Allergies and Adverse Reactions
Both SNOMED CT and HL7 differentiate an isolated reaction event from the condition of being
allergic or intolerant. For instance, the following hierarchy is present in SNOMED CT (July
2005 release):

Allergic disorder (SCTID 127072000)
o Allergy (SCTID 106190000)
 Drug Allergy (SCTID 416098002)
o Allergic reaction (SCTID 212999007)
 Allergic reaction to drug (SCTID 416093006)
Different SNOMED CT value sets may apply, depending on the application context. Potential
value sets include:


Substance/Product value set8 <= Substance (SCTID 105590001) and/or Pharmaceutical /
Biologic product (SCTID 373873005): Might be used where the context is clearly the
recording of allergies (e.g. a data entry box labeled "ALLERGIES").9
Findings value set <= Context-dependent finding (SCTID 413350009) and/or Clinical finding
(SCTID 404684003): Might be used where the context is an encounter diagnosis or a problem
list.
In addition, the RIM supports the sequential determination of primary and secondary
observations relating to discovery and analysis of adverse reactions. For example, a reaction
event is preceded by some type of substance exposure. Following the exposure is the
development of signs or symptoms that may not be immediately recognized as being due to the
exposure, and thus captured as discrete observations. Later, the patient or clinician may associate
the signs or symptoms with the exposure.
Example 10. Reactions coded with Substance/Product value set
<observation classCode="OBS" moodCode="EVN">
<code code="418038007"
codeSystem="2.16.840.1.113883.6.96"
displayName="Propensity to adverse reaction to
substance">
<qualifier>
<name code="246075003"
displayName="Causative agent"/>
<value code="373270004"
displayName="PCN (substance)"/>
</qualifier>
</code>
<text>Allergy to PCN manifesting as hives</text>
<actRelationship typeCode="MFST"
inversionInd="true" contextConductionInd="true">
<observation classCode="OBS" moodCode="EVN">
<code code="247472004"
codeSystem="2.16.840.1.113883.6.96"
displayName="Hives"/>
</observation>
</actRelationship>
</observation>
Where the Substance/Product value set is used, there may be no ability in the application to
differentiate an allergic reaction from the condition of being allergic, and so a broad
observation.code is used.
Where the clinician fills in both the substance/product and the reaction, context can propagate
across the MFST relationship. The manifestation should not be post-coordinated with the allergic
disorder into a single observation.code.
Example 11. Reactions coded with Findings value set
<observation classCode="COND" moodCode="EVN">
<code code="91936005"
codeSystem="2.16.840.1.113883.6.96"
displayName="Allergy to penicillin"/>
<text>Allergy to PCN</text>
</observation>
In this case, the selected finding indicates the condition of being allergic.