To date, most hospitalized patients undergo fingertip capillary blood glucose testing as the primary method of glucose monitoring. However, the accuracy of blood glucose meters varies greatly [13] and may be affected by several factors [14]. Moreover, frequent measurements in those patients who use insulin will simultaneously increase the workload of medical workers and the risk of infection, especially in the context of the COVID-19 pandemic. On the other hand, there is a large body of evidence that isCGM can improve glucose control and quality of life [15–17]. In a randomized controlled multicenter study, Bolinder et al. found that using isCGM (freestyle Libre) for 2 weeks reduced the whole-day hypoglycemic events (<70 mg/dL) by 25.8%, the nocturnal hypoglycemic events by 33.2% and the severe hypoglycemic events (<40 mg/dL) by 55% in patients with type 1 diabetes with good glycemic control (HbA1c ≤ 7.5% [58.5 mmol/mol]) [18]. This suggests that isCGM may improve blood glucose control in critically ill patients.

The accuracy of isCGM in critically ill patients has been reported in some studies, but these studies have some limitations, such as a small sample size, few matched pairs, the single-center and/or retrospective study design, and inconsistent reference methods, which also partially explains for the conflicting results concerning the MARD values (14–41%). For instance, Ancona et al. analyzed 185 pairs in 8 critically ill patients with type 2 diabetes and showed that the MARD was 14% [8]. The analysis of the accuracy of isCGM in the hypoglycemic range was precluded in that study due to the lack of hypoglycemic glycose pairs. Several previous studies evaluated the accuracy of isCGM in a pediatric ICU (n = 16, 711 pairs) [10], a cardiology ICU (n = 15, 149 pairs) [11], and COVID-19 patients in an ICU (n = 17, 171 pairs) [12], respectively, but the results could not be extended to all critically ill patients.

In this prospective study, we analyzed 3416 pairs from 122 patients, and the MARD was 18.0%, implying suboptimal overall accuracy for critically ill patients. Furthermore, we found that the accuracy of blood glucose monitoring decreased as vBG decreased but was not influenced by patients’ characteristics, such as age, gender, pH, APACHE II score, or the use of vasopressors, suggesting that these results could be generalized to a wide range of critically ill patients.

Of note, together with the current glucose value, CGM systems provide trend arrows to indicate the direction and rate of glucose change, to inform decision making. Therefore, the ideal assessment of CGM accuracy should include both point accuracy and trend accuracy [19]. However, there is no well-accepted metric for evaluating trend accuracy. Interestingly, in a small study investigating the accuracy of isCGM, visual inspection revealed that FGM values closely followed the trend of the reference method (arterial blood gas) despite the poor point accuracy [9]. Taken together, although the point accuracy of isCGM in our study does not justify the nonadjunctive use of this device in ICU, it is possible that patients may benefit from a hybrid management protocol incorporating both isCGM (including glucose values and trend) and point-of-care measurements while reducing the workload and infection risk of health providers. The impact of such protocols on the outcomes of critically ill patients warrants further investigations.

The advantage of this study lies in the inclusion of a mixed population of medical-surgical ICU patients, the multicenter study design, a large number of glucose pairs comprising the full range of glycemia, the use of venous plasma glucose as the only reference standard, and the unified equipment used to measure vBG. Nevertheless, there are some limitations to this research. First, most of the samples in this study were taken during the day. Therefore, we could not examine the difference in the accuracy of isCGM between daytime and nighttime. However, the relatively large sample size enabled us to investigate the accuracy of FGM in the full range of glucose levels (i.e., hypoglycemic, euglycemic, and hyperglycemic ranges), which partially mitigates this concern. Second, trend accuracy was not evaluated in this study, which may be an interesting issue to address in the future. Finally, the effect of isCGM on glycemic outcomes could not be determined in this study.

In summary, our study demonstrated suboptimal overall accuracy of isCGM for critically ill patients. Whether the adjunctive use of isCGM could improve glucose management and health outcomes in the critically ill needs further investigation.