A report into how a Somerset care home manager was able to kill a resident has said it would have been very difficult to stop it happening even if extra checks had been in place.
Nurse Rachel Baker was jailed for 10 years for the manslaughter of 97-year-old resident Lucy Cox last year.
The nurse, who worked at Parkfields Residential home in Butleigh, siphoned off drugs meant for her patients.
A serious case review said it was difficult to prevent criminal acts.
Baker, 49, of Boundary Way, Glastonbury, was convicted at Bristol Crown Court.
She was cleared of two counts of murder and acquitted of the manslaughter of another resident, Frances Hay, 85.
Baker admitted 10 counts of possessing Class A and C drugs and one count of perverting the course of justice.
The court was told Baker gave Mrs Cox lethal doses of medication at the care home.
She started abusing opiates in 2005 and was only found out more than a year later when one of the home's carers, Sarah Barnett, raised concerns and told the authorities.
The review concluded that:
- It would always be very difficult for regulations to guard against the actions of a trusted professional concealing a serious criminal act. Rachel Baker was "determined and adept at covering up her actions and went to great lengths to do this"
- Even the best systems would have limitations in picking up criminal behaviour - they are designed to identify incompetence and poor practice not criminal activity
- People working in the care home system need to have a "respectful uncertainty", and should not be scared to report suspicions
- There was a conflict of interest between Baker and the residents having the same GP, this should have been identified by the GP practice
- Unusual patterns of prescribing were not picked up. High levels of prescriptions were queried by some health professionals - but there was no way to bring these together.
'Suspend professional respect'
The review was voluntarily commissioned by the Somerset Safeguarding Adults Board, with input from NHS Somerset, the Care Quality Commission and Somerset County Council.
It was independently chaired by Margaret Sheather, the former group director of Community and Adult Care at Gloucestershire County Council.
She said: "This review did not set out to apportion blame. Its purpose was to establish if there are lessons to be learnt from these very sad events that could help improve the way professionals and agencies work together.
"Where areas of improvement have been identified they have either been addressed or will be addressed in the actions plans.
"Unfortunately it will always be very difficult for any form of regulation to guard against the actions of a trusted professional who has set out to commit and conceal a very serious criminal act.
"However, the improvements already made and the actions in this report should lessen the chance of such actions remaining undetected."
She added it was important for whistle blowers to come forward but said it took a lot for colleagues to "suspend that sense of respect and say 'actually, I don't think this is OK and I'm going to pursue it'."
NHS Somerset said in 2007 it had appointed a senior pharmacist to monitor patterns of prescribing by GPs.
This was a national requirement after Harold Shipman, the serial killer GP, rather than as a direct result of Baker, so these patterns would be picked up now.