Hospital standardized mortality ratios: sensitivity analyses on the impact of coding

Abstract

Introduction: Hospital standardized mortality ratios (HSMRs) are derived from administrative databases and cover 80 percent of in-hospital deaths with adjustment for available case mix variables. They have been criticized for being sensitive to issues such as clinical coding but on the basis of limited quantitative evidence.

Methods: In a set of sensitivity analyses, we compared regular HSMRs with HSMRs resulting from a variety of changes, such as a patient-based measure, not adjusting for comorbidity, not adjusting for palliative care, excluding unplanned zero-day stays ending in live discharge, and using more or fewer diagnoses.

Results: Overall, regular and variant HSMRs were highly correlated (ρ>0.8), but differences of up to 10 points were common. Two hospitals were particularly affected when palliative care was excluded from the risk models. Excluding unplanned stays ending in same-day live discharge had the least impact despite their high frequency. The largest impacts were seen when capturing postdischarge deaths and using just five high-mortality diagnosis groups.

Conclusions: HSMRs in most hospitals changed by only small amounts from the various adjustment methods tried here, though small-to-medium changes were not uncommon. However, the position relative to funnel plot control limits could move in a significant minority even with modest changes in the HSMR.

Figure 1

Differences in Hospital Standardized Mortality Ratios (HSMRs) for 2008/2009 for Acute Non-Specialist Trusts between the Regular Models and Those without Adjustment for Comorbidity (Expected Counts Were Derived from Models Using Data for 2005/2006 to 2008/2009)

Figure 2

Hospital Standardized Mortality Ratios (HSMRs) for 2008/2009 for Acute Non-Specialist Trusts: Taking the First Admission per Patient versus Taking the Last Admission per Patient (Expected Counts Were Derived from Models Using Data for 2005/2006 to 2008/2009)