One in 10 NHS patients comes to harm while in hospital as a result of their clinical care, suggests a study in Quality and Safety in Health Care.
The findings are based on a review of the case notes of a random sample of just over 1000 patients admitted to one large teaching hospital in the north of England during the first six months of 2004.
The findings are likely to be typical of other similar facilities, however, say the authors.
The researchers used a six point scale to find out just how strongly an incident directly caused harm and how easily this could have been prevented in eight specialties. A score of 4 or more was a higher filtering score and a score of 2 was a lower filtering score, so giving a higher figure for what might have been prevented.
These were: surgery; urology; orthopaedics; general medicine; medicine for the elderly; cancer; ear nose and throat problems; and eye disease.
Surgical patients were more likely to come to harm, but these incidents were less preventable. Diagnostic errors, on the other hand, were less common, but more preventable, the findings showed.
Incidents causing harm lengthened hospital stay by an average of eight days
A score of 4 or more showed that almost one in 10 admissions (8.7%) involved at least one of these incidents, of which almost one in three (31%) could have been prevented.
A score of 2 or more suggested that over half might have been prevented.
By far the largest category of problematic incidents involved an unplanned admission or readmission as a result of previous treatment at the hospital.
Injuries sustained while in hospital, such as falls, burns, or pressure sores, and other complications, such as a heart attack or deep vein thrombosis were the next most common categories.
One in seven incidents (15%) were sufficient to interfere with daily life or cause temporary disability for six months or more.
One in 10 caused permanent disability and a further one in 10 contributed to the patient's death.
"In the light of the findings from this study and [previous UK research], it is now clear that 8 to 10% of patients in NHS hospitals may experience some kind of adverse events," say the authors.
"..between 30% and 55% are to some extent preventable," they add.