Assessing the validity of post-discharge readmission and mortality as a composite outcome among newborns in Uganda
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Background
Composite outcomes, which include mortality and readmission rates, are often used in risk prediction models following hospital discharge when event rates for the primary outcome of interest, mortality, are low. However, increased readmission rates may result in decreased mortality making interpretation of the composite outcome difficult. We assess the usefulness of a composite outcome of post-discharge readmission and mortality as a target outcome in this context.
Methods
This was a secondary analysis of data collected among mothers and their newborn(s) admitted for delivery at two regional referral hospitals in Uganda. Six-week post-discharge mortality (all-cause) and readmission in newborn infants were analyzed using a competing risk framework. The Sub distribution Hazard Ratios (SHRs) were compared across predictor variables to examine the relationship between the two outcomes.
Results
Of the 206 predictors, 81 had a consistent association with both outcomes. These include a higher weight (Mortality SHR: 0.14, Readmission SHR: 0.68) and length of the baby (Mortality SHR: 0.85, Readmission SHR: 0.91). However, 125 variables depicted an association in opposing directions for both outcomes which may be linked to social and financial barriers to care-seeking. These include a travel time to the hospital of greater than 1 hour (Mortality SHR: 1.4, Readmission SHR: 0.28).
Conclusion
While mortality is unequivocally a negative outcome, readmission may be a positive outcome, reflecting health seeking, or a negative outcome, reflecting recurrent illness. This directional dichotomy is reflected to varying degrees within different variables. When using a composite outcome for a prediction model, caution should be exercised to ensure that the model identifies individuals at risk of the intended outcomes of interest, rather than merely the proxies used to represent those outcomes. Identifying predictors with a consistent relationship for both outcomes may yield a more optimized and less biased prediction model for use in clinical care.
