Withholding metformin prior to ¹⁸F-FDG positron emission tomography (PET) oncology studies

Objectives This prospective, randomized, single-blinded trial was conducted to investigate the effects of witholding metformin on bowel ¹⁸F-FDG uptake in Type II diabetic patients.

Methods 90 Type II diabetic patients on metformin therapy undergoing ¹⁸F-FDG PET/CT for oncology participated in the study. The subjects were randomized into 3 groups: group A and B (n=30 each) were instructed to discontinue metformin 24h and 48h prior to the scan, respectively. Group C acting as controls continued regular metformin regimen (n=30). Two nuclear medicine physicians blindly assessed ¹⁸F-FDG uptake in the bowel with a visual grading system and semi-quantitatively with maximum bowel SUV. The fasting blood glucose levels were recorded by patients 3 days before the examination (baseline), and measured prior to the ¹⁸F-FDG injection.

Results ¹⁸F-FDG uptake was lower in the small bowel of patient in groups A and B compared to the group C controls (p<0.0001). Similar significant decreased uptake was noted in the colon between groups A / B and group C controls (p<0.0001). No significant difference was noted in the small bowel when groups A and B were compared (p=0.77). However, uptake in the colon was lower in groups B compared to group A (p<0.01). Bowel uptake patterns in subjects continuing metformin was intense, diffuse and segmental along the bowel wall and lumen, particularly in the colon. Baseline blood glucose levels were similar among the 3 groups. At the time of the PET/CT examination, the blood glucose levels were 7.5 ± 1.9 for group A (mean ± st. dev.), 8.4 ± 2.9 mmol/L for group B and 6.8 ± 2.1 mmol/L for the control group. The difference was statistically significant between group B and the control group (Kruskal-Wallis test; adjusted p = 0.0026).

Conclusions Our study confirms increased uptake of ¹⁸F-FDG in the colon and to a lesser extent the small bowel in patients taking metformin. Withholding metformin 48 hours prior to ¹⁸F-FDG PET/CT can reduce uptake in the small bowel and colon, at the expense of a small increase in blood glucose levels.