HbA1c and wound infection
A chronic state of impaired glucose metabolism has long been demonstrated to affect components of the immune system, thereby meaning HbA1c may impact the rate of post-operative infections. Post-operative wound infection affects patient morbidity and length of stay in hospital following cardiac surgery and hence it is important to analyse the relationship between HbA1c levels and infection rate. Some studies that have been carried out have shown that HbA1c level has no effect on the rate of wound infection 28,30,32,39,41. By contrast, far more studies have demonstrated that elevated HbA1c levels are associated with increased rate of wound infection20,21,23,26,29,34–37,40,42. The main outcomes from these studies are summarised in Table 3 and Table 4.
Biskupski et al 39 analysed outcomes in 350 patients who were stratified into three groups according to HbA1c levels: <7.0%, 7.0-8.0%, and >8.0%. The study found that there was no significant difference in the incidence of wound infection for the group with HbA1c <7.0% when compared to those with HbA1c 7.0-8.0% and to those with HbA1c >8.0% (1.53% vs 3.40%, P= 0.57 and 1.53% vs 5.97%, P= 0.13 respectively). These findings were replicated in two further studies which showed that HbA1c level had no effect on the incidence of wound infection. In a study where the primary outcome analysed was the incidence of wound infections, it was found that there was no statistically significant difference in the rate of all subset of wound infection in patients with HbA1c <7.0% and in those with HbA1c >7.0% (P=0.431 and P=0.744 for sternal and non-sternal infections respectively) 42.
By contrast, Gatti et al 41 specifically looked at whether HbA1c was a risk factor for sternal wound infection following CABG surgery in 2,130 patients and found that the mean baseline HbA1c level was significantly higher in patients who had sternal wound infection (54±17mmol/mol vs 45±13mmol/mol; P< 0.0001). Logistic regression showed that HbA1c level was an independent risk factor for sternal wound infection (OR 1.04; 95% CI 1.02-1.05;P< 0.0001) with an HbA1c level greater than 8.6% being associated with the highest risk of sternal wound infection (OR 5.01; 95% CI 2.47-10.15). Similar results were shown in a range of other studies which all showed that pre-operative elevated HbA1c was associated with increased incidence of both sternal and non-sternal wound infection.
Overall the evidence suggests that elevated HbA1c level is associated with an increased incidence of wound infection in cardiac surgery patients but it is unclear if there is a specific level of HbA1c associated with this increased incidence which could serve as a target for pre-operative glycaemic control as studies used a range of HbA1c cut-offs in their analyses. Yet, further work is required to determine the exact cut-off level and association of HbA1C level and non-sternal wound infections.