Minimum volume thresholds for rare diseases: insights through the transfer of evidence

To improve the quality of complex, higher-risk and elective hospital treatments, minimum volumes apply in Germany for certain services, i.e. interventions. The Federal Joint Committee (G-BA), the highest decision-making body in the German healthcare system, determines which procedures are subject to this regulation. These decisions are based on reports by the Institute for Quality and Efficiency in Health Care (IQWiG), Germany’s health technology assessment agency. These reports evaluate studies that examine whether a relationship between the volume of services (VoS) and the quality of treatment is likely. In this context, the number of procedures per hospital and/or per doctor is taken into account.

To date, no minimum volume regulations have been established in Germany for higher-risk hospital treatments for specific rare diseases. The prerequisite for setting a minimum volume is the existence of precise and reliable studies indicating a probable relationship between the VoS and the quality of treatment. Such studies are often lacking for higher-risk services and procedures for rare diseases because the number of cases per hospital is low. Nevertheless, patients requiring complex and higher-risk but rare procedures should also be guaranteed the best possible care.

Against this background, IQWiG has developed a methodological approach on behalf of the G-BA to assess the relationship between the VoS and the quality of treatment, even for procedures applied in rare diseases with correspondingly low case numbers. Claudia-Martina Messow from IQWiG's Medical Biometry Department explains: “In these cases, we propose transferring evidence from sufficiently similar therapeutic indications to the therapeutic indication under investigation, if such evidence is available.” This involves examining whether results from studies on similar patient groups or similar procedures can be transferred to the rare disease in question. External experts are to contribute the necessary clinical and procedural expertise.

Methodologically uncharted territory

In the course of its search for international literature, IQWiG did not identify any documents on methodological derivations of minimum volume requirements for rare diseases, nor on criteria for hospital certification to treat specific rare diseases. Likewise, no overarching methodological literature on minimum volumes for rare diseases was found.

The IQWiG team therefore developed its own approach: where there are no or no reliable studies on the relationship between the VoS and the quality of treatment outcomes, evidence from other populations or interventions should be transferred to the rare procedure. Clinical and procedural considerations based on the respective population and intervention under investigation are decisive for transferability.

For this purpose, IQWiG developed a list of criteria that considers differences between populations and interventions and evaluates transferability. This evaluation should generally be carried out with the help of external experts.

Claudia-Martina Messow notes: “If a population or intervention has been identified for the possible transfer of evidence, and in the absence of direct evidence on the relationship between the VoS and the quality of treatment, conclusions can potentially be derived from the transfer of evidence that could inform the G-BA’s deliberations on minimum volume regulations for rare procedures.”

Process of report production

On 9 October 2024, the G-BA commissioned IQWiG to develop a method to assess the relationship between the VoS and the quality of treatment outcomes in rare diseases with correspondingly low case numbers and an insufficient body of evidence. According to the commission, the report was to be prepared in an accelerated procedure as a so-called rapid report, which was sent to the G-BA on 30 April 2025. The rapid report was published in May 2025. The English translation was published in November 2025.