Clinical Laboratory Strategies: November 11, 2010

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Exploring the Relationship between Hyperglycemia and Surgical Site Infection
Study Suggests Potential Glycemic Target
By Genna Rollins

Numerous studies have examined the impact of tight glycemic control in different populations of hospitalized patients, with varying results. Some have found benefits such as reduced surgical site infections (SSI), while others have reported harms like increased mortality. Now, new research examines the impact of perioperative hyperglycemia on the incidence of SSI in general and vascular surgery patients. This issue of Strategies explores those findings.

The effects of tight glycemic control have been studied in several populations of hospitalized patients, including cardiac, medical, and surgical intensive care units (ICUs), and in specific subgroups of these populations, such as diabetics undergoing abdominal or cardiac surgery, or individuals who had infrainguinal vascular surgery. The results of these analyses have been mixed. Some studies have reported higher infection rates in hyperglycemic patients compared to those with lower glucose levels. On the flip side, and perhaps most notably, the Normoglycemia in Intensive Care Evaluation-Survival Using Glucose Algorithm Regulation (NICE-SUGAR) trial recently found that intensive glycemic control with a target of 81-108 mg/dL in medical and surgical ICU patients was associated with higher mortality.

Despite the controversy surrounding tight glycemic control, intensive insulin therapy is common in cardiac and surgical ICUs, according to the authors of a new study that examined whether perioperative hyperglycemia was associated with increased incidence of SSI, independent of pre-existing diabetes (Arch Surg 2010;145:858-864). “Quite a while ago, there was a breakthrough study that showed aggressive treatment of post-op hyperglycemia was associated with significantly reduced infections and other favorable outcomes,” explained lead author Ashar Ata, MBBS, MPH, research instructor at Albany Medical College in Albany, N.Y. “Since then, it’s become normal in cardiovascular surgery that post-op glucose is excessively controlled.”

Ata also pointed out that post-operative SSI is one of the most common nosocomial infections among surgery patients and that much attention nationally and in individual hospitals has been focused on reducing SSI. For example, the Surgical Care Improvement Project, a national quality partnership of organizations aimed at improving surgical care by significantly reducing surgical complications, put forth six performance improvement measures that involve SSI prevention. The Centers for Medicare and Medicaid Services has proposed using these measures for value-based purchasing and payment to hospitals.

Given both the controversy surrounding intensive glycemic control in hospitalized patients and the concern about reducing SSI, Ata and his colleagues performed a retrospective study on general and vascular surgery patients at Albany Medical Center to look at the relationship between hyperglycemia and SSI. The researchers abstracted data from the American College of Surgeon’s National Surgical Quality Improvement Program (ACS-NSQIP) database for patients who had surgery at Albany Medical Center during a 2 ½-year period. Because ACS-NSQIP does not collect data on pre-, intra-, or post-operative blood glucose levels, the investigators abstracted this information through medical records reviews. Overall, they found post-operative serum glucose levels for 74.7% of patients, 51.4% of which had been obtained within 12 hours after surgery.

Across the entire study population, the SSI rate was 7.42%, but rates were significantly higher for colorectal (14.11%) and vascular (10.32%) surgery patients than for noncolorectal general surgery patients (4.36%). The researchers considered numerous risk factors for SSI, including increasing age, emergency surgery, and diabetes, but when they adjusted for postoperative serum glucose levels, all the other risk factors were no longer significant predictors. Furthermore, a subanalysis of colorectal surgery patients found that a postoperative serum glucose level ≥140 mg/dL was the only significant predictor of SSI.

For the whole study cohort, the researchers found a graded relationship between rising serum glucose levels and SSI incidence rates. Starting with glucose levels ≤110 mg/dL, the incidence of SSI was 1.8%, but rose to 17.7% at glucose levels ≥220 mg/dL. In comparison to patients with a first post-operative glucose level ≤110 mg/dL, the likelihood of SSI increased 3.61 fold in patients with a first post-operative glucose level 111-140 mg/dL up to 12.13 fold in those with levels ≥220 mg/dL.

“Before our study, physicians would become worried or intervene when glucose levels reached 220 mg/dL, or sometimes at 180 mg/dL. Intervening before the patient had those levels was referred to as aggressive or intensive management,” said Ata. “But we found in our study that maybe 140 mg/dL is the time when you’d want to intervene, and maybe that’s what the threshold should be.”

Based upon these findings, Albany Medical Center already has started a prospective study of general surgery patients using a cutpoint of 140 mg/dL. Patients with glucose levels >140 mg/dL will be evaluated by a consulting endocrinologist. The prospective study also has strict criteria for obtaining samples for serum glucose tests to address a shortcoming of the retrospective study. “We hope that by obtaining all the post-op glucose readings within the first six hours after surgery, it’ll make the picture clearer,” said Ata.

The study results already have prompted greater scrutiny of serum glucose levels within the surgical community, according to Joseph Frankhouse, MD, a colorectal surgeon at Legacy Health System in Portland, Ore., and author of an editorial that accompanied Ata’s study. “The study had some drawbacks, but it’s laid a good foundation as we move forward in trying to implement protocols to decrease surgical site infections and improve quality control,” he observed. Frankhouse indicated that a consortium of hospitals in Oregon and Washington are considering adding glucose levels as a parameter for surgical quality improvement initiatives. “Even though this article suggested 140 mg/dL as an important goal, I don’t think the data is strong enough to suggest that it’s a magic number,” he said. “But it’s a starting point that can be further refined. We need to look at further studies down the road to see if other cutpoints are better.”

Ata agreed that more data will be needed before any definitive conclusions can be drawn. “It’s critical that the results of our study be interpreted with caution,” he said. “It’s important to know that it’s one hospital’s experience and our findings were based on retrospective data. It may ultimately prove to be the truth, but it has to be validated by more studies in more diverse settings.”

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