Discussion
Our study revealed that hyperglycemia portends worse outcomes in patients hospitalized for COVID-19, regardless of prior diagnosis of diabetes. Several other observational studies have examined the relationship between hyperglycemia and outcomes in patients hospitalized with COVID-19. However, our study is unique in that it examines this relationship in a largely African American cohort, a group disproportionately affected by the virus.
Our results are consistent with several other studies published examining the relationship between serum glucose levels and outcomes in patients with COVID-19. Studies conducted by Wang et al and Alahmad et al examined the effect of fasting blood glucose (FBG) on outcomes in patients with COVID-19, and found that those with FBG ≥126 had increased odds of 28-day mortality and ICU admission, respectively.19 20 An Italian study of Coppelli et al analyzed their cohort by three groups: normoglycemia defined as admission glucose <140 mg/dL (<7.78 mmol/L), hyperglycemia without prior diagnosis of DM defined as glucose >140 mg/dL and prior diagnosis of DM regardless of admission glucose levels.21 They found that the group with glucose >140 mg/dL on admission without prior diagnosis of DM had two times greater inpatient mortality than those of the normal glycemia group, and 30% increased mortality when compared with those with known DM.21 These findings are consistent with our results which reveal that individuals without diabetes with admission BG >140 mg/dL have twofold increased odds of mortality when compared with those with admission BG <140 mg/dL. Similarly, another retrospective study conducted in northern Italy found that elevated FBG on admission was associated with increased risk of ICU admission or death in patients with COVID-19. This association was stronger among patients without pre-existing diabetes when compared with those with diabetes.22 A large retrospective cohort study (n=11 312) from Spain found admission hyperglycemia (BG >180 mg/dL) to be an independent risk factor for mortality, mechanical ventilation, and ICU admission regardless of pre-existing diabetes in patients with COVID-19 when compared with those with BG <140 mg/dL.12 Like our study, the studies described above analyzed outcomes according to admission BG levels, thus excluding patients with hyperglycemia which have resulted from inpatient therapies (ie, steroids).12
The association between hyperglycemia (without a prior diagnosis of diabetes) and worse outcomes in hospitalized patients suffering from acute illness is a well-known phenomenon demonstrated prior to this pandemic. Specifically, a large retrospective study conducted in 1998 by Umpierrez et al found that patients admitted to the hospital with new hyperglycemia (FBG >126 mg/dL or random BG >200 mg/dL) without a prior diagnosis of diabetes had increased mortality, longer LOS, and higher ICU admission rates, when compared with patients with a history of diabetes and those with normoglycemia.23 Another study conducted by Frisch et al24 reported a significant association between perioperative hyperglycemia and mortality rate in patients who underwent non-cardiac surgery.24 However, as with the association between COVID-19 and hyperglycemia, it is unclear whether hyperglycemia initiates worsening disease, or if it occurs as a result of severe illness itself.
While the pathogenesis of the novel COVID-19 (SARS-CoV-2) remains unclear, the other coronaviruses, SARS-CoV and Middle East respiratory syndrome (MERS-CoV), have been studied more closely to date. Hyperglycemia was found to be an independent predictor of mortality in SARS-CoV and MERS-CoV.4 One study found that SARS directly damages the islet cells of the pancreas, including the beta cells, which results in impaired insulin secretion and transient insulin-dependent diabetes, which resolves with resolution of disease.25 If the pathogenesis of SARS-CoV-2 is similar, this would support the hypothesis that hyperglycemia is a result of severe COVID-19 infection itself, rather than the cause. Additionally, inflammation, which occurs because of infection, causes an increase in counter-regulatory stress hormones which leads to insulin resistance and thus hyperglycemia.21 This phenomenon is often referred to as stress hyperglycemia.
On the contrary, hypotheses have been proposed to explain why hyperglycemia may trigger worsening illness in patients with COVID-19. One hypothesis is that the severity of COVID-19 infection is directly correlated with the concentration of glycosylated SARS-CoV-2 viral particles and glycosylated ACE-2 receptor in the lung epithelium.26 This receptor is thought to be the primary entry point for the SARS-COV-2 virus into the epithelium. Thus, persistent hyperglycemia causing a direct increase in the glycosylated viral particles and ACE-2 receptors in the lung epithelium would enhance viral propagation and could potentially increase disease severity.26 Hyperglycemia alone is known to induce a state of metabolic inflammation, ultimately generating a cytokine storm, which is hypothesized to cause multiorgan failure in individuals with severe COVID-19 infection.27 The retrospective cohort study conducted by Zhu et al7 found that patients with diabetes hospitalized for COVID-19 who had well-controlled BG throughout their stay had decreased inflammatory markers including lower neutrophil count, and lower levels of interleukin-6, C-reactive protein (CRP), and lactate dehydrogenase (LDH) when compared with those with poorly controlled glucose.7 Similarly, the study conducted by Mazori et al (n=133) which found that among critically ill patients admitted to the ICU with COVID-19, individuals without DM and early-onset hyperglycemia (EHG), defined as BG >180 mg/dL during the first 2 days after ICU admission, had increased 14 and 60-day mortality when compared with those without DM and no EHG.28 These patients were found to have elevated levels of inflammatory markers including CRP, D-dimer, procalcitonin, and lactate.
If hyperglycemia is in fact an independent risk factor for disease severity, the next question to be answered is whether improving glucose control would improve outcomes in patients with COVID-19. A few studies to date have examined the impact of adequate glucose control on clinical outcomes in these patients. The study conducted by Zhu et al7 in China (n=7337) found that among patients with diabetes hospitalized for COVID-19, well-controlled glucose defined as serum glucose 70–180 mg/dL during patient admission was associated with significantly decreased mortality compared with those with poorly controlled glucose defined as the upper limit of glycemic variability exceeding 180 mg/dL.7 Specifically, those in the well-controlled BG group had a decreased all-cause mortality when compared with those in the poorly controlled cohort. Additionally, the patients in the well-controlled BG group had a decreased frequency of acute respiratory distress syndrome (ARDS), acute heart injury, AKI, septic shock, and disseminated intravascular coagulation (DIC) when compared with the poorly controlled glucose group.7 However, this study excluded patients without diabetes, thus the effect of hyperglycemia in this population is unknown. Similarly, a study in Italy found that patients with COVID-19 and hyperglycemia treated with insulin infusions had a decreased risk of severe disease and death.29
Another study performed by Bode et al on 1122 patients hospitalized for COVID-19 within 88 hospitals in the USA studied the clinical outcomes of patients with diabetes and/or uncontrolled hyperglycemia (defined as ≥2 BG values >180 mg/dL over a 24-hour period).5 Their study found that patients with uncontrolled hyperglycemia without DM had a significantly increased mortality rate when compared with those with DM, 41.7% and 14.8%, respectively (p<0.001). These studies suggest that glucose control during admission might improve outcomes in patients with COVID-19.
It should be noted that while our findings are consistent with others published on the relationship between hyperglycemia and outcomes in COVID-19, our study did not reveal significantly worse outcomes in patients with diabetes compared with those without, except for AKI and LOS, which is different from previous studies (see table 1). Our study revealed similar mortality rates in patients with diabetes compared with those without (42% vs 38%, p=0.32) whereas the retrospective cohort study by Zhu et al found that patients with DM had a significantly greater mortality rate than those without (HR 1.49, p=0.005).7 Since only 30% of our study cohort (n=205) had available HbA1c values within 6 months of admission, the diagnosis of diabetes was largely based on patient history and/or prior documentation. Thus, some patients were likely to be misclassified.
This study has several other limitations. Although our study sample size is larger than the studies conducted by Coppelli et al, Wang et al, Mazori et al, and Alahmad et al, our study cohort remains relatively small which could have predisposed to error. In terms of statistical analysis, evaluating the associations between patient characteristics and outcomes with a multivariate logistic regression model is suboptimal given that it only allows for the calculation of adjusted ORs. It is known that ORs can exaggerate an association between variables. Therefore, there is a chance that the ORs found may have overestimated the strength of the association.30 Additionally, it should be noted that it is possible that some patients with DKA on admission were missed as not all patients had urine or plasma ketone level collected on admission, one of which is needed to diagnose DKA. However, the prevalence of DKA in our population (4.4%) was similar to what was found by Khan et al31 (3.1%) in their study of a similar population.31
Despite these limitations, our study has several strengths. Our analysis included patients with and without diabetes and analyzed the effects of two different admission glucose cut-offs (>140 and >180 mg/dL). While other studies have included minority populations, our study is unique in that it is the first study to explore the effects of admission hyperglycemia on outcomes in COVID-19 in a primarily African American population (n=628, 89%). This is important as African American, Black populations are disproportionately affected by the COVID-19 pandemic. A study conducted by Acosta et al across the USA found that blacks had a 3.17 times increased risk of ICU admission and 2.58 times increased risk of death when compared with whites.32 Another study conducted in California found that hospital admissions in African Americans were more than two times that of whites.33 While both studies among others explored outcomes in African Americans, neither examined the effect of admission hyperglycemia in this population.
In conclusion, our study revealed that hyperglycemia portends worse outcomes in patients with COVID-19, with and without diabetes. These findings suggest that patients with hyperglycemia require closer observation and more aggressive therapies, emphasizing the importance of screening for hyperglycemia in all patients who test positive for COVID-19. However, whether hyperglycemia is a marker or a cause of more severe COVID-19 is unclear at this time. This raises the testable hypothesis that intensive glucose control may improve outcomes in patients with COVID-19. Future research must be conducted to determine the role of hyperglycemia in the pathogenesis of COVID-19.