From the OMOP v5.3 Spec
The FACT_RELATIONSHIP table contains records about the relationships
between facts stored as records in any table of the CDM. Relationships can
be defined between facts from the same domain (table), or different
domains. Examples of Fact Relationships include: Person relationships
(parent-child), care site relationships (hierarchical organizational
structure of facilities within a health system), indication relationship
(between drug exposures and associated conditions), usage relationships (of
devices during the course of an associated procedure), or facts derived
from one another (measurements derived from an associated specimen).
Field Required Type Description
domain_concept_id_1 The concept representing the domain of fact one, from
which the corresponding table can be inferred.
fact_id_1 The unique identifier in the table corresponding to the domain
of fact one.
domain_concept_id_2 The concept representing the domain of fact two, from
which the corresponding table can be inferred.
fact_id_2 The unique identifier in the table corresponding to the domain
of fact two.
relationship_concept_id A foreign key to a Standard Concept ID of
relationship in the Standardized Vocabularies.
Conventions
• All relationships are directional, and each relationship is represented
twice symmetrically within the FACT_RELATIONSHIP table. For example, two
persons if person_id = 1 is the mother of person_id = 2 two records are in
the
Given your example of a specimen to one or more measurement
Specimen to measurement:
domain_concept_id_1 = 36 (Specimen) The Domain_id is ‘Metadata’ and
Concept Class is ‘Domain’
fact_id_1 = the specimen_id
domain_concept_id_2 = 27 (Observation)
fact_id_2 = the observation_id
relationship_concept_id = 2,000,000,001 ( these are custom concepts we
defined it as ‘Specimen to Observation’)
Then the reverse for Measurement to Specimen:
domain_concept_id_1 = 27 (Observation)
fact_id_1 = the observation_id
domain_concept_id_2 = 36 (Specimen)
fact_id_2 = the specimen_id
relationship_concept_id = 2,000,000,002 ( these are custom concepts we
defined it as ‘Observation to Specimen’)