About half of the adverse events were preventable with current standards of care

British Medical Journal March 4, 2001 517; 548; 562