An investigation is underway after a North West hospital recorded four times as many cases of septicaemia as expected.
Bolton NHS Foundation Trust's acting chief executive, Dr Jackie Bene, has stepped aside after an unusually high number of patients had been "coded" as having had blood poisoning in a year.
Septicaemia, a severe infection that enters the blood stream, 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.
In the wake of the findings, NHS medical director Professor Sir Bruce Keogh called on all hospitals to "behave openly and honestly about their performance" and said they would be "monitoring the situation".
The Trust recorded more than 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.
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 blood stream.
"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 Foundation Trust in January.
BCCG is now working with the trust to further investigate the interim findings "urgently", Dr Bhatiani said.
He added: "We must establish the facts and then we will be able to provide more information.
"When the final report is available on March 6, we will present the findings openly and transparently, reflecting the fact that the Bolton public are our main concern and priority."
In a statement the trust said: "We have received early information from an initial audit about potential discrepancies in the way the trust coded patient mortality information in 50 cases from between April 2011 and March 2012.
"The audit has not reported but we have decided to act on this information because we want to rule out any serious issues as soon as possible.
"As a precaution, the 50 cases will be reviewed by clinical staff and if any concerns arise with the care provided in them we will directly contact their families.
"When the full audit report is available to us in the coming weeks we will be able to have a clearer understanding of specific issues that need addressing in how we code patient information.
"But we want to ensure that they way the trust now codes patient outcomes meets the highest quality standards."
To achieve this the trust said: it will work with the BCCG to jointly review all the relevant information and with the trust's regulator Monitor; bring in an independent team to look at its current coding practices; extend the existing work that Deloitte - which is currently part of a #1 million financial turnaround plan at the trust - is doing on "quality governance".
The statement continues: "While this work is taking place, Dr Jackie Bene who as a trust consultant and medical director oversaw coding, has agreed to step aside as acting chief executive, to allow a fully independent view to be taken.
"We do not believe that there are any clinical concerns regarding the care of patients, but rather there are questions that need answering about how the trust reports information about their care for administrative and financial purposes.
"The steps we have taken will provide the trust and our community with information about what has happened, any steps that need to be taken and any lessons that may need to be learned."
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 (Care Quality Commission) 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."