Background: Reprocessing of medical devices, being classified as semi-critical B is recommended to be performed in a washer-disinfector. In order to estimate, whether the expected contaminants of the various medical disciplines can be effectively removed by this washer-disinfector, different so called “test soils” have been proposed to be tested as a marker of cleaning efficacy of the disinfector. Todays described test soils are optimised for the testing of contaminations occurring in surgical procedures, but not for dental procedures. Methods: In this study the test soils being proposed in the EN 15883-5 (e.g. KMNE soil, recipe by Koller and coagulated sheep’s blood) were compared with 8 reference substances used in the conservative-prosthetic dental practice. The success of the cleaning efficacy in the washer-disinfector was checked visually and by determining the residual protein concentration on the contaminated instruments after the cleaning procedure. Results: It could be shown that in contrast to the proposed test soils of the EN 15883-5, the used reference substances of the dental practice could not be removed by the washer-disinfector. Removal of these reference substances was only possible after manual or ultrasonic cleaning. Conclusions: Since blood plays a subordinate role as a contaminant of instruments during conservative-prosthetic dental treatments, testing of the cleaning efficacy of the washer-disinfector with test soils according to the proposals of the EN 15883-5 is not representative in this discipline of dentistry. Most of the materials used in dental practice can only be removed manually or with the help of the ultrasound bath.