###### Abstract  
The coding system Unified Code for Units of Measure (UCUM) serves the unambiguous electronic communication of physical quantities and their measurements and has faced a slow uptake. Despite being closely related to popular healthcare standards such as LOINC, laboratories still majorly report results using proprietary unit terms. Currently available methods helping users create mappings between their units and UCUM are not flexible and automated enough to be of great use in trying to remedy this. We propose the \"LOINC to UCUM Mapping Assistant\" (LUMA) as a tool able to overcome the drawbacks of existing approaches while being more accessible even to inexperienced users. By mapping LOINC's _Property_ axis to representations within UCUM reflecting its semantics, we were able to formalize the association between the two. An HL7 FHIR back-end provides LUMA with UCUM unit recommendations sourced from existing lookup tables simply by providing it with a LOINC code. Additionally, the mappings users created may be used to perform unit conversions from proprietary units to UCUM. The tool was evaluated with five participants from the LADR laboratory network in Germany, who valued the streamlined approach to creating the mappings and particularly emphasized the utility of being able to perform unit conversions within the tool. U 2022 202  
conjoined standardization of locally used units with UCUM is not mandatory and therefore not pursued wholeheartedly, although the idiosyncrasies of proprietary terms have already been shown to impede the aspired integration of otherwise interoperable lab data [6; 7]. Additionally, there is still a divide in the way certain measurements are handled even after the reunification of Germany, with the former West Germany having adopted the SI system in the late 1970s, whereas East Germany did not [8]. Still, the lack of motivation to perform a time-consuming and seemingly optional mapping does not come as a surprise, seeing as currently available methods for doing so have noticeable drawbacks. As for existing approaches, users may try consulting the LOINC data set via either RELMA or the web assistant SearchLOINC [9] in order to ascertain whether an example for a UCUM unit is provided for a specific LOINC code, but this is very inflexible considering the fact that there is often more than one way to report the unit of a laboratory result. For example, measuring a mass could be performed in grams or the non-metric pound, as is customary in the US. Additionally, metric units can differ in magnitude from the prefix they are preceded by. The other approach is manual in nature and requires the user to have at least some expertise in the matters of UCUM by using aggregated lists of UCUM units as guidance for finding or formulating an appropriate match for their proprietary unit. On the official LOINC web presence, users can find a reference to one such document with data gathered by the Regenstrief Institute [10] but there are also other resources such as the \"UCUM-Common\" ValueSet provided by Fast Healthcare Interoperability Resources (FHIR) [11]. While offering a broader range of possible units to choose from, these lists are not associated with specific LOINC codes in any way and users have to make an informed guess regarding the mapping they deem appropriate. Ideally, one would like to combine the flexibility of these extensive lists with the more automated approach of RELMA and SearchLOINC. In light of the lack of readily available support for such endeavors and a complete absence of formalized associations between LOINC and UCUM that could facilitate digital processing, we present the \"LOINC to UCUM Mapping Assistant\" (LUMA) as a tool to close this gap. For this project, LOINC's _Property_ axis acts as a bridge between the proprietary units and UCUM. This approach requires a user's proprietary units to already be associated with a LOINC code, as we mapped all quantitative LOINC _Properties_ to a representative base unit combination in UCUM.