Author: Jan van der Laan, Corine Witvliet-Penning, Suzanne Gerritsen, Agnes de Bruin
HSMR 2022 Methodological Report

2. Method changes

This chapter summarizes the changes in the HSMR method (HSMR 2022) compared to the method used last year (HSMR 2021). For previous changes see the respective methodological reports (CBS, 2011, …, 2022). Overall, the method has remained the same. Only a few changes related to selection of the data and related to the covariates have been implemented. These changes are described below.

2.1 Inclusion of COVID-19 admissions of 2022

The COVID-19 pandemic significantly impacted hospital care in 2020 and 2021. Because of the different nature of admissions with COVID-19 as a main diagnosis compared to other admissions and the lack of specific expertise for treating COVID-19 during the first months of the pandemic, the COVID-19 admissions were excluded from the HSMR models of 2020 and 2021. This decision was made in agreement with the Dutch Healthcare Authority (NZa) and the Health and Youth Care Inspectorate (IGJ). In 2022 Statistics Netherlands developed a COVID-19 SMR model using 2021 data, which had good predictive power (CBS, 2022). Because of this, and since hospitals have gained experience in treating COVID-19 patients and COVID-19 admissions have been more integrated in regular hospital care in 2022, the newly developed COVID-19 model has now been integrated in the overall HSMR model. The definitions of the covariates in the COVID-19 model is the same as for the models of the other diagnosis groups, except for the covariates “Severity of the main diagnosis” and “Month of admission” (see section 3.4).

Admissions with a main diagnosis of COVID-19 include those with ICD-10 code U07.1 (COVID-19, virus identified (lab confirmed)), U07.2 (COVID-19, virus not identified (clinically diagnosed)) and U10.9 (Multisystem inflammatory syndrome associated with COVID-19, unspecified). In accordance with the HSMR 2020 and 2021, the COVID-19 admissions of 2020 and 2021 were excluded from the present HSMR model which comprises the years 2019-2022. The COVID-19 admissions of 2020 and 2021 were also excluded from the dataset that was used to evaluate data quality and case mix (see section 3.5). For 2022 the COVID-19 admissions were included in the HSMR model and the additional analyses. 

Since COVID-19 admissions were only included for the year 2022 (and not for the years 2020 and 2021), the HSMR value of 2022 might be less comparable to the values of the previous years. In section 4.5 the influence of the inclusion of the COVID-19 admissions on the HSMR outcomes is described.

2.2 Exclusion of admissions of healthy persons

From now on all admissions of so-called ‘healthy persons’ are excluded from the HSMR model (for all model years) and additional analyses. 

Admissions of healthy persons are admissions of healthy newborns, healthy parents accompanying sick children, or other healthy boarders. These admissions are identified by the main diagnosis of the admission (ICD-10 code Z76.2-Z76.4) and/or the registration of procedure codes for a stay of a healthy newborn or healthy mother for each individual day of the admission ((Dutch procedure codes 190032, 190033 (‘Zorgactiviteiten’ codes), 339911 or 339912 (‘CBV’ codes)).

Since the admissions of healthy persons are non-medical admissions it is not relevant to include these admissions in the HSMR. These admissions are also excluded from other indicators based on the LBZ, such as the hospital readmission rate. 

2.3 Updated socioeconomic status (SES) scores for all model years

In the previous HSMR model of 2021 a new variable was used for the socioeconomic status (SES) covariate (see CBS, 2022). This new variable, based on the so-called ‘SES-WOA’ score calculated by Statistics Netherlands (Arts et al., 2022), was introduced because the old variable of the Netherlands Institute for Social Research (SCP) is not updated anymore by SCP. In the HSMR 2021 model the new variable was only applied to the data of 2021. For the earlier years in the model the SES covariate was still based on the SCP variable. However, due to differences in the calculation of the scores, the SCP and SES-WOA scores are not fully comparable.

Therefore, in the current HSMR 2022 model the new SES-WOA variable was applied to all model years (2019-2022). For the model years 2019-2021 the SES-WOA scores of 2019 were used and for model year 2022 the SES-WOA scores of 2022 were applied. Further details of the SES-WOA variable are described in section 3.4.