Cannock Hospital is run by the trust

Data showed death rates from the Mid Staffordshire NHS Foundation Trust were "out of normal range".

The investigation will also look at the quality of care provided across the trust, in particular to older people, after patients raised concerns.

The trust said it believed its rates were normal for a trust of its size.

'Improve data'

The Healthcare Commission's probe will apparently focus on patients admitted as emergencies.

Figures showed the trust's standardised mortality rate (SMR) was 127 in 2005/06. Nationally, it is set at 100.

The trust said its SMR was down to 101 between May and October 2007, with an SMR for emergency admissions over that time period of 100.4.

"We worked with the Strategic Health Authority and investigated this apparently high mortality rate and concluded that it was due to problems in the way we were recording and coding information about patients," Martin Yeates, the trust's chief executive, said.

The figures at Mid Staffordshire Foundation Trust are out of normal range which is why we are carrying out this investigation to get a clear picture of what is going on Nigel Ellis, Healthcare Commission

"We have, over the last year, employed more clinical coding experts to work within the various specialty departments to help staff to record information and improve the quality of data."

But the commission said it would investigate after an alert system suggested a higher than normal death rate among the data.

Nigel Ellis, the commission's head of investigations, said: "The figures at Mid Staffordshire Foundation Trust are out of normal range which is why we are carrying out this investigation to get a clear picture of what is going on."

But he added that the apparently high mortality rates did not necessarily highlight safety concerns. It could be the way information was recorded.

He reassured patients that action would have already been taken if they thought the trust was unsafe.

Health check

The inquiry will also look at protocols for protecting the safety of patients at the two hospitals managed by the trust - Cannock Chase and Stafford.

Mid Staffordshire became a foundation trust in February and serves about 300,000 people.

It scored "fair" on quality of services in the commission's annual health check last year.

A helpline has been set up for anyone with concerns.