Spurious inferences about long-term outcomes: the case of severe sepsis and geriatric conditions

Abstract

Rationale: Survivors of critical illness suffer significant limitations and disabilities.

Objectives: Ascertain whether severe sepsis is associated with increased risk of so-called geriatric conditions (injurious falls, low body mass index [BMI], incontinence, vision loss, hearing loss, and chronic pain) and whether this association is measured consistently across three different study designs.

Methods: Patients with severe sepsis were identified in the Health and Retirement Study, a nationally representative cohort interviewed every 2 years, 1998 to 2006, and in linked Medicare claims. Three comparators were used to assess an association of severe sepsis with geriatric conditions in survivors: the prevalence in the United States population aged 65 years and older, survivors' own pre-sepsis levels assessed before hospitalization, or survivors' own pre-sepsis trajectory.

Measurements and main results: Six hundred twenty-three severe sepsis hospitalizations were followed a median of 0.92 years. When compared with the 65 years and older population, surviving severe sepsis was associated with increased rates of low BMI, injurious falls, incontinence, and vision loss. Results were similar when comparing survivors to their own pre-sepsis levels. The association of low BMI and severe sepsis persisted when controlling for patients' pre-sepsis trajectories, but there was no association of severe sepsis with injurious falls, incontinence, vision loss, hearing loss, and chronic pain after such controls.

Conclusions: Geriatric conditions are common after severe sepsis. However, severe sepsis is associated with increased rates of only a subset of geriatric conditions, not all. In studying outcomes after acute illness, failing to measure and control for both preillness levels and trajectories may result in erroneous conclusions.

Figure 1.

Prevalence of geriatric conditions among survivors of severe sepsis, unadjusted. The −1 survey is the last survey conducted before severe sepsis (median, 1.19 yr before sepsis; interquartile range [IQR]: 0.7–1.7 yr), the −2 is the second-to-last (median, 3.21 yr before sepsis; IQR, 2.7–3.7 yr), and the −3 survey was a median of 5.16 yr before sepsis (IQR, 4.7–5.6 yr). The post-sepsis survey was a median of 0.92 yr after severe sepsis (IQR, 0.4–1.4 yr). BMI = body mass index.

Figure 2.

Reciprocal causation. In long-term outcome studies, our primary interest is typically in estimating X1. Note that X2 is usually neglected, and X3 and X4 are of key interest when considering truncation by death.