A typical scenario for clinical decision support is when the physician is ordering a drug through the EHR’s prescription order entry system, and being alerted of a potential drug allergy. SNOMED CT can be used to implement allergy and intolerance checking. The prerequisite is that the drugs in the order entry system are mapped to SNOMED CT substances. If the allergy or intolerance information is stored in the allergy list, which is already mapped to SNOMED CT substances, allergy checking will be relatively straightforward. If the information is stored as a SNOMED CT-encoded problem, the causative agent can be derived from the concept model. For example, e.g., Allergy to amoxicillin (finding) has an attribute Causative agent with value Amoxicillin (substance).

When SNOMED CT is used in allergy and intolerance checking, the following should be noted:

• Drug class - when the causative agent is recorded as a SNOMED CT drug class, the SNOMED CT hierarchy can be used to check for members of the class. It is recommended to use drug classes that are descendants of Substance categorized by structure (substance) and Substance categorized by disposition (substance), and avoid therapeutic role-based classes.
• Excipient or adjuvant – in some rare cases, the adverse sensitivity is caused by an excipient rather than the active ingredient of a pharmaceutical product. Excipients are not well documented in most drug terminologies, including SNOMED CT. Similarly, allergy to vaccines could be caused by preservatives, egg protein, gelatin, latex, or vaccine adjuvants. In such cases, the SNOMED CT substance hierarchy will not be as useful for decision support, and the allergy list will be populated with the medical product concept.
• Cross reactivity – this is either mediated by immunologic or non-immunologic mechanisms. The former kind is usually explained by the presence of common structural antigenic determinants in the cross-reacting drugs. In the case of compounds provoking non-immunologic hypersensitivity reactions, cross-reactivity is explained by a common pharmacological characteristic, such as the inhibitory effect of non-steroidal anti-inflammatory drugs on cyclooxygenase-1 and the capability of muscle relaxants or contrast media to release histamine through a non-immunologic mechanism. While structural similarity is reflected in the structural categorization of SNOMED CT Substances, the threshold of similarity that should trigger cross reactivity alerts is not well defined. Generally, an external knowledge source outside of SNOMED CT is required for reliable cross reactivity checking.
• Bidirectional allergy checking – Documenting allergy to a drug product containing multiple substances in which the causative substance is known through testing or a high probability requires that documentation of an allergy to the product will trigger an alert if a drug containing only that substance is ordered. Similarly, an allergy to a drug containing a single substance should trigger an alert if a drug product containing that substance is ordered. Decision support systems based on the SNOMED CT product and substance hierarchies will need to be designed to traverse these relationships in both directions in order to trigger the kinds of alerts just described.