HbA1c and mortality rates
As the incidence of Diabetes Mellitus (DM) increases, the proportion of people with DM undergoing cardiac surgery has also increased17. There exists a large body of evidence which has looked at the association between patient HbA1c levels and mortality following cardiac surgery. The evidence on the effect of HbA1c levels on mortality is contradictory with some studies reporting that increased HbA1c levels are associated with increased mortality18–25. But by in large, most studies seem to show that increased HbA1c is not predictive of increased mortality as solo indicator 26–40. The findings from all these studies are summarised in Table 1 and Table 2.
The study with the largest patient cohort looked at outcomes in 6,313 patients with type 2 DM who underwent CABG surgery between 2003 and 201318. The study found that HbA1c was associated with an increased risk of death in patients with HbA1c level between 9.1-10.0% (Hazard ratio (HR) 1.26; 95% CI 1.04-1.53), and this risk was even greater in patients with HbA1c >10.0% (HR 1.33; 95% CI 1.05-1.69). Other studies similarly showed that elevated HbA1c was associated with increased mortality 19–25. One study showed that 30-day mortality was significantly higher in patients with HbA1c greater than 6.5% compared to those with Hb1ac <6.5% (4.22% vs 3.07%; P= 0.0035), however following multivariable adjustment this association was lost and there was no significant difference in the mortality rates 26.
All these studies, overall, demonstrates that increased HbA1c increases mortality in cardiac surgery patients and all of them, with the exception of one, had large sample sizes meaning they were all by in large sufficiently statistically powered to make their findings significant. However, all the aforementioned studies were retrospective in nature meaning they were liable to biases inherent in the study design and the studies do not demonstrate causality.
The largest cohort study demonstrating that there is no relationship between HbA1c levels and mortality looked at outcomes in 6,393 patients undergoing cardiac surgery 27. The study found that HbA1c was not a significant predictor of post-operative mortality (P= 0.88). Further studies similarly showed that there was no significant difference in mortality following cardiac surgery according to HbA1c levels, regardless of the HbA1c cut-off levels utilised28–32,34–40.
Overall the evidence showing that there is no significant difference between higher HbA1c levels in the incidence of mortality outweighs evidence showing that elevated HbA1c is associated with increased mortality, suggesting that HbA1c alone may not be predictive of mortality following cardiac surgery.