Logical Observation Identifiers Names and Codes (LOINC®)
A universal code system for identifying laboratory and clinical observations.
From serum levels of hepatitis B surface antigen to diastolic blood pressure, LOINC has standardized terms for all kinds of observations and measurements that enable exchange and aggregation of electronic health data from many independent systems.
It's free, but invaluable. Both LOINC and the RELMA mapping program that helps link your local codes to LOINC terms are distributed at no cost by the Regenstrief Institute. LOINC is your key to interoperable data exchange.