Discussion
In this study, we found that lower SES, using the surrogate of median area income, was strongly associated with DKA readmission. Similarly, insurance status, an established social determinant of health that is closely linked to SES, was the strongest predictor of DKA readmission. To place these findings in context, a patient from an area with the lowest income quartile would be expected to have a 46% increase in the odds of four or more DKA readmissions in a given calendar year, while a patient with Medicare insurance would have over a 3-fold increased odds of this outcome.
The inverse association with median income and DKA readmission in this study expands on previous findings from the Type 1 Diabetes Exchange Registry, which showed nearly 2-fold increased odds of at least one DKA admission in those with a household income of US$35 000 or less compared with those with US$75 000 or more.12 Unlike that study, which was conducted in an ambulatory population with type 1 diabetes where the outcome was a single DKA admission (most of which were patient reported), our study was focused on confirmed DKA readmissions, which represents a higher risk outcome. Additionally, in that study, extremes of income were evaluated, while the present study used a greater number of income categories, which are normalized to the general US population over time and allow inferences to be drawn regarding the magnitude and strength of the association. Our findings also align with those obtained in a UK population which showed a 2-fold risk of DKA admission in patients from more deprived areas.10
With respect to insurance status, only one other study has evaluated the association of DKA readmission and payer in adults with diabetes; however, this study included a mix of adult patients with type 1 and type 2 diabetes limited to the Chicago area and only reported proportions of patients with different payers without any adjusted measure of association. In that study, among patients with 2–3 DKA admissions, 19% were insured by Medicaid and 22% by Medicare, while only 18% had private insurance. In comparison, among the equivalent group in our study (1–3 readmissions), 34% were insured by Medicaid, 17% by Medicare, and 23% by private insurance. Thus, compared with the Chicago study, there was a much larger proportion of patients insured by Medicaid nationally who had a moderate number of DKA readmissions.
In the type 1 diabetes population (Type 1 Diabetes Exchange Registry), non-private insurance status was associated with 1.5-fold and 2-fold increases in odds of at least one admission for DKA in pediatric and adult patients, respectively; again, DKA readmissions was not an outcome in those studies and no further classification of the non-private insurance group was reported.12 22 Interestingly, in our study, patients with Medicare or Medicaid were at higher risk of readmission than those who likely had no insurance coverage (self-pay). Similar patterns have been noted in other patient populations. In a mixed type 1 and type 2 diabetes population, Mays et al found that patients with recurrent DKA admission (eg, ≥4 admissions) were more likely to have Medicare (25.6%) and Medicaid (29.4%) insurance than self-pay (10.9%). Hasegawa et al showed that in chronic obstructive pulmonary disease (COPD), when compared with patients with private insurance, Medicaid patients had the highest risk of repeated emergency department (ED) visits (OR 1.77 and 2.92 for >2 and >3 ED visits, respectively). Patients with Medicare had only a slightly lower odds of readmission than Medicaid patients, and self-pay patients had an OR of 1.11 and 1.05 of >2 and >3 ED visits.23 In both our study and this COPD study, self-pay patients had a lower odds than those with public insurance which has policy implications. Given that the uninsured population is thought to be an even higher risk population, the reason for this finding is unclear.
Other notable predictors of DKA readmissions identified in this study were age, female sex, and leaving the hospital AMA. Each 1-year increase in age was associated with a 7% decrease in the odds of four or more DKA readmissions. The inverse association of age and DKA admission has been previously described, although the effect sizes were not as marked as in this study,10 24 likely owing to the difference in outcome as we selected for a higher risk group. Female sex has generally been shown to be a risk factor for DKA admissions.10 24 The reasons for this are not readily apparent. Several possible explanations include (1) men are less likely to access medical services, (2) women may intentionally avoid glycemic control for weight gain, and 3) hormonal effects on glucose regulation. Lastly, the caregiver role of many women can sometime inadvertently result in self-neglect.25
In contrast to the pediatric population, considerably less attention has been devoted to DKA readmissions in adults with type 1 diabetes, especially in the USA.2 4–6 9 26–29 While recurrent DKA admissions has been thought to be a phenomenon that typically resolves by adulthood,27 we demonstrated that 20% of adults with type 1 diabetes had at least one DKA readmission in a given calendar year. Despite the fact that recurrent DKA admissions occur more often in younger adults, mortality in a Chicago study was reported to be 16% in ≥4 DKA admissions at 6-year follow-up2 and in a UK study mortality was found to be 23% in those with ≥5 DKA admissions at a median 2.4-year follow-up.1 Given the staggering mortality statistics and implications of long-term complications from chronically poor glycemic control, identifying and characterizing this high-risk population of patients and intervening may improve clinical outcomes, limit acute care utilization, and reduce healthcare expenditures.
A main strength of this study was the use of a very large sample dataset that is representative at a national level of adults with type 1 diabetes. Hospital records were used to determine DKA admissions, serving as a more reliable source of DKA admission than self-report. The main limitation of this study was that we were only able to follow readmissions within a calendar year given the design of the NRD; thus, this study likely underestimates the number of readmissions in patients who were admitted in the latter portions of the calendar year and may mask seasonal variation in readmission trends. Furthermore, the lack of a consistent patient identifier across calendar years precluded our ability to report unique patients admitted across the study period. The NRD did not include information on race; thus, we were unable to evaluate interactions between race and SES. This is particularly important since ketosis-prone diabetes is more prevalent in ethnic minorities. Although these patients are often phenotyped as type 2 diabetes, it is possible that they may be misclassified as type 1 diabetes, which may result in bias, as their risk for recurrent DKA may be different. The absence of medications and laboratory data in the NRD precluded evaluation of severity of DKA, precipitating causes, and management approaches and their role in readmissions. Lastly, given the large database, some findings may be statistically significant but not necessarily be clinically relevant. Although it may be difficult to qualitatively describe the clinical impact of an individual variable, at a population level differences in proportions on the order of 5% or more may be relevant or at least draw attention to the need for further investigation.
In conclusion, lower median income and Medicare and Medicaid insurance were strong predictors of recurrent DKA admissions in adults with type 1 diabetes. Further studies are needed to understand how these socioeconomic factors mediate this association and to identify strategies to overcome disparities in this potentially life-threatening outcome.