The Logical Observation Identifiers Names and Codes terminology (LOINC) has been proposed as a nomenclature for clinical laboratory tests. We present a formal representation of LOINC using a Semantic Web-based ontology that defines LOINC concepts in terms of the six main LOINC axes and their relationships with UMLS Semantic Types and the UMLS Metathesarus. This representation may enable automated information integration and decision support in public health surveillance.