In June the CQC launched an investigation into the provision of out-of-hours services by Take Care Now, the company providing evening and weekend cover in East Cambridge and Fenland when Mr Gray died after being accidentally given 10 times the usual painkiller dose.
Dr Daniel Ubani, who was employed by Take Care Now through a locum agency to cover some out-of-hours shifts, was given a nine-month suspended sentence by German courts for causing death by negligence.
After visits to five PCTs which commission out-of-hours care from Take Care Now, the watchdog found trusts were failing to consistently and routinely look at the quality of care provided.
Christine Braithwaite from the Care Quality Commission says primary care trusts must ensure the quality of our-of-hours care
And variations between trusts in how the services are monitored suggest it could be a nationwide problem, it said.
PCTs must look far more closely at details like the quality of clinical decisions, the efficiency of call handling, the number of unfilled shifts and the proportion of shifts covered by non-local doctors, the watchdog warned.
'Scratching the surface'
CQC chief executive Cynthia Bower said: "Our visits to the five trusts that commission Take Care Now's services showed they are only scratching the surface in terms of how they are routinely monitoring the quality of out-of-hours services.
"Although we are still in the early stages of our enquiries, we believe this may point towards a national problem."
Take Care Now has since withdrawn 100mg doses of diamorphine to prevent the same mistake happening again, the CQC found.
But it warned there were still issues with staffing problems and medicines management procedures.
David Cocks, chief executive of Take Care Now, said he welcomed the interim progress report and it would continue to work with the CQC in its investigation.
It is not the first time concerns have been raised about evening and weekend cover.
TRUSTS REVIEWED BY CQC
Great Yarmouth and Waveney
South West Essex
In 2007, a report into the death of journalist Penny Campbell found serious flaws in the out-of-hours system, which led to her dying of multiple organ failure after consulting eight doctors over four days.
An investigation criticised the actions of one private company, Camidoc, but also concluded there were weaknesses that needed to be addressed nationally.
Health minister Mike O'Brien said: "Patient safety is paramount and PCTs have a clear legal responsibility to provide safe, high quality out-of-hours care and are required to have in place robust performance management arrangements to ensure their out-of-hours services are delivering against contractual requirements.
Health Minister Mike O'Brien: "Safety needs to be paramount"
"A failure to do so can result in investigation by the independent regulator and enforcement action by primary care trusts where providers are not meeting their contractual obligations."
Dr Laurence Buckman, chairman of the BMA's GPs Committee, said PCTs had a duty to ensure doctors were properly trained and that recruitment procedures were robust enough to ensure patient safety.
But he added many have been "too keen to cut costs at the expense of investing in a quality out-of-hours service".
Katherine Murphy, director of the Patients Association said the recommendations from the CQC were important but should not be necessary.
"Why do NHS managers need to be told that they should ensure out-of-hours care is safe?
"Where is the accountability for the managers that use taxpayers' money to employ unsuitable doctors, or pay agencies that do the same thing?"
David Stout, director of the Primary Care Trust Network, said: "All PCT boards should ensure their PCT have robust mechanisms in place to scrutinise the quality of out-of-hours care, with input from local clinicians."
The final report from the CQC is due early in 2010.
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