Assessment of the FreeStyle Libre 2 interstitial glucose monitor in hypo- and euglycemic cats

Abstract

Background: Continuous glucose monitoring systems have been validated for eu- and hyperglycemic cats. The FreeStyle Libre 2 (FSL2) is sufficiently accurate in people during hypoglycemia to guide critical treatment decisions without confirmation of blood glucose concentration (BG).

Objectives: Assess FSL2 accuracy in cats with hypoglycemia.

Animals: Nine healthy, purpose-bred cats.

Methods: Hyperinsulinemic-hypoglycemic clamps were performed by IV infusion of regular insulin (constant rate) and glucose (variable rate). Interstitial glucose concentration (IG), measured by FSL2, was compared to BG measured by AlphaTrak2. Data were analyzed for all paired measurements (n = 364) and separately during stable BG (≤1 mg/dL/min change over 10 minutes). Pearson's r test, Bland-Altman test, and Parkes Error Grid analysis respectively were used to determine correlation, bias, and clinical accuracy (P < .05 considered significant).

Results: Overall, BG and IG correlated strongly (r = 0.83, P < .0001) in stable glycemia and moderately at all rates of change (r = 0.69, P < .0001). Interstitial glucose concentration underestimated BG in euglycemia, but the BG-IG difference was progressively smaller as BG decreased (12.9 ± 12.2, 8.8 ± 11.2, -3.2 ± 7.4, and -7.8 ± 5.2 mg/dL in the ranges of 80-120 [n = 64], 60-79 [n = 29], 50-59 [n = 71], and 29-49 mg/dL [n = 53], respectively).

Conclusions: Although IG underestimates BG throughout most of the hypo-euglycemic range, IG generally overestimates BG in marked hypoglycemia (<60 mg/dL). It is therefore imperative to evaluate FSL2 results in this critical range with caution.

Keywords: FreeStyle Libre 2; cat; continuous glucose monitor; diabetes; euglycemia; feline; flash glucose monitor; hyperglycemia; hypoglycemia; insulin; interstitial glucose monitor; veterinary.

FIGURE 1

Correlation between blood glucose (BG) and interstitial glucose (IG) concentrations in healthy cats (n = 9) in hypo‐ and euglycemia at all rates of glycemic change. The solid line represents best fit with dashed lines representing the 95% CI of the best fit. Blood glucose measured by AlphaTrak2. Interstitial glucose measured by FreeStyle Libre 2 continuous interstitial glucose monitor.

FIGURE 2

Parkes Error Grid analysis exhibiting excellent clinical accuracy of FSL2 at hypo‐ and euglycemia in healthy cats (n = 9) at all rates of glycemic change. All data points fall within zone A (indicating no change in clinical action) or zone B (indicating change in clinical action unlikely to affect outcome), with 91.5% (n = 333) in A and 8.5% (n = 31) in B. BG, blood glucose as measured by AlphaTrak2. IG, interstitial glucose as measured by FSL2, FreeStyle Libre 2 continuous interstitial glucose monitor.

FIGURE 3

Bland Altman plot of agreement between blood glucose (BG) and interstitial glucose (IG) concentrations in hypo‐ and euglycemia in healthy cats (n = 9) at all rates of glycemic change. The middle, solid line represents best fit with dashed lines representing the 95% CI of the best fit. Blood glucose measured by AlphaTrak2. Interstitial glucose measured by FreeStyle Libre 2 continuous interstitial glucose monitor.

FIGURE 4

Correlation between blood glucose (BG) and interstitial glucose (IG) concentrations in healthy cats (n = 9) in hypo‐ and euglycemia during stable BG. The middle, solid line represents best fit with dashed lines representing the 95% CI of the best fit. Blood glucose measured by AlphaTrak2. Interstitial glucose measured by FreeStyle Libre 2 continuous interstitial glucose monitor.

FIGURE 5

Parkes Error Grid analysis exhibiting excellent clinical accuracy of FSL2 at hypo‐ and euglycemia in healthy cats (n = 9) during steady blood glucose (BG) concentrations. All data points fall within zone A (indicating no change in clinical action) or zone B (indicating change in clinical action unlikely to affect outcome), with 94% (n = 204) data points in zone A and 6% (n = 13) in zone B. BG, measured by AlphaTrak2. IG, interstitial glucose as measured by FSL2, FreeStyle Libre 2 continuous interstitial glucose monitor.

FIGURE 6

Bland Altman plot of agreement between blood glucose (BG) and interstitial glucose (IG) concentrations in hypo‐ and euglycemia in healthy cats (n = 9) during steady BG. The solid line represents best fit with dashed lines representing the 95% CI of the best fit. BG, blood glucose as measured by AlphaTrak2. IG, interstitial glucose as measured by FreeStyle Libre 2 continuous interstitial glucose monitor.

FIGURE 7

Difference between blood glucose (BG) and interstitial glucose (IG) concentrations, stratified based on BG range in healthy cats (n = 9) during steady BG. Central horizontal lines represent median values, boxes represent quartiles, whiskers represent 10% to 90% percentiles, and asterisks represent statistically significant differences [Correction added after first online publication on 8 August 2023. Figure 7 has been replaced.].