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This article presents methods for using administrative data to study multimorbidity in hospitalized individuals and indicates how the findings can be used to gain a deeper understanding of hospital multimorbidity.

A Dutch nationwide hospital register (n=4,521,856) was used to calculate age- and sex-standardized observed/expected ratios of disease-pairing prevalences with corresponding confidence intervals.

The strongest association was found for the combination between alcoholic liver and mental disorders due to alcohol abuse (observed/expected=39.2). Septicemia was found to cluster most frequently with other diseases. The consistency of the ratios over time depended on the number of observed cases. Furthermore, the ratios also depend on the length of the time frame considered.

Using observed/expected ratios calculated from the …