In Germany antibiotic usage in farm animals is constantly captured by various systems: The documentation in the HI-Tier-database, which is mandatory by the German drug law and the documentation within privately organized quality-assurance systems like the system of the "QS Qualitat und Sicherheit GmbH" Additional data are provided by research projects, such as VetCAb ("Veterinary Consumption of Antibiotics"). These monitoring systems make their calculation of antibiotic usage based on the treatment frequency or equivalent metrics. Since these metrics have varying definitions within the systems and a detailed description is often missing, there is confusion about the interpretation of results from different sources. In order to provide an overview, this study presents detailed descriptions of four of the used metrics and related variables in Germany: The animal treatment index, the therapy index according to the definition of QS Qualitat und Sicherheit GmbH and the treatment frequency of the German drug law and the project "VetCAb''. Comparisons of the metric's definitions as well as selected variables show fundamental differences making it impossible to compare results directly. However, all indices are suitable to analyse and compare trends in antimicrobial usage within each system.