Blood tests after weight-loss surgery

For people who have had a gastric bypass, sleeve gastrectomy, duodenal switch, gastric band or another weight-loss (bariatric) operation.

12 tests explainedAI-verified against cited sources. Awaiting expert reviewLast reviewed 2026-10-19

Weight-loss surgery changes how much you eat and, after some operations, how well the gut absorbs vitamins and minerals. Low iron, B12, folate, vitamin D, thiamine, zinc and copper can show up months or years later, even with supplements. Guidelines suggest regular checks, and which tests matter most depends on the operation you had.

The tests that matter most

Most important first. Each one links to its full page, where the note for this group sits near the top.

Ferritin
Low iron is common after weight-loss surgery, from eating less iron, absorbing less of it and, for some women, periods. It can happen after any operation, even with routine supplements. ASMBS suggests checking iron within 3 months of surgery, every 3 to 6 months until 1 year, then yearly.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and British Obesity and Metabolic Surgery Society (2020)
Men, 20–59
30–400 ng/mL
Men, 60+
31–409 ng/mL
Hemoglobin
Hemoglobin is the main number on a blood count for spotting anemia, and anemia from low iron is common after bariatric surgery. BOMSS suggests a blood count and ferritin at 3, 6 and 12 months, then at least yearly.
Source: British Obesity and Metabolic Surgery Society (2020)
Men, 16+
13–17.7 g/dL
Women, 16+
11.1–15.9 g/dL
Vitamin B12 (Cobalamin)
Absorbing B12 needs stomach acid and a stomach protein called intrinsic factor, and gastric bypass and sleeve gastrectomy reduce both. Guidelines suggest regular B12 checks after these operations and after duodenal switch. The body's B12 stores can last about 2 years, so a shortage can build slowly.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and Nutrients (2025)
Usual adult range
232–1,245 pg/mL
Folate (Vitamin B9), Serum
Folate can run low after surgery, partly from eating fewer folate-rich foods or missing multivitamins. ASMBS suggests checking it in everyone, with particular attention for women who could become pregnant.
Source: American Society for Metabolic and Bariatric Surgery (2017)
Usual adult range
3 and above ng/mL
Vitamin D (25-Hydroxy)
Low vitamin D is very common in people with obesity before surgery and stays common afterward. ASMBS suggests checking it in everyone, and BOMSS suggests checks at 3, 6 and 12 months, then at least yearly.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and British Obesity and Metabolic Surgery Society (2020)
Usual adult range
30–100 ng/mL
Parathyroid Hormone (PTH), Intact
PTH is the hormone that keeps blood calcium steady. When calcium and vitamin D absorption fall, PTH can rise to hold calcium up, so it is read with calcium and vitamin D. BOMSS suggests checking PTH after surgery if it wasn't checked before, to rule out an overactive parathyroid gland, and calcium at 3, 6 and 12 months, then at least yearly.
Sources: British Obesity and Metabolic Surgery Society (2020) and Nutrients (2025)
Usual adult range
18–59 pg/mL
Vitamin B1 (Thiamine)
Low thiamine (vitamin B1) is an early risk after surgery, especially with ongoing vomiting, fast weight loss or eating very little. Guidelines say suspected low thiamine is treated right away, without waiting for the blood result.
Sources: American Society for Metabolic and Bariatric Surgery (2017), British Obesity and Metabolic Surgery Society (2020) and Nutrients (2025)
Usual adult range
70–180 nmol/L
Zinc
Zinc is absorbed mostly in the first part of the small intestine, which gastric bypass and duodenal switch go around. ASMBS suggests checking zinc at least yearly after these operations, and BOMSS suggests the same after sleeve gastrectomy too.
Sources: American Society for Metabolic and Bariatric Surgery (2017), British Obesity and Metabolic Surgery Society (2020) and Nutrients (2025)
Usual adult range
60–106 µg/dL
Copper
Copper can run low after gastric bypass and duodenal switch, and high doses of zinc can make that more likely. ASMBS suggests checking copper at least yearly after these operations, even without symptoms, and BOMSS includes sleeve gastrectomy too.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and British Obesity and Metabolic Surgery Society (2020)
Men, 18+
70–140 µg/dL
Women, 18+
80–155 µg/dL
Vitamin A (Retinol)
Vitamin A is one of the fat-soluble vitamins, and low levels are most common after duodenal switch and gastric bypass. ASMBS suggests checking it in the first year after surgery, especially after duodenal switch.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and British Obesity and Metabolic Surgery Society (2020)
Usual adult range
32.5–78 µg/dL
Albumin
Albumin is the main protein in blood. It can fall when the body runs short of protein, and also with infection or inflammation. BOMSS uses routine liver blood tests partly to watch for low albumin.
Sources: British Obesity and Metabolic Surgery Society (2020) and Nutrients (2025)
Usual adult range
3.5–5 g/dL
Hemoglobin A1c (HbA1c)
For people who had diabetes before surgery, BOMSS suggests keeping track of A1c afterward. It suggests the same for cholesterol in people who had high levels before.
Source: British Obesity and Metabolic Surgery Society (2020)
Usual adult range
4.8–5.6 %

Ranges shown are the usual adult ranges labs print. The range on your own report is the one your result is judged against.

Getting results you can compare

ASMBS describes routine checks after surgery as every 3 to 6 months in the first year and yearly after that, unless a test has its own schedule.
Source: American Society for Metabolic and Bariatric Surgery (2017)
Many people already have at least one low vitamin or mineral before surgery, so results from before the operation make later ones easier to read.
Source: American Society for Metabolic and Bariatric Surgery (2017)
Which tests are suggested depends on the operation. Zinc, copper and B12 checks are tied to gastric bypass, sleeve gastrectomy and duodenal switch, and vitamin A checks mostly to duodenal switch and gastric bypass, so it helps to know exactly which one you had.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and British Obesity and Metabolic Surgery Society (2020)
A B12 result in the usual range can still miss a shortage. ASMBS suggests adding methylmalonic acid (MMA), with or without homocysteine, to check B12 status.
Source: American Society for Metabolic and Bariatric Surgery (2017)

Good to know

Ongoing vomiting after weight-loss surgery isn't normal and is always worth reporting. BOMSS notes it raises the risk of low thiamine, which can affect the brain and nerves.
Sources: British Obesity and Metabolic Surgery Society (2020) and Nutrients (2025)
Low levels can show up years after surgery, even in people taking supplements. That is why guidelines keep yearly checks going rather than stopping once the first year goes well.
Sources: American Society for Metabolic and Bariatric Surgery (2017) and Nutrients (2025)

Questions you could ask

Which tests make sense for the operation I had, and how often?
Should we check methylmalonic acid along with my B12?
Are any of these results worth rechecking before my next yearly visit?
How might my supplements affect these results?

Sources

American Society for Metabolic and Bariatric Surgery Integrated Health Nutritional Guidelines for the Surgical Weight Loss Patient 2016 Update: Micronutrients, American Society for Metabolic and Bariatric Surgery (2017)
British Obesity and Metabolic Surgery Society Guidelines on perioperative and postoperative biochemical monitoring and micronutrient replacement for patients undergoing bariatric surgery: 2020 update, British Obesity and Metabolic Surgery Society (2020)
Preventing and Managing Pre- and Postoperative Micronutrient Deficiencies: A Vital Component of Long-Term Success in Bariatric Surgery, Nutrients (2025)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-19. How we build and check these pages.

Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.