Accuracy and Clinical Safety of the FreeStyle Libre 2 System in an Internal Medicine Intermediate Care Unit

Hyperglycemia and glycemic variability in hospitalized patients are strongly associated with adverse outcomes, including increased mortality and infection rates. While continuous glucose monitoring (CGM) offers the potential for real-time surveillance, its accuracy in critical patient settings remains under scrutiny. This study evaluates the real-world accuracy and clinical safety of the FreeStyle Libre 2 system in an Internal Medicine Intermediate Care Unit (IMICU).

We conducted a prospective observational study of adult patients admitted to the IMICU at Unidade Local de Saúde de São João, located in Porto, Portugal. Participants utilized the FreeStyle Libre 2 system, and sensor glucose values were paired with reference capillary point-of-care (POC) measurements. Accuracy was assessed using the mean absolute relative difference (MARD) and point accuracy thresholds (percentage of readings within 15%/15 mg/dL, 20%/20 mg/dL, and 30%/30 mg/dL of reference). Clinical safety was evaluated using the Parkes (Consensus) Error Grid Analysis (EGA). Subgroup analyses were performed to assess performance across glycemic ranges, diabetes status, and clinical comorbidities.

Twenty-three patients (mean age 72.39 ± 15.51 years; 14 (60.9%) male) were included, providing 414 paired glucose readings. The overall MARD was 18.62 ± 11.87%. On the Parkes Error Grid, 226 (99.03%) paired readings fell within Zones A and B, with one (0.24%) in Zone E. Point accuracy analysis showed 46.14% of readings within 15%/15 mg/dL, 61.11% within 20%/20 mg/dL, and 85.75% within 30%/30 mg/dL. MARD values did not differ significantly across hypoglycemic, euglycemic, and hyperglycemic ranges (p = 0.843) or between patients with and without diabetes (p = 0.401). Subgroup analysis revealed lower MARD in patients receiving intravenous fluid therapy (p < 0.001) and who presented with shock (p < 0.001).

Despite a higher numerical MARD typical of comparisons against POC reference methods, the FreeStyle Libre 2 system demonstrated a high degree of clinical safety (99.03% in acceptable zones) in the IMICU. Its consistent performance in both diabetic and non-diabetic cohorts supports its potential as an adjunctive surveillance tool for detecting glycemic trends in critical patient settings.