CGM Shows When Bariatric Surgery Patients Need Diabetes Therapy Adjustments

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The study used the Freestyle Libre to track blood glucose levels in patients with and without type 2 diabetes (T2D) who had different surgical procedures; the CGM showed a drop in glucose on the third day after surgery among patients with T2D who had gastric bypass.

In 2017, as advocates and researchers discussed the potential for continuous glucose monitoring (CGM) to become a tool in clinical trials, most of the discussion involved testing in new therapies.1,2 The discussion culminated in an international consensus on CGM, published in December 2017, that included standards for assessing hypoglycemia in clinical trials.3

From the start, the Abbott Freestyle Libre Pro, which allows data to be downloaded by a healthcare professional, appeared the default CGM choice for clinical trials because it is factory calibrated and does not require patient interaction. It was approved in September 2016. (In September 2017, FDA approved the Freestyle Libre with a 10-day sensor for consumer use; it approved the 14-day sensor in July 2018.)

Now, a small study published last week in the journal Obesity Surgery4 shows how CGM technology can give researchers the ability not only to track minute-by-minute data but also to customize surgical or therapeutic interventions for individual patients.

In the study, researchers from Jagiellonian University Medical College in Krakow, Poland, used the Freestyle Libre CGM to study patients with and without type 2 diabetes (T2D) who underwent 2 different methods of bariatric surgery, laparoscopic sleeve gastrectomy (sleeve) and laparoscopic Roux-en-Y, also known as gastric bypass. Of the group, 16 patients had T2D and 16 did not; 18 patients had the sleeve procedure and 14 had gastric bypass. Patients were tracked for the 10-day postoperative period, from the day of surgery onward.


Patients in the study had body mass index of 35 to 50 kg/m2. Diabetes for patients in the study was very well controlled: the median glycated hemoglobin (A1C) in the patients with T2D was 5.5%, which did not differ from the control group (P = 0.460.) Preoperative daily blood glucose concentration between the 2 groups was also similar (P = 0.622).

While the mean postoperative blood glucose concentration over the length of the study was similar between the 2 types of surgical groups as well as between those with and without T2D, use of CGM during the process revealed different patterns, giving physicians insights into when diabetes therapy should be altered to prevent hypoglycemia.

The research team found the following:

  • Among the 32 patients, glucose concentration was within the target range of 3.9-6.7 mmol/L 68% of the time during the 10-day postoperative period.
  • Researchers recorded 12±6 low glucose events (<3.9 mmol/L) per patient, which were more frequent among the T2D patients than those without the disease (14±5 events vs 9±6 events, P = 0.035).
  • After gastric bypass, T2D patients had more low blood glucose episodes that lasted significantly longer than patients with T2D who had the sleeve procedure (P = 0.035 and P = 0.049 respectively).
  • While mean daily glucose concentrations did not differ between the groups on days 1-3, by day 4 the CGM showed significantly lower CGM concentrations for T2D patients who received gastric bypass, compared with those who had the sleeve procedure; symptoms were seen in 4 T2D patients who had gastric bypass that required glucose infusions.
  • Only 2 patients did not have any hypoglycemia; both had the sleeve procedure. Low glucose events, most occurring without symptoms, were seen in all the other patients at some point.

The authors noted that prior studies had tracked glucose status after bariatric surgery, but for shorter periods; this pilot study was one of the first to use CGM to compare 2 different surgical methods and to follow patients for a full 10 days. The CGM showed that patients with and without T2D both had rapid rise in interstitial glucose concentration right after surgery, with a larger increase seen in those who had the sleeve procedure (although difference was not statistically significant). This was consistent with prior studies showing hyperglycemia immediately after surgery, which can result in complications and hospitalization.

The study’s chief finding—pinpointing the drop in blood glucose levels after the third postoperative day for T2D patients undergoing gastric bypass—is likely explained by greater incretin release following the procedures, the authors concluded.

“This finding, if replicated in a larger-scale, longer-term study, preferably a randomized controlled trial, would indicate a need for an early reduction in the dose of [T2D] medication,” they wrote. Thus, they concluded, for these patients, therapy should be cut back even before weight loss begins.

More studies will be forthcoming that make use of the Freestyle Libre Pro. Joslin Diabetes Center is leading a study that examines the relationship between blood glucose levels and retinopathy and nephropathy risk; participants who wear the CGM are asked to mail the sensor to the coordinating center after wearing it for 14 days.5 According to, data collection on the study is expected to be finished June 30, 2019.


  1. Caffrey M. Beyond A1C pinpoints metrics that matter to patients for future FDA approvals. The American Journal of Managed Care® website. Published July 21, 2017. Accessed February 14, 2019.
  2. Schnell O, Barnard K, Bergenstall R, et al. Role of continuous glucose monitoring in clinical trials: recommendations on reporting. Diabetes Technol Thera. 2017; 19(7): 391—399. doi: 10.1089/dia.2017.0054.
  3. Danne T, Nimri R, Battelino T, et al. International consensus on use of continuous glucose monitoring. Diabetes Care. 2017;40(12):1631-1640. doi: 10.2337/dc17-1600.
  4. Wysocki M, Szopa M, Stefura T, et al. Continuous glucose monitoring in bariatric patients undergoing laparoscopic sleeve gastrectomy and laparoscopic Roux-en-Y gastric bypass [published online February 8, 2019]. Obesity Surgery. 2019; doi:10.1007/s11695-018-03684-5.
  5. PERL Continuous Glucose Monitoring (CGM) Study. Updated November 8, 2017. Accessed February 14, 2019.