Two influential organizations in the development of medical data exchange standards in the United States of America are ASTM and HL7. A shared message structure has been adapted to diverse applications including hospital information systems and clinical laboratories. The basic approach is the exchange of ASCII messages. Messages segments relate to clinical entities and support a hierarchy of entities. Individual data fields are of variable length and bounded by delimiter characters. The feasibility of this approach has been confirmed by its adoption by a number of medical institutions and information system vendors.