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Parameters used in the API

The Castor EDC API Documentation page (,, gathers the methods and requirements of the fields in Castor to be used with an API. There are a handful of parameters that are used to locate the data in the database and retrieve it or update it. See the table below:




Numeric identifier of the user (UUID)


Numeric identifier of the country (use the GET method for a list of IDs)


ID of the study (found in Settings > Study)


Identifier of the field


RecordID as shown in the Records tab


Identifier of a report (structure) 


Identifier of a report added for a Record


Identifier of a survey (structure) 


Identifier of a survey added for a Record


Identifier of a survey package (structure)


Identifier of a survey package added for a Record


Identifier of a survey step

What is the difference between ID and instance ID?

Each component of the study is identified by a unique ID: field, user, etc. Some of them are simply referenced by "ID", like the studyID and the recordID. In the user interface these items locate in the Structure and Forms tab:

However, some components can appear repeatedly in the study even within the same record. Such is the case of reports, surveys and survey packages. For this reason we need a unique ID to identify each of the occurrences of these components across the study. This unique identifier is the instanceID

These items that are referenced with an "instanceID" are always located in the menu inside each record, in the Records tab of the user interface:

The following chart exemplifies the relation between ID and instance ID for a report called "Medication":

See also Where can I find IDs for the study/field/report instance/... for the API?  to learn how to find the IDs for the API, and The structure of Castor for a comprehensive description of the parts of a study.

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