Who qualifies for bariatric surgery: BMI criteria, contraindications and tests

Who qualifies for bariatric surgery: the short answer

Bariatric surgery is considered for adults whose obesity is severe or is already damaging their health, and who have not reached a lasting result with diet, exercise and medical treatment. The decision is never based on weight alone.

A surgeon looks at four things together:

• body mass index (BMI) and how long the excess weight has lasted;
• weight-related conditions such as type 2 diabetes, high blood pressure or sleep apnoea;
• previous attempts to lose weight without surgery and why they did not last;
• general health, surgical risk and readiness to change eating habits for life.

If most of these point towards surgery, the next step is a full examination. The final answer comes from the surgeon and the team after that, not from a calculator.

BMI criteria in international guidelines

BMI is weight in kilograms divided by height in metres squared. It is a rough screening tool: it does not show where fat is stored or how healthy the metabolism is, but it helps to sort patients into groups.

In 2022 the American Society for Metabolic and Bariatric Surgery (ASMBS) and the International Federation for the Surgery of Obesity and Metabolic Disorders (IFSO) updated the long-standing criteria and lowered the thresholds. Many national health systems and clinics still use the older, stricter limits. In practice both are taken into account:

A figure in this table is a reason to talk to a surgeon, not an automatic yes. On our bariatric surgery page the clinic describes the same approach: surgery is usually justified with a BMI above 40, or above 35 with serious related conditions, and lower values are discussed individually.

Related conditions that make surgery more likely

Excess weight rarely comes alone. When a doctor links a condition to obesity and expects it to improve with weight loss, this weighs in favour of surgery. The most common ones are:

• type 2 diabetes or prediabetes that is hard to control;
• high blood pressure and raised cholesterol;
• obstructive sleep apnoea;
• fatty liver disease;
• joint pain and osteoarthritis of the knees and hips;
• varicose veins and swelling of the legs;
• fertility problems and polycystic ovary syndrome in women;
• reflux, shortness of breath and reduced mobility.

Improvement of these conditions after surgery is common, but it is not guaranteed and varies from person to person. That is why regular check-ups continue after the operation.

When surgery is not advised: contraindications

Some situations rule surgery out, and others mean it has to wait until the problem is treated. The surgeon weighs each case, but these are the usual reasons for a no or a not yet:

• pregnancy, or less than about six months since breastfeeding ended;
• age under 18 at City Medical Centre (teenagers are assessed only in specialised programmes);
• active cancer;
• an acute illness or a flare-up of a chronic disease;
• severe heart, lung, liver or kidney disease that makes anaesthesia too risky;
• blood clotting disorders;
• severe mental illness that is not stable, active alcohol or drug misuse, or an untreated eating disorder such as bulimia;
• inability or unwillingness to follow the diet, take lifelong supplements and attend follow-up.

Many of these are temporary. After treatment, stabilisation or the end of breastfeeding, the question of surgery can be raised again.

Tests and examinations before surgery

A full check-up has two aims: to confirm that surgery is safe and to choose the right operation. The exact list depends on your health, but it usually includes:

• Blood and urine tests: complete blood count, urinalysis, biochemistry, HbA1c (average blood sugar) and a clotting test.
• Instrumental tests: chest X-ray, ECG, ultrasound of the abdomen and kidneys, and a Doppler scan of the leg veins.
• Endoscopy: gastroscopy is required to check the stomach and oesophagus; colonoscopy is done if there are indications.
• Specialist opinions: a general physician, endocrinologist, cardiologist, psychiatrist and, for women, a gynaecologist.

Results you already have can be sent in advance, which often shortens this stage. The team will say which tests still need to be done.

Psychological readiness and life after surgery

Bariatric surgery changes the size of the stomach, and sometimes the route food takes, but it does not change habits by itself. Long-term results depend largely on what happens after discharge. That is why readiness is assessed as carefully as physical health.

Before agreeing to surgery, it helps to be honest with yourself about the following commitments:

• moving step by step from liquids to purées and then to solid food over several weeks;
• eating small portions slowly, and limiting sugary drinks and high-calorie liquid food;
• taking vitamin and mineral supplements as prescribed, often for life;
• regular blood tests and check-ups, especially in the first year;
• regular physical activity once the doctor allows it;
• stopping smoking before surgery and not returning to it.

Rapid weight loss also brings emotional changes. Some people need support with emotional eating or body image, and asking for it is part of normal care, not a sign of failure.

How the surgeon chooses the operation

Once it is clear that a patient qualifies, the next question is which operation. The surgeon considers BMI, diabetes and its duration, reflux, previous abdominal surgery, eating habits and how ready the patient is for lifelong supplements. At City Medical Centre the main options are:

• sleeve gastrectomy, which turns the stomach into a narrow tube;
• Roux-en-Y gastric bypass, often preferred with type 2 diabetes or significant reflux;
• mini gastric bypass, a bypass with a single connection;
• SADI-S, usually reserved for very high BMI, severe diabetes or as a second stage after a sleeve.

We compare the sleeve and the bypass in detail in a separate article. The key point here is that there is no single best operation: the method is chosen for the individual patient after the examination.

Consultation at City Medical Centre

City Medical Centre works with international patients. The first consultation is free and can be held online: share your height, weight, weight history, conditions you are treated for and any test results you already have. The surgeon assesses whether bariatric surgery is right for you, which method suits your case and what preparation is needed, and names the cost, which is then fixed in the contract. A personal coordinator stays with you from your first message to the end of recovery, and documents and questions can be sent by WhatsApp. Find out more on the bariatric surgery page.

Frequently asked questions

What BMI do I need to qualify for bariatric surgery?

Under the 2022 ASMBS/IFSO guideline, surgery is recommended from a BMI of 35 and, with type 2 diabetes, from 30. Many clinics still use the older limits of 40, or 35 with a serious related condition. The surgeon makes the final decision after examining you.

Can I have bariatric surgery with a BMI under 35?

Sometimes. With a BMI of 30 to 34.9, surgery may be considered if you have type 2 diabetes or if non-surgical treatment has not brought lasting weight loss. For people of Asian origin lower thresholds apply. Such cases are always assessed individually.

Is there an age limit for bariatric surgery?

At City Medical Centre bariatric surgery is performed on adults only. There is no strict upper age limit: in older patients the surgeon looks mainly at heart and lung health, anaesthetic risk and the expected benefit, rather than at age alone.

Do I have to lose weight before surgery?

Often a short pre-operative diet is advised in the weeks before surgery. It can reduce the size of the liver and makes the operation safer and easier. Your surgeon decides whether it is needed and for how long, based on your examination.

What can stop me from having bariatric surgery?

Common reasons are pregnancy or recent breastfeeding, active cancer, an acute illness, severe heart or lung disease, clotting disorders, unstable mental illness, alcohol or drug misuse and an untreated eating disorder. Many of these only postpone surgery until they are treated.