Patients undergoing operations in Northern Ireland were not assessed for their risk of developing hypothermia, a report said.
Although they were routinely warmed if their time in theatre was expected to exceed 30 minutes and staff were aware of their responsibilities, there was tno formal guidance on defining those who were most vulnerable, inspectors said.
The Regulation and Quality Improvement Authority (RQIA) called for full compliance with international standards for surgical safety.
Chief executive Glenn Houston said: "Inspectors found that while baseline assessments had been undertaken, no trust had a formal definition of a high-risk patient, or a formal risk assessment to identify a patient's risk of developing hypothermia.
"Theatre care pathways or documentation do not record whether a patient is considered to have an increased risk of developing hypothermia."
The review team found that significant work was required in all trusts to ensure this clinical guideline was fully implemented, Mr Houston said.
The report recorded that no health trust had an overarching policy to prevent hypothermia, although the Northern Trust had a protocol to guide staff.
Various actions had been taken by trusts in response to a Department of Health notice on the issue.
The document added: "However, any improvements put in place had not ensured that this guideline had been fully implemented."
Most patients were taken to theatre in a bed but occasionally walked as day cases.
Those observed by inspectors were kept warm throughout their journey, in theatre and on transfer back to the ward. During inspections, most staff appeared to be aware of their role and responsibility in the prevention of hypothermia.
The review team also noted good practice in relation to regional healthcare hygiene and cleanliness standards but said all hospitals need to ensure that all staff adhered to their trust's policy surrounding zero tolerance of health care associated infection.
Staff stated that they knew which patients were at risk of hypothermia and the type of patient that would require a warming device.
Notes reviewed indicated that the most commonly used grading system to predict morbidity and mortality was not always recorded by anaesthetists, the report said.
Documentation did not include a designated area to record if the patient was considered to be a high-risk of developing hypothermia.
The anaesthetist identifies which patients are high-risk, and the need for warming devices when a procedure is less than 30 minutes.
Thirteen recommendations were made including that consent forms filled out by patients before operations should be fully completed and legible.
Inspectors also said all trusts should ensure the World Health Organisation (WHO) Surgical Safety Checklist should be consistently applied with full engagement, participation and completion by the relevant professional staff.