Preoperative Laboratory Testing for Gastric Bypass Surgery
All patients undergoing gastric bypass surgery require comprehensive preoperative laboratory testing to identify and correct existing nutritional deficiencies before surgery, as these patients have extremely high baseline prevalence of deficiencies that will worsen postoperatively due to reduced oral intake and malabsorption. 1, 2
Essential Preoperative Laboratory Panel
Hematologic Studies
- Complete blood count (CBC) to evaluate for anemia and assess platelet count, as iron deficiency and anemia occur in 0-47% of patients preoperatively 2, 3
- Ferritin to screen for iron deficiency, which is extremely common in this population 2, 3
- Vitamin B12 to identify deficiency (prevalence 0-23% preoperatively), as untreated deficiency causes irreversible neurological damage including subacute combined degeneration of the spinal cord 1, 2
- Folate to screen for deficiency (prevalence 0-63% preoperatively), but always check B12 first before supplementing high-dose folic acid, as folate can mask B12 deficiency while irreversible neurological damage progresses 1, 2
Bone and Mineral Metabolism
- Serum 25-hydroxyvitamin D to assess vitamin D status, as deficiency is reported in up to 99% of patients with obesity 2, 3
- Calcium to establish baseline levels 2, 3
- Parathyroid hormone (PTH) along with calcium, as elevated PTH with elevated calcium may indicate primary hyperparathyroidism requiring specialist referral 1
Metabolic and Organ Function
- HbA1c and fasting plasma glucose (FPG) for all patients to screen for diabetes, even in those without known diabetes, with optional oral glucose tolerance test 1
- Comprehensive metabolic panel including liver function tests to assess for non-alcoholic fatty liver disease (NAFLD), which is the most common liver abnormality in this population and improves dramatically after surgery 1
- Renal function tests to establish baseline kidney function 1
- Fasting lipid profile for patients with pre-existing treated dyslipidemia to establish baseline for postoperative assessment 1
Selective Testing Based on Procedure Type
- Vitamin A, zinc, copper, and selenium should be considered preoperatively for malabsorptive procedures such as biliopancreatic diversion with duodenal switch (BPD/DS), as these procedures have increased incidence of these deficiencies 1
- Thiamine levels are not routinely recommended preoperatively unless there is clinical suspicion of deficiency 1
- Magnesium is not routinely recommended preoperatively due to low reported prevalence of deficiency 1
Critical Clinical Considerations
Diabetes Management
- For patients with known type 2 diabetes, optimize glycemic control before surgery and review all diabetes medications for perioperative adjustments 1
- Document the presence and extent of diabetes complications preoperatively, as long-term monitoring of existing microvascular complications may still be required 1
- Distinguish between type 1 and type 2 diabetes preoperatively, as type 1 patients will not achieve remission or insulin withdrawal and have increased risk of both diabetic ketoacidosis and hypoglycemia postoperatively 1
- Be aware that abrupt glycemic improvement following surgery can exacerbate proliferative diabetic retinopathy in some patients 1
Preoperative Deficiency Correction
- All identified nutritional deficiencies must be treated and corrected preoperatively, as bariatric surgery impacts oral intake and absorption, increasing the risk of worsening deficiencies 1
Common Pitfalls to Avoid
- Never prescribe high-dose folic acid without first checking vitamin B12, as folate supplementation can mask B12 deficiency while irreversible neurological damage progresses 2, 3, 4
- Do not overlook the need to check both PTH and calcium together, as isolated PTH elevation with hypercalcemia suggests primary hyperparathyroidism requiring specialist evaluation 1
- Do not assume normal nutritional status in obese patients—vitamin D deficiency occurs in up to 99% of this population preoperatively 2, 3