Severity of Illness Scores May Misclassify Critically Ill Obese Patients

Rodrigo Octávio Deliberato, MD, PhD; Stephanie Ko, MBBS; Matthieu Komorowski, MD; M. A. Armengol de La Hoz, MA; Maria P. Frushicheva, PhD; Jesse D. Raffa, PhD; Alistair E. W. Johnson, DPhil; Leo Anthony Celi, MD, MPH; David J. Stone, MD


Crit Care Med. 2018;46(3):394-400. 

In This Article

Abstract and Introduction


Objective: Severity of illness scores rest on the assumption that patients have normal physiologic values at baseline and that patients with similar severity of illness scores have the same degree of deviation from their usual state. Prior studies have reported differences in baseline physiology, including laboratory markers, between obese and normal weight individuals, but these differences have not been analyzed in the ICU. We compared deviation from baseline of pertinent ICU laboratory test results between obese and normal weight patients, adjusted for the severity of illness.

Design: Retrospective cohort study in a large ICU database.

Setting: Tertiary teaching hospital.

Patients: Obese and normal weight patients who had laboratory results documented between 3 days and 1 year prior to hospital admission.

Interventions: None.

Measurements and Main Results: Seven hundred sixty-nine normal weight patients were compared with 1,258 obese patients. After adjusting for the severity of illness score, age, comorbidity index, baseline laboratory result, and ICU type, the following deviations were found to be statistically significant: WBC 0.80 (95% CI, 0.27–1.33) × 109/L; p = 0.003; log (blood urea nitrogen) 0.01 (95% CI, 0.00–0.02); p = 0.014; log (creatinine) 0.03 (95% CI, 0.02–0.05), p < 0.001; with all deviations higher in obese patients. A logistic regression analysis suggested that after adjusting for age and severity of illness at least one of these deviations had a statistically significant effect on hospital mortality (p = 0.009).

Conclusions: Among patients with the same severity of illness score, we detected clinically small but significant deviations in WBC, creatinine, and blood urea nitrogen from baseline in obese compared with normal weight patients. These small deviations are likely to be increasingly important as bigger data are analyzed in increasingly precise ways. Recognition of the extent to which all critically ill patients may deviate from their own baseline may improve the objectivity, precision, and generalizability of ICU mortality prediction and severity adjustment models.


Obesity is currently a global pandemic, responsible for 3–4 million deaths per year,[1] with an increasing prevalence in adults as well as children and adolescents.[2] Obesity is overrepresented in the ICU, comprising approximately one-third of patients,[3] compared with the 20% prevalence of being overweight or obese worldwide. ICUs commonly use severity of illness scores, such as Acute Physiology and Chronic Health Evaluation (APACHE), Simplified Acute Physiology score version II (SAPS-II), or Sequential Organ Failure Assessment (SOFA) to predict mortality,[4–6] but none of these scoring systems incorporates obesity into their risk adjustment variables.

These scores rely on the assumptions that patients have the same normal physiologic values at baseline and that similar severity of illness scores represent the same degree of deviation from the baseline state. However, this may not always hold true for different population groups in different intensive care settings. In obese patients, prior studies have shown abnormal laboratory markers including WBC and liver enzymes,[7,8] platelet counts,[9] and respiratory physiological values.[10,11]

Therefore, although obese and normal weight patients may present to the ICU with a similar physiological "snapshot or phenotype" as reflected by the same severity of illness scores, these identical scores may actually represent inherently different levels of deviation from the prior baseline state. This may inadvertently result in misclassification, leading to potential errors in mortality prediction and severity adjustment. The case of obese ICU patients is made more complex in that, despite their higher prevalence of chronic diseases that would be expected to result in generally higher all-cause mortality compared with normal weight individuals,[2] critically ill obese patients have been reported to paradoxically have better clinical outcomes than nonobese patients.[12]

We postulated that prognostic severity misclassification may be an artifact of applying the same scoring system to these two diverse populations without regard to divergences in evolving anomalies. This question is critical in determining whether the use of conventional scoring systems produce reliable predictions in conditions associated with diverse physiologies, including obesity.

To investigate this question, we analyzed a large ICU database (which included baseline laboratory results prior to hospital admission) to compare the deviation of laboratory tests utilized in scoring systems from baseline to ICU admission in both obese and normal weight patients.