It is customary for individual vendors to provide tools to acquire measurements that aren't currently defined in a Standard measurement template. In the normal evolution of the Standard, standard measurement sets are periodically updated to reflect the state of medical practice. Often, individual vendors and/or clinical users are first to implement the acquisition of new measurements.

Some measurements may be defined and used within a particular clinical institution. For maximum interoperability, if there exists a Standard or vendor-defined measurement concept code for that measurement, the Standard or vendor-defined concept code should be used instead of creating a custom measurement code unique to that institution.

Determining whether two or more different measurement definitions pertain to the same real-world quantity is a non-trivial task. It requires clinical experts to carefully examine alternative measurement definitions to determine if two or more definitions are equivalent. This task is greatly simplified if the distinct characteristics of the non-standard measurement are explicitly stated and conveyed. If two measurements differ in one or more critical characteristics then it can be concluded that the two measurement definitions describe different real-world quantities. Only those measurements that share all the critical clinical characteristics need to be carefully examined by clinical experts to see if they are equivalent.

It may be determined that two measurements that share all specified clinical characteristics are actually distinct real-world quantities. If this occurs, it may be an indication that not all relevant clinical characteristics have been isolated and codified. In this case, the convention for defining the measurement should be extended to include the unspecified clinical characteristic.