The Potential for Providing Treatment on an Outpatient Rather Than Inpatient Basis

A Nationwide Analysis of Hospital Discharge Data in Germany for the Year 2022

Background: In an expert analysis by the IGES Institute, it was examined which inpatient hospital treatments could also be carried out on an outpatient basis. A method of determining the feasibility of performing any particular treatment in the outpatient setting based on routine documentation was proposed in the report. A new version of the German catalogue of operations that can be performed on an outpatient basis and other outpatient procedures (the AOP catalogue) was issued accordingly. In addition, sector-blind (hybrid DRG) reimbursement of selected treatments was introduced, i.e., the amount of the reimbursement is now the same regardless of whether the treatment is provided on an inpatient or outpatient basis.

Methods: The present analysis is based on German nationwide hospital discharge data from 2022. It was calculated how many inpatient treatments could have been carried out on an outpatient basis according to the criteria of the IGES report and the AOP catalogue, and how many cases would fall under hybrid DRGs.

Results: Of 14.8 million inpatient treatment cases (excluding deliveries and neonates), 7.2% were identified that might have been provided on an outpatient basis according to both models, and a total of 17.3% and 21.8%, respectively, according to the IGES model or the AOP catalogue. The estimated number of potential hybrid DRG cases was 153 000, or 1.0%.

Conclusion: Both the IGES model and the AOP catalogue identified a relevant potential for providing treatment on an outpatient rather than inpatient basis. However, the overlap between the two approaches is small. Further development of the AOP catalogue appears necessary for the more accurate identification of treatment cases that can be provided in the outpatient setting. The extent to which the new AOP catalogue and hybrid DRG remuneration can promote the provision of outpatient rather than inpatient care will need to be determined in future analyses.

Cite this as:

Pioch C, Busse R, Mansky T, Nimptsch U: The potential for performing procedures on an outpatient rather than inpatient basis: A nationwide analysis of hospital discharge data in Germany for the year 2022. Dtsch Arztebl Int 2025; 122: 151–5. DOI: 10.3238/arztebl.m2025.0012

Hospital care in Germany is characterized by the large number of hospital beds and acute inpatient hospital treatment. In 2019 there were six hospital beds (1) and 252 inpatient treatment cases per 1000 population in Germany (2), which is significantly above the European and OECD average of four hospital beds and 146 treatment cases (1, 2). In view of the statutory prioritization of outpatient care (Section 39 of the German Social Code V), staff shortages, and insufficient investment cost funding in the hospital sector, there are calls for more inpatient services to be diverted to the outpatient setting (3, 4, 5, 6).

In 2020, in accordance with Section 115b German Social Code V—Hospital-Based Outpatient Surgery, the contract parties (National Association of Statutory Health Insurance Funds, German Hospital Federation, National Association of Statutory Health Insurance Physicians) commissioned an expert analysis to provide recommendations for expanding the catalogue of operations which can be performed on an outpatient basis and other procedures to replace inpatient care (AOP catalogue). This analysis was drawn up by the IGES Institute and has identified numerous treatment services so far provided on an inpatient basis which could, however, also be feasible without the patient having to stay in hospital overnight (potential for providing treatment on an outpatient rather than inpatient basis). Examples of this include diagnostic endoscopy of the upper gastrointestinal tract, which in 2019 was documented around 1.3 million times under inpatient services, and conservative treatments of cancer, such as radiotherapy, nuclear medicine therapy, and chemotherapy which were performed around 450 000 times on an inpatient basis (7).

The report also developed a process by which the feasibility of outpatient provision on a case-related basis was reviewed using routine documentation. This so-called context factor analysis uses documented diagnoses and procedures to assess whether there are any reasons militating against outpatient treatment.

This then formed the basis for expanding the AOP catalogue and supplementing it with the context factor analysis. This came into effect in 2023 and was again expanded in 2024, yet without fully implementing the recommendations proposed in the IGES report. For example, not all exclusion criteria recommended for context factor analysis were adopted.

At the same time, hybrid DRGs (hybrid diagnosis-related groups) were introduced together with Section 115f of German Social Code V—Special Sector-Specific Reimbursement. This means that, as of 2024, selected treatment services will be reimbursed at the same level, regardless of whether they are provided on an outpatient or inpatient basis. The remuneration amount for a hybrid DRG is calculated based on the relative proportion of outpatient or inpatient services provided and adjusted over time.

The aim of these measures is to encourage the outpatient provision of services that were previously provided unnecessarily on an inpatient basis. In 2022, hospitals in Germany provided a total of 1.9 million operations in the outpatient setting, including procedures replacing inpatient care, while 16.8 million cases were treated on an inpatient basis (8).

The present analysis is based on nationwide hospital discharge data in Germany for the year 2022 to estimate the proportion of treatment cases which can potentially be provided in the outpatient setting according to the criteria of the IGES report and compare it with the proportion based on the 2024 AOP catalogue. This analysis draws on the previous analysis covering the years 2017 to 2021 (9). In addition, the case figures of hybrid DRGs according to the criteria of the 2024 DRG catalogue are also estimated. The aim is to inform clinicians about the potential for providing treatment on an outpatient rather than inpatient basis and to create a data set that could help draw up further measures to allow treatment on an outpatient so far provided on an inpatient basis.

Methods

Data

The microdata of the DRG statistics for 2022 were provided by the Research Data Center of the Federal Statistical Office and evaluated using controlled remote data processing (10). The data cover all acute inpatient cases treated in hospitals involved in the DRG reimbursement system and include for each treatment case its demographic data, reason for admission and discharge, specialist department(s) providing the treatment, primary and secondary diagnoses (International Statistical Classification of Diseases and Related Health Problems, 10th Revision, German modification of the ICD-10), and procedures (Operation and Procedure Codes, German procedure classification [OPS]). Treatment cases relating to deliveries or those involving neonates were excluded (11). This is based on the understanding that outpatient care of a neonate would only be possible if the mother’s delivery were also conducted in the outpatient setting. However, the available data do not allow a link to be created between the treatment cases of mother and neonate.

Potential for providing treatment on an outpatient basis using the IGES model

Using the IGES model (7), treatments that can potentially be provided in the outpatient setting were first identified from defined inclusion lists (Lists A1–A4) containing selected diagnoses, procedures, and DRG codes (Box). In a second step of the context factor analysis, cases were again excluded which are not considered feasible in the outpatient setting based on certain diagnoses, procedures, DRG case groups, mechanical ventilation, or other factors (Lists K1–K3, K5–K8, and the ICD observation status list). Lists K4 (Complexity arising from combinations of mild interventions) and K9 (Inpatient treatment indicated based on social circumstances) were not used due to the lack of sufficient implementation. Instead, the Hospital Frailty Risk Score developed by Gilbert et al. was applied for the context factor “frailty” (K8) (12). The Patient Clinical Complexity Level (PCCL; K7: Complex comorbidity warrants inpatient treatment), which is not included in the DRG statistics, was determined by applying the complication index of the German Inpatient Quality Indicators (13) and when “death” was the reason for discharge (eTable 1).

Box

Criteria for providing outpatient services

AOP catalogue

Using the AOP catalogue, treatments that can potentially be provided in the outpatient setting were identified from the catalogue’s inclusion procedures (OPS codes). In a second step, the exclusion criteria of the AOP catalogue were applied for context factor analysis and cases not feasible for the outpatient setting were subsequently excluded. The context factors of the AOP catalogue contain proportionate codes drawn from the IGES lists K2, K3, K6, and K8. Unlike the IGES model, lower age limits are used as context factors in the 2024 AOP catalogue: This applies to children up to the age of one year and children aged two to 12 with a care level of 2 to 5 or who have a heart defect. Furthermore, OPS codes for bilateral eye procedures and eye operations for certain ocular comorbidities are also defined as exclusion criteria (eTable 1).

eTable 1

Inclusion and exclusion criteria for identifying treatment cases which can potentially be provided in the outpatient setting and (estimated) hybrid DRGs based on Pioch et al. (9)

Hybrid DRGs

Hybrid DRG cases were identified in approximation, based on the corresponding (non-hybrid) DRGs and applying hybrid DRG-specific inclusion and exclusion criteria (including, amongst others, certain diagnoses, procedures, length of stay, ventilation) in accordance with the 2024 G-DRG definition manual. These three approaches are listed in eTable 1 for comparison.

Analysis

Treatment cases identified as being potentially available in the outpatient setting according to the IGES model and the AOP catalogue were presented as proportions of the total number of inpatient treatment cases and stratified according to age groups, specialist department groups, reason for admission, and hospital stay categories.

The proportion of hybrid DRG cases was also presented. SAS Version 9.3 was used for analysis, with due consideration of Good Practice in Secondary Data Analysis (14).

Results

Of 14.8 million inpatient cases during the year 2022 (excluding deliveries and neonates), 10.4 million (70.3%) were initially included that could potentially have been treated in the outpatient setting according to the IGES model. Of these, 7.8 million (75.4%) were then excluded by context factor analysis, so that 2.6 million (17.3% of all inpatient treatment cases) were ultimately identified that could potentially have been provided in the outpatient setting. Initially, 4.6 million cases were included (31.3%) based on the AOP catalogue, of which 1.4 million (30.5%) were subsequently excluded by context analysis. Hence, 3.2 million cases (21.8% of all treatment cases) were identified that could have potentially been treated in the outpatient setting (eFigure). At around 153 000 cases, a proportion of 1.0% of all treatment cases were identified as potential hybrid DRGs (Table, eTable 2). Based on the IGES model, the highest proportions of treatment cases potentially suitable for outpatient treatment were observed among children and adolescents up to the age of 19 (31.3%), followed by adults of working age (24.3%). The highest proportion based on the AOP catalogue was accounted for by adults of working age (26.7%), followed by the age group 65 to 84 years (21.5%). The specialist departments radiotherapy, urology, ears, nose and throat medicine, and ophthalmology demonstrated the highest potential for providing treatment on an outpatient rather than inpatient basis according to the IGES model (29.1 to 33.9%). Based on the AOP catalogue, the highest potential was found in urology, gynecology, and ears, nose and throat medicine (35.4 to 44.0%), and in ophthalmology and dentistry and oral medicine (around 30% each). The identified hybrid DRGs were almost entirely accounted for by the specialist departments of gynecology, surgery, and urology (Table, eTable 2).

Table

Proportion of treatment cases which can potentially be provided in the outpatient setting according to the IGES model and the AOP catalogue as well as the proportion of hybrid DRGs, data year 2022

eFigure

Inclusions and exclusions for identifying treatment cases which can potentially be provided in the outpatient setting according to the IGES model and the AOP catalogue, data year 2022 based on Pioch et al. (9)

eTable 2

Outpatient potential based on the IGES model and the AOP catalogue and hybrid DRGs, data year 2022

As regards cases with the admission reason “hospital referral”, both the IGES model and the AOP catalogue revealed that the proportion of those treatments that can potentially be provided in the outpatient setting was higher (22.6% and 32.8%, respectively) than for cases with the admission reason “emergency” (around 13% each; Table, eTable 2).

With regard to short-stay cases (admission and discharge on the same day), the IGES model had a higher potential for providing treatment on an outpatient rather than inpatient basis (21.3% of all short-stay cases) as compared with the AOP catalogue (7.7%). However, the highest proportions of treatment cases that can be provided in the outpatient setting were found for both approaches in cases with hospital stays of one to three days (IGES 28.9%; AOP 30.4%; Table, eTable 2).

The IGES model and the AOP catalogue considered more than two thirds of all treatment cases (68.9%) as being neither potentially suitable for outpatient treatment nor as hybrid DRGs. Of the overall 4.5 million cases (30.1% of all treatment cases) that could have been provided in the outpatient setting as identified either by the IGES model or the AOP catalogue, 1.1 million cases (7.2% of all treatment cases) were similarly classified as feasible for outpatient treatment (Figure).

Figure

Overlap of the proportion of treatments which can potentially be provided in the outpatient setting based on the IGES model and the 2024 AOP catalogue with the proportion of 2024 hybrid DRGs, data year 2022

More than 60% of all cases identified by the criteria of the AOP catalogue as potentially suitable for outpatient treatment demonstrated at least one of the exclusion criteria that are only included in the IGES model (not presented).

Discussion

With percentages of 17.3% (IGES) and 21.8% (AOP) of all inpatient treatment cases, the potential in Germany for providing treatment on an outpatient rather than inpatient basis is substantial. This means that in 2022, depending on the calculation model applied, more than 2.6 (IGES) and 3.2 million (AOP) inpatient treatment cases can be carried out in the outpatient setting. However, only 1.1 million treatment cases were classified as suitable for outpatient treatment by both approaches.

The differences between the IGES model and the AOP catalogue lay primarily in their inclusion and exclusion criteria. More than two thirds of all treatment cases were initially included by the IGES model, but three quarters of these were subsequently excluded by context factor analysis. Just under one third of all treatment cases was initially included by the AOP catalogue, of which another one third was subsequently excluded by context factor analysis (which includes fewer exclusion criteria than suggested by the IGES model). This means that treatments that can potentially be provided in the outpatient setting and identified by only using the AOP catalogue but not the IGES model are predominantly cases that have context factors not included in the context factor analysis of the AOP catalogue (9).

This applies, for example, to the list of excluded procedures (K2: Inpatient treatment indicated as based on the OPS) which comprises around 19 400 OPS codes in the IGES model, yet only 5600 OPS codes in the AOP catalogue. It must therefore be assumed that, due to context factors not being taken into consideration (for example, intensive care complex treatment was only considered as an exclusion criterion based on the IGES model), not all cases identified by the AOP catalogue as being feasible for the outpatient setting can actually be performed as outpatient cases. As far as hospitals are concerned, this means that the health insurance funds must be provided with additional reasons to justify inpatient provision of care for such cases, otherwise checks for inappropriate admissions to hospital could be triggered. With its more comprehensive context factor analysis, the IGES model therefore appears to identify treatment cases that can be provided in the outpatient setting more reliably than the AOP catalogue.

The stratified results show that, based on the IGES model, the specialist department radiotherapy, for example, has a significantly higher potential for providing treatment on an outpatient rather than inpatient basis. This is due to the fact that, unlike the AOP catalogue, DRGs for the medical treatment of neoplasms and DRGs with the primary service “radiotherapy” are explicitly included as services that can potentially be provided in the outpatient setting (7). Specialist departments in which a higher proportion of treatments that can potentially be provided in the outpatient setting, as identified according to both the IGES model and the AOP catalogue, are urology, ears, nose and throat medicine, and ophthalmology.

As regards duration of hospital stay, it appears that the IGES model identifies a three-fold higher proportion of short-stay cases suitable for outpatient treatment than the AOP catalogue. This may in part be due to the fact that the IGES model also explicitly includes those medical DRG case groups as suitable for the outpatient setting which represent common unscheduled treatments with a generally shorter duration of treatment. These include, for example, the treatment of gastrointestinal disorders, cardiac arrhythmia, and mild head injuries (7).

In the present study, the IGES model indicates that 17.3% of inpatient treatment cases could potentially be treated in an outpatient setting, which is slightly lower than the approximately 20% of treatment cases potentially suitable for outpatient treatment in analyses based on data from the Barmer health insurance fund (15). This discrepancy could be due to the different data source and different methods of operationalization of individual context factors. An analysis of the differences between the IGES model and the AOP catalogue with regard to inclusion criteria and context analysis yielded similar results to those of the present article (16). Only a comparatively small number of treatment cases were rated as potential hybrid DRG cases. However, it should be noted that the present article only considered the services of the “start catalogue” of the hybrid DRG regulation. More services were added to the hybrid DRG catalogue in 2025 (17).

The present analysis identified only a “calculated” potential for providing treatment on an outpatient rather than inpatient basis. In each case, the feasibility of outpatient treatment must be assessed on an individual basis, taking into account not only medical criteria but also the care situation at home, compliance, and access to aftercare services (18). Ensuring post-operative care on leaving the health service site is of critical concern (7). From the patient’s point of view, the outpatient provision of suitable treatment services has its advantages, such as remaining in familiar surroundings. However, risks can also arise as a result of shortened follow-up times. If, for example, post-operative and emergency care is not ensured, this could place additional burdens on emergency rooms, emergency services, and on-call services provided by the Associations of Statutory Health Insurance Physicians (19).

Limitations

Some limitations must be borne in mind when interpreting the results. The data were evaluated retrospectively, and this could have overestimated the potential for providing treatment on an outpatient rather than inpatient basis as no context factor analysis was in place in 2022. In the future, hospitals could adapt their coding practices to capture context factors as fully as possible and so justify inpatient stays and avoid checks for inappropriate hospital admission (9). This could reduce the “calculated” outpatient potential. The process of approximating hybrid DRGs is not without its uncertainties either.

The potential for providing treatment on an outpatient rather than inpatient basis was identified exclusively by using information available in the DRG statistics. It was not possible to include potential social circumstances, such as lack of home care services or absence of health awareness (7), which also results in a further overestimation of outpatient treatment potential.

It was only possible to reproduce some context factors in approximation, such as the PCCL value, while others such as frailty and cancer stage are not (yet) operationalized. The IGES report suggests introducing new ICD codes specifically for this purpose. These codes are not yet in place, so they were not included in the present retrospective analysis. The context factor “Complexity arising from combinations of interventions” is also not yet operationalized and was therefore not considered (9). This is supposed to cover combinations of minor procedures that, alone, could be performed on an outpatient basis but become more complex when combined (7).

The context factor “ocular comorbidities” is not defined by ICD codes in the context factor analysis of the AOP catalogue. For this reason, all procedures named in the corresponding OPS list (extracapsular cataract extractions) were excluded from being considered outpatient services. This could have led to the outpatient potential in the field of ophthalmology tending to be underestimated according to the criteria of the AOP catalogue.

Only those inpatient treatment cases actually provided in 2022 were taken into consideration. The fact that some specialist departments, such as ophthalmology, already have a more developed focus on outpatient treatment and could therefore have less additional potential for providing treatment on an outpatient rather than inpatient basis compared with the past (20) was not taken into account.

Conclusions

Although the two approaches of the IGES model and the AOP catalogue indicate a similarly high potential for providing treatment on an outpatient rather than inpatient basis, the overlap of outpatient treatment cases identified by the two methods is small. Further development of the AOP catalogue appears necessary to identify treatment cases that can be provided in the outpatient setting more accurately and to expedite the transfer of inpatient services to the outpatient setting. The recommendations of the IGES report should be adopted more extensively, since the AOP catalogue does not fully encompass the recommended context factors in particular. The (at present still of little significance in terms of numbers) hybrid DRGs enable hospitals to tailor their service provision to individual medical requirements, irrespective of billing regulations.

The extent to which the new AOP catalogue and hybrid DRG remuneration can promote the provision of outpatient rather than inpatient care against the background of the hospital reform will need to be determined in future analyses.

Conflict of interest statement
RB is member of the Government Commission for a Modern and Needs-based Hospital Sector.

TM was involved as advisor and co-author in the preparation of the IGES report on services feasible for the outpatient setting.

The other authors confirm that there are no conflicts of interest.

Manuscript received on 30 August 2024, revised version accepted on 16 January 2025.

Translated from the original German by Dr. Grahame Larkin

Corresponding author:
Carolina Pioch, MSc

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