By Michael Vlessides

San Diego—Although the American Society of Anesthesiologists’ physical status classification system has proven to be an important predictive tool, its value may be degraded by consistent “up-classification” of obese patients. A recent analysis of more than 300,000 patients found that obese individuals are more likely to be classified as ASA II to IV than their normal-weight counterparts, regardless of their overall health.

“One of the things that has haunted ASA classification over the years is the question of interobserver variability,” said Christopher H. Guerry, MD, an anesthesia resident at Virginia Commonwealth University in Richmond. “And it is in the setting of obesity in which we find some of the greatest interobserver variability.”

Given that such inconsistencies can degrade the predictive value of surgical outcome models that use ASA physical status as a risk input, Dr. Guerry and his colleagues examined trends in the classification system across body mass index (BMI)-based categories between 1986 and 2010 in both emergent and nonemergent surgical cases. Researchers stratified 302,829 patients (ages 18-89 years; mean, 48) according to BMI, after which they determined the relative risk for up-classification according to ASA physical status.

As Dr. Guerry reported at the 2013 annual meeting of the International Anesthesia Research Society (abstract S-153), 10.9% of patients were classified as ASA I, 47.7% as ASA II, 32.5% as ASA III, 8.4% as ASA IV and 0.6% as ASA V. Over the study period, the percentage of surgical patients who were of normal weight fell from 42% to 29%, whereas the proportion of obese patients rose from 20% to 39%.

Compared with normal-weight patients, individuals with higher BMI were more likely to be up-classified on the ASA scale.

“One of the arguments you can make is that an increasing prevalence of comorbidities is a big part of increasing BMI, which would obviously be part of the classical definition of ASA physical status,” Dr. Guerry said. “So we looked at a subgroup of patients aged 18 to 25, the ones in which a diagnosis of comorbidities would be less likely.”

Yet the up-classification association was more pronounced for these younger patients.

“Although the ASA classification system was not originally designed to be a risk predictor, there has been a lot of interest recently in coming up with complex models for predicting outcomes with it,” Dr. Guerry said. And as it turns out, the system has been highly predictive.

“But obesity adds inconsistency to the classification system, and with obesity rates increasing, we’re introducing a certain amount of uncertainty into these predictive models,” he continued. “So we can spend time debating whether or not it’s legitimate to up-classify obese patients, but the most important thing is that as a professional community we need to be consistent. Once consistent, then we can go back and look at the predictive value of the ASA classification system in these patients.”

Charles B. Watson, MD, chief of anesthesia and deputy surgeon in chief at Bridgeport Hospital in Bridgeport, Conn., said there are legitimate reasons for counting marked obesity as a factor in assigning an ASA physical status.

“The ASA [classification] was designed to predict the difficulty of the anesthetic, not outcome, and many of us believe that very obese patients are more difficult to care for,” Dr. Watson said. “Many arguments for and against the issue of whether obese patients, smokers, infants and others should be ASA I or II have been bandied about over the years. Most of us believe that very heavy patients are more likely to have comorbidities and worse outcomes, although there are published series showing ‘acceptable’ preoperative morbidity in this population.”