We included a total of 32,835 unique individuals in the study cohort (Table 1; eFig. 1, Online Appendix). The mean age was 62 ± 17 years; over half were women, and individuals were diverse in race/ethnicity and primary insurance status. Most individuals had non-elective admissions, no previous ED visits or hospitalizations in the prior year, and had a Charlson comorbidity index of 0. The overall median length of stay was 4 days (IQR 2–6 days).
Types and Rates of Vital Sign Instabilities on Discharge
Overall, 6127 individuals (18.7 %) had one or more instabilities on discharge. Elevated heart rate was the most common, affecting 10.4 % of the study population (Table 2). The frequency of other specific instabilities and combinations of instabilities is shown in eTable 1 (Online Appendix). Combinations involving elevated temperature had the lowest unadjusted 30-day composite and mortality rates, but only a small number of individuals had a combination of instabilities, limiting further analysis. There were no significant differences in rates of instability on discharge among the six study sites (range 17.9 %-20.3 %).
In the 30 days after discharge, 4484 (13.7 %) individuals had a readmission or death. The median time to the composite outcome was 12 days (IQR 5–20 days). A total of 4163 individuals were readmitted within 30 days (12.7 %), with a median time to readmission of 11 days (IQR 5–19 days). Only 535 individuals died within 30 days of discharge (1.6 %), with a median time to death of 16 days (IQR 9–24 days).
Associations Between Vital Sign Instability on Discharge and Outcomes
Figure 1 shows that the greater the number of instabilities on discharge, the greater risk of death and/or readmission. Overall, 12.8 % of individuals with no instabilities on discharge died or were readmitted, compared to 16.9 % with one instability, 21.2 % with two instabilities, and 26.0 % with three or more instabilities (p < 0.001).
Even after controlling for numerous other prognostic factors and potential confounders, including demographic, clinical, and utilization characteristics, we found a dose-dependent relationship between the number of vital sign instabilities and odds for 30-day post-discharge adverse events (Table 3; eTable 2, Online Appendix). The relationship was most striking for mortality, with the odds of death doubling, tripling, and quadrupling with each additional instability. Individuals with any instability on discharge had higher adjusted odds of the composite outcome (AOR 1.36, 95 % CI 1.26–1.48), death (AOR 2.36, 95 % CI 1.97–2.83), and readmission (AOR 1.36, 95 % 1.26–1.47) compared to those discharged with stable vital signs (Table 3).
Associations Between Instability on Discharge and Disposition Status
The greater the number of vital sign instabilities an individual had on hospital discharge, the more likely they were to be discharged to a post-acute care facility (which included nursing homes, skilled nursing facilities, and long-term acute care facilities; eFig. 2, Online Appendix). Only 18.1 % of individuals with no instabilities were discharged to post-acute care, compared to 21.5 % of those with one instability, 26.7 % of those with two instabilities, and 42.7 % of those with three or more instabilities (p value <0.001 for trend). Although instability on discharge remained a significant predictor of death and readmission irrespective of disposition status, rates of post-discharge adverse events were uniformly much higher among those discharged to a post-acute care facility compared to those discharged home (p value <0.001 for interaction). For those with two instabilities, 12.3 % of individuals discharged to a post-acute care facility died and 18.7 % were readmitted within 30-days, compared to 30-day mortality and readmission rates of 1.6 % and 17.1 %, respectively, among those discharged home (p < 0.001 for both comparisons).
Onset of Vital Sign Instabilities
The proportion of patients with the same instability present on admission ranged from 38 % for fever to 76 % for tachycardia (eTable 3, Online Appendix). However, rates of post-discharge adverse outcomes did not differ by whether the instability was also present on admission.
Assessment of Instability on Discharge as a Diagnostic Test for Adverse Events
From a clinical perspective, individual physicians or hospitals may want to use a specific definition of instability to help gauge safety and appropriateness of discharge. Thus, vital sign instability criteria may be considered a type of diagnostic test for future adverse events. The test characteristics, including sensitivity, specificity, positive and negative predictive values, and positive and negative likelihood ratios, for various cutoffs used to define instability are displayed in Table 4. A greater number of vital sign instabilities is more helpful in predicting death within 30 days (LR+ 3.03, 95 % CI 2.25–4.11 for ≥2 instabilities) than readmission (LR+ 1.50, 95 % CI 1.26–1.78 for ≥2 instabilities). However, the absence of vital sign instability does not necessarily rule out these outcomes (i.e., LR− range of 0.8–1.0).