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October 2007

Improving the Assessment of the Relationship Between Body Mass Index and Hospital Discharge After Ischemic Stroke

Arch Neurol. 2007;64(10):1545-1546. doi:10.1001/archneur.64.10.1545-c

Razinia et al1 report that “[e]levated BMI [body mass index] is associated with a lower likelihood of being discharged home and a trend toward extended hospital stay.” However, this study suffers from several methodological drawbacks. Patients are divided into 4 prespecified groups based on the US Preventive Services Task Force, and statistical analyses are based on this categorization. The categorization of continuous exposures like BMI (calculated as weight in kilograms divided by height in meters squared) can lead to a biased estimation of the exposure-outcome relationship.2 Filardo et al3 have shown that using categorization when describing the relationship between BMI and postsurgical outcomes can misrepresent this relationship completely. Moreover, low patient counts for specific BMI categories can affect the statistical power of the analysis. For example, the adjusted comparison of discharge outcome between class I obesity and normal (which compares 61 patients with 208 patients) is significant at the .05 level (odds ratio, 0.46; 95% confidence interval, 0.22-0.96) while the smaller odds ratio describing the comparison between class II obesity and normal (which compares only 19 patients with 208 patients) is not (odds ratio, 0.42; 95% confidence interval, 0.13-1.37). Also, all subjects with BMIs of less than 25 are contained in the normal group, including any subjects with cachexia. Because patients with cachexia may suffer worse outcomes than patients with less extreme BMIs,4 the cutoff of 25 may combine patients with very different risk profiles. Smoothing techniques can avoid these inherent drawbacks of categorization.2,3 Finally, the suggestion that “higher BMI at the time of hospital admission for ischemic stroke is associated with a lower likelihood of being discharged directly home and longer stay in the hospital stroke service” contrasts with the fact that the overweight group has the best estimated risk (adjusted) for all 3 discharge outcomes.

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