An investigation into death rates at a hospital has been launched after an independent watchdog discovered an "unusually high number" of deaths due to septicaemia, it has emerged.

Bolton NHS Foundation Trust's Acting Chief Executive, Dr Jackie Bene, has "stepped aside" following the discovery that an unusually high number of patients had been "coded" as having had septicaemia in a year.

Septicaemia, a severe infection that enters the bloodstream, is coded, or tagged, in a different way to other illnesses and infections, which means deaths caused by septicaemia do not affect hospital mortality figures.

According to a report in The Bolton News, the Trust recorded 800 cases of septicaemia between March 2011 and April 2012 - a similar sized trust would expect to have just 200. Dr Bene, as former medical director, was in charge of recording the information.

The independent watchdog which is auditing the data is understood to be investigating 200 cases, The Bolton News reported. Its interim findings for 50 of the cases show "cause for concern".

Dr Wirin Bhatiani, chairman of the Bolton Clinical Commissioning Group (BCCG) said in a statement: "In October, we became aware of an unusually high number of patient episodes at Bolton NHS Foundation Trust that were coded as being due to septicaemia - a severe infection that enters the bloodstream. As a result, we spoke to the medical director at the trust, and agreed to look into this further by commissioning a company - Dr Foster, a specialist in this area - to carry out an independent audit to look at these cases.

"Although the final report from this audit is not due to be given to us until March 6, we have received some interim findings that indicate some cause for concern. While we are keen to wait for the final report, we are sufficiently concerned by the interim findings to commence further investigations, and to inform the chair of the trust."

The initial concerns about coding and the impending Dr Foster audit were discussed at a "quality risk summit" involving the commissioners and regulators of Bolton NHS FT in January. BCCG is now working with the Trust to further investigate the interim findings "urgently", said Dr Bhatiani.

NHS Medical Director Professor Sir Bruce Keogh said: "If we're to have an open and accountable NHS, where patients and the public know how NHS hospitals are doing, those hospitals must behave openly and honestly about their performance.

"These allegations have been picked up by the local NHS working together and scrutinising what has been going on - with support from the CQC and Monitor. As yet, there is no evidence that any patient has been harmed. But we will be monitoring the situation and learning any lessons. There are clear national guidelines on how death rates should be recorded, and I expect all NHS hospitals to follow them."