Interpretation of laboratory results after gastric bypass surgery: the effects of weight loss and time on thirty blood tests in a five years follow up program

Abstract Background Long-term follow-up with blood-tests is essential for bariatric surgery to be a successful treatment for obesity and related comorbidities. Adverse effects, deficiencies and metabolic improvements need to be controlled. Our objective was to investigate the impact of time and weight-loss on laboratory results in each postoperative phase after laparoscopic Roux-en-Y gastric bypass (LRYGB). Setting Bariatric center of excellence, general hospital. Methods We retrospectively evaluated results of thirty blood-tests, preoperative and six months, one, two and five years after LRYGB. The 2019 Dutch bariatric chart was used to define weight-loss responses ‘outstanding' (>p+1SD), ‘average' (p+1SD to p-1SD) and ‘poor' ( Results Results of 4835 patients were analyzed. Five-year follow-up was 58%. Blood levels of ferritin, mean-corpuscular-volume, thrombocytes, vitamin-D, parathyroid-hormone, hemoglobin-A1c, triglyceride, total-cholesterol, C-reactive-protein, gamma-glutamyl-transferase, alkaline-phosphatase, creatinine, vitamin-B1 and total-protein were related with weight-loss response. All thirty blood-tests were also related with time. For several blood-tests weight-loss and time did not only influence median results, but also 5th and 95th percentile cut-off values. Many patients had better vitamin levels after the operation. We observed an increase of parathyroid-hormone and ongoing iron depletion up to five years post-surgery. Conclusions Presenting results of thirty routine blood-tests, including cut-off values based on 5th and 95th percentile, grouped by weight-loss response and postoperative time interval after gastric bypass surgery is new. The elaborate tables and graphs could serve as practical guide for proper interpretation of laboratory results in post-bariatric surveillance. Results underline the need for long-term follow-up, including blood-tests.