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.