What laboratory tests should be checked pre- and post-operatively for patients undergoing bariatric surgery?

Laboratory Testing for Bariatric Surgery

All patients undergoing bariatric surgery require comprehensive preoperative laboratory screening and lifelong postoperative monitoring, with essential tests including CBC, ferritin, folate, vitamin B12, 25-hydroxyvitamin D, calcium, PTH, comprehensive metabolic panel (renal and liver function), HbA1c, and lipid profile. 1, 2

Preoperative Laboratory Testing

Essential Tests for All Patients

Hematologic Parameters:

• Complete blood count (CBC) to detect anemia (prevalence 0-47% preoperatively) 1, 2
• Ferritin to assess iron stores 1, 2
• Folate (deficiency ranges 0-63% preoperatively) 1, 2
• Vitamin B12 (deficiency ranges 0-23% preoperatively) 1, 2

Bone Health Markers:

• Serum 25-hydroxyvitamin D (deficiency reported in up to 99% of patients with obesity) 1, 2, 3
• Serum calcium 1, 2
• Parathyroid hormone (PTH) to detect primary hyperparathyroidism if calcium is elevated 1

Metabolic Assessment:

• HbA1c and fasting plasma glucose for diabetes screening (all patients, even without known diabetes) 1
• Fasting lipid profile (total cholesterol, LDL, HDL, triglycerides) 1, 2
• Liver function tests (ALT, AST, albumin) to assess for NAFLD 1, 2
• Comprehensive metabolic panel including renal function (creatinine, BUN, electrolytes) 1, 2

Procedure-Specific Additional Tests

For Malabsorptive Procedures (BPD/DS):

• Vitamin A levels 1
• Zinc levels 1
• Copper levels 1
• Selenium levels 1

These additional tests are recommended because malabsorptive procedures carry higher risk of fat-soluble vitamin and trace mineral deficiencies postoperatively. 1

Preoperative Correction

Any identified nutritional deficiencies must be corrected before surgery, as patients have increased risk of worsening deficiencies postoperatively due to reduced oral intake and absorption. 1, 2

Postoperative Laboratory Monitoring

First Year Intensive Monitoring

Test at 3,6, and 12 months postoperatively:

• Complete blood count 4
• Ferritin 4
• Vitamin B12 4
• Folate 4
• 25-hydroxyvitamin D 4
• Calcium 4
• PTH 4
• Comprehensive metabolic panel (renal function, liver function, albumin, electrolytes) 1, 4

The first year requires intensive monitoring because nutritional deficiencies develop most rapidly during this period, and patients may struggle with adequate fluid intake leading to dehydration. 1, 4

Annual Lifelong Monitoring (After Year 1)

Minimum annual testing includes:

• Complete blood count (anemia occurs in up to 50% of post-bypass patients) 5
• Ferritin (iron deficiency is common due to reduced absorption) 5
• Folate 5
• Vitamin B12 (deficiency affects up to 61.8% of patients and can cause irreversible neurological damage) 1, 5
• 25-hydroxyvitamin D (maintain >75 nmol/L for bone health) 5
• Calcium 5
• PTH 5
• Comprehensive metabolic panel (renal and liver function) 5
• HbA1c (for patients with preoperative diabetes) 5
• Lipid profile (for patients with preoperative dyslipidemia) 5

Lifelong annual monitoring is mandatory as part of shared-care management, with follow-up remaining at the bariatric surgery center for the first 2 years. 1, 4

Procedure-Specific Additional Annual Testing

For Roux-en-Y Gastric Bypass (RYGB):

• Add selenium monitoring annually due to malabsorptive component 4

For BPD/DS or One-Anastomosis Gastric Bypass with biliopancreatic limb >150 cm:

• Add vitamin A monitoring annually due to significant fat malabsorption 4
• Zinc (deficiency affects up to 40.5% of patients) 5
• Copper (must be monitored simultaneously with zinc as they compete for absorption) 5

Symptom-Triggered Additional Testing

Immediate testing required for specific clinical presentations:

Prolonged vomiting, dysphagia, or poor oral intake:

• Check thiamine (vitamin B1) immediately and begin treatment without waiting for results to prevent Wernicke's encephalopathy 4

Unexplained anemia or fatigue:

• Protein levels 4
• Zinc 4
• Copper 4
• Selenium 4

Special Population Considerations

Women of reproductive age:

• Require vigilant monitoring of ferritin due to menstrual blood loss combined with reduced absorption 4

Pregnant women post-bariatric surgery:

• Increase monitoring frequency to every trimester for ferritin, folate, vitamin B12, calcium, vitamin D, and vitamin A 4, 5

Critical Pitfalls to Avoid

Never prescribe high-dose folic acid without first checking vitamin B12 levels. Folate can mask B12 deficiency while irreversible neurological damage progresses (megaloblastic anemia and irreversible neuropathies). 1, 4, 5

Always monitor zinc and copper simultaneously. Supplementation of one affects levels of the other due to competitive absorption. 4, 5

Do not overlook the need for more frequent monitoring in patients with malabsorptive procedures (BPD/DS, long-limb bypass), as they require additional fat-soluble vitamin monitoring. 4

Vitamin B12 levels are not a good predictor of deficiency due to methodological problems affecting sensitivity and specificity. When in doubt about B12 deficiency, it is better to treat. 1