Safe HbA1c levels for surgery are a critical consideration for anyone with diabetes undergoing a planned procedure. HbA1c — a blood test reflecting average glucose control over two to three months — is used by surgical and anaesthetic teams across the UK to assess perioperative risk. Poorly controlled diabetes significantly increases the likelihood of wound infections, delayed healing, and cardiovascular complications. This article explains the recommended HbA1c thresholds for elective surgery, what UK guidelines advise, how to improve your levels before an operation, and how blood glucose is managed throughout the perioperative period.

What HbA1c Level Is Considered Safe Before Surgery?

An HbA1c of 69 mmol/mol (8.5%) or below is the widely accepted UK threshold for elective surgery, though many centres prefer 53 mmol/mol (7%) or lower, particularly for major procedures.

HbA1c (glycated haemoglobin) is a blood test that reflects your average blood glucose levels over the preceding two to three months. It is expressed as a percentage or in millimoles per mole (mmol/mol), and it serves as one of the most important markers used by surgical and anaesthetic teams when assessing a patient's fitness for an operation.

For elective (planned) surgery, the widely accepted threshold in UK practice — as set out in guidance from the Centre for Perioperative Care (CPOC) and the Joint British Diabetes Societies for Inpatient Care (JBDS-IP) — is an HbA1c of 69 mmol/mol (8.5%) or below. Many surgical centres and anaesthetists prefer levels closer to 53 mmol/mol (7%) or lower, particularly for major procedures, as lower HbA1c is associated with a reduced risk of complications. The strength of this association varies by procedure type and individual patient factors, and specific thresholds may differ according to local policy. Relevant considerations include:

• The type and urgency of surgery

• The patient's age and overall health

• How long the person has had diabetes

• The presence of other conditions such as cardiovascular disease or renal impairment

Elevated HbA1c before surgery is associated with a higher risk of surgical site infections, poor wound healing, prolonged hospital stays, and cardiovascular events. Persistently high blood glucose can impair immune function, reduce collagen synthesis, and compromise microvascular circulation — all of which are important to recovery.

Important caveat: HbA1c results can be unreliable in certain situations, including haemoglobinopathies (such as sickle cell trait), significant anaemia, chronic kidney disease, or following a recent blood transfusion. In these circumstances, your clinical team may use alternative measures of glycaemic control, such as recent self-monitored blood glucose (SMBG) or continuous glucose monitoring (CGM) trends, or fructosamine testing.

It is also important to note that HbA1c alone does not capture short-term glucose fluctuations. Patients with well-controlled diabetes may still experience perioperative hyperglycaemia, which is why blood glucose monitoring before, during, and after surgery remains essential regardless of HbA1c values.

NICE and UK Guidelines on Diabetes and Preoperative Assessment

CPOC/JBDS-IP guidance recommends HbA1c below 69 mmol/mol (8.5%) before elective surgery, with deferral advised if this threshold is exceeded and optimisation is clinically feasible.

Several UK bodies have issued guidance emphasising the importance of optimising glycaemic control before elective surgery. The primary UK standard is the CPOC guideline: Perioperative Care for People with Diabetes (2021, with updates), developed in collaboration with the JBDS-IP and endorsed by the Association of Anaesthetists. This is supported by NICE NG45 (Routine Preoperative Tests for Elective Surgery) and NICE NG180 (Perioperative Care in Adults), which provide the broader framework for preoperative assessment and perioperative management.

Key recommendations from CPOC/JBDS-IP and NICE guidance include:

• HbA1c should ideally be below 69 mmol/mol (8.5%) before elective surgery proceeds — this threshold is set by CPOC/JBDS-IP guidance, not NICE directly

• HbA1c should have been measured within the preceding three months; if not, testing should be arranged promptly

• If HbA1c exceeds this threshold, surgery should be deferred where clinically safe to allow time for glucose optimisation

• Patients with diabetes should be prioritised first on the morning operating list to minimise fasting time and reduce the risk of hypoglycaemia or hyperglycaemia

• A structured diabetes perioperative care plan should be in place, involving the diabetes team, surgeon, and anaesthetist

The Association of Anaesthetists and JBDS-IP have also published perioperative guidelines advising that patients with poorly controlled diabetes should be referred to a diabetes specialist nurse or consultant for review before surgery is scheduled.

Preoperative assessment clinics play a central role in identifying patients at risk. If your HbA1c is found to be elevated during this assessment, your surgical team should communicate with your GP or diabetes care team promptly. Patients are encouraged to attend preoperative appointments well in advance of their planned procedure to allow sufficient time for any necessary adjustments to their diabetes management.

How to Lower Your HbA1c Before a Planned Procedure

Dietary changes, regular physical activity, and medication review — coordinated with your GP or diabetes team — can meaningfully reduce HbA1c over eight to twelve weeks before surgery.

If your HbA1c is above the recommended threshold, there are several evidence-based strategies that can help bring it down before your operation. Because HbA1c reflects a two-to-three-month average, meaningful improvements typically require at least eight to twelve weeks of consistent effort. This is why early preoperative assessment is so valuable — it creates a window of opportunity for optimisation.

Lifestyle modifications are the foundation of glycaemic improvement:

• Dietary changes: Reducing refined carbohydrates, sugary drinks, and processed foods can have a rapid and significant impact on blood glucose levels. A referral to a dietitian may be beneficial.

• Physical activity: Regular moderate exercise (such as brisk walking for 30 minutes most days) improves insulin sensitivity and helps lower blood glucose.

• Weight management: Even modest weight loss of 5–10% of body weight can substantially improve HbA1c in people with type 2 diabetes.

Medication review is equally important. Your GP or diabetes team may consider:

• Intensifying existing oral medication (e.g., increasing metformin dose or adding another agent such as a GLP-1 receptor agonist)

• Initiating or adjusting insulin therapy

• Reviewing medications that may inadvertently raise blood glucose, such as corticosteroids

If surgery is within eight to twelve weeks, any new or intensified therapy should be coordinated carefully with your diabetes team. In particular, if a GLP-1 receptor agonist (such as semaglutide or liraglutide) is being considered or is already prescribed, your team will need to review perioperative management in line with local CPOC and Association of Anaesthetists policy, as these medicines can slow gastric emptying and may affect aspiration risk at the time of anaesthesia.

It is important not to make changes to your diabetes medication without medical supervision, as this can increase the risk of hypoglycaemia. Always contact your GP or diabetes nurse before altering your treatment regimen. Regular SMBG or CGM with agreed glucose targets can provide useful real-time feedback to guide adjustments; if you are experiencing frequent hypoglycaemia or persistently high readings, escalate to your diabetes specialist nurse promptly.

If you experience any suspected side effects from a change in your diabetes medicines, you can report these to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

What Happens If Your HbA1c Is Too High for Surgery

If HbA1c exceeds the safe threshold, elective surgery is commonly deferred to reduce the risk of serious complications including wound infection, cardiovascular events, and acute kidney injury.

If your HbA1c is found to be above the safe threshold during preoperative assessment, your surgical team will need to make a clinical judgement about how to proceed. For elective procedures, surgery is commonly deferred until glycaemic control has improved. This decision is made in the interest of patient safety, as operating on someone with poorly controlled diabetes is associated with a significantly increased risk of serious complications.

Complications associated with elevated HbA1c at the time of surgery include:

• Surgical site and wound infections — high glucose is associated with impaired neutrophil function and reduced ability to fight bacteria

• Delayed wound healing — hyperglycaemia disrupts collagen formation and tissue repair

• Cardiovascular events — including myocardial infarction and arrhythmias during or after the procedure

• Acute kidney injury — particularly in patients with pre-existing diabetic nephropathy

• Longer intensive care or hospital stays

In high-risk patients — particularly those with type 1 diabetes or those taking SGLT-2 inhibitors — preoperative ketone assessment is recommended. Diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS) should be identified and treated before any elective procedure proceeds.

If surgery cannot be safely deferred — for example, in cases of cancer, limb-threatening conditions, or other urgent clinical need — the surgical and diabetes teams will work together to manage the risk as effectively as possible. This may involve a variable rate intravenous insulin infusion (VRIII) during the perioperative period, in line with JBDS-IP VRIII guidance, along with closer postoperative monitoring.

Patients should be reassured that a deferral is not a refusal of treatment — it is a proactive step to ensure the best possible outcome. During the deferral period, your GP or diabetes specialist will work with you to optimise your HbA1c. Contact your GP promptly if you have been told your surgery has been deferred due to your HbA1c, so that a structured plan can be put in place without delay.

Managing Blood Glucose During and After Your Operation

Perioperative blood glucose is targeted at 6–10 mmol/L; specific medications including SGLT-2 inhibitors and metformin require adjustment or temporary cessation around the time of surgery.

Even when HbA1c is within an acceptable range before surgery, careful blood glucose management during and after the procedure remains essential. The physiological stress of surgery triggers the release of counter-regulatory hormones such as cortisol and adrenaline, which can cause blood glucose to rise significantly — even in patients whose diabetes is usually well controlled.

On the day of surgery, the following general principles apply:

• Patients with diabetes are typically scheduled first on the morning operating list to minimise fasting duration

• Specific guidance on which diabetes medications to take or omit on the day of surgery will be provided by your care team — do not assume your usual regimen is safe without checking

• Metformin is often withheld on the day of surgery and for 48 hours afterwards if there is a risk of renal impairment; it should be restarted only when renal function is confirmed to be stable and you are eating and drinking normally

• SGLT-2 inhibitors (such as dapagliflozin, empagliflozin, or canagliflozin) should generally be stopped at least three days before surgery (at least four days for ertugliflozin) due to the risk of euglycaemic diabetic ketoacidosis (DKA) — a serious condition where ketones accumulate even when blood glucose appears normal. This is in line with MHRA Drug Safety Update advice. Ketone levels should be checked perioperatively in patients who have recently taken an SGLT-2 inhibitor. These medicines should only be restarted once you are eating and drinking normally, are clinically well, and ketone levels are confirmed to be normal — follow your local hospital policy

• GLP-1 receptor agonists (such as semaglutide or liraglutide): perioperative management varies across UK centres. Some centres advise omitting the dose on the day of surgery due to the potential for delayed gastric emptying and associated aspiration risk; others continue with appropriate precautions. Follow your local CPOC or Association of Anaesthetists policy and discuss any gastrointestinal symptoms with your anaesthetist

• Insulin regimens are usually adjusted according to local hospital protocols

• If you use an insulin pump (CSII) or continuous glucose monitor (CGM), discuss management with your diabetes team well in advance. For short procedures, continuation may be feasible; for longer or more complex surgery, a switch to VRIII may be recommended in line with CPOC/JBDS-IP guidance

During surgery, blood glucose is monitored regularly by the anaesthetic team, with a target range typically of 6–10 mmol/L in the perioperative period. A VRIII may be used for tighter control in high-risk cases. Ketone monitoring is recommended if blood glucose exceeds 12 mmol/L or if the patient is at elevated risk of DKA.

After surgery, blood glucose monitoring continues on the ward. Patients should resume eating and drinking as soon as it is safe to do so, and diabetes medications are reintroduced in a stepwise manner according to clinical stability and renal function. Before discharge, ensure you have clear written instructions about restarting your medications, and arrange a follow-up with your GP or diabetes nurse within one to two weeks to review your glucose control and overall recovery.

If you experience any suspected side effects from your diabetes medicines during this period, please report them to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

Frequently Asked Questions