SNOMED logic-based definitions represent terminological knowledge. In other words, they represent what is always necessarily true about the meaning of a code. The logic definitions are not intended to cover the entire range of medical knowledge, and are not intended to include probabilistic or uncertain knowledge. Such knowledge is beyond the scope of SNOMED CT's logic definitions.
For example, consider myocardial infarction (MI). terminological knowledge about this entity includes the fact that it must involve the myocardium, and it must involve an infarction. Additional knowledge that is not terminological, and therefore not included in the SNOMED logic definition, might include the fact that an MI is usually associated with crushing substernal chest pain, diaphoresis, arrhythmia, ST-segment elevation on EKG, and elevated levels of cardiac enzymes. Not every case of MI will have chest pain, nor will every case show ST segment elevation, etc. While these are valuable clues to the diagnostician, they are not necessarily always present, and therefore they are not part of the terminological knowledge base. As another example, consider appendicitis. terminological knowledge about this entity includes the fact that it is a kind of inflammatory disorder, and that it involves the appendix. Additional knowledge that is not terminological might include the fact that it often involves central abdominal pain that migrates to the right lower quadrant, and that it is associated with anorexia, nausea, elevated white blood count, and rebound tenderness over McBurneys point. These additional pieces of knowledge are variably present and therefore represent uncertain or probabilistic knowledge. Such variable or probabilistic knowledge is highly valuable for decision support algorithms, but is beyond the scope of SNOMED CT's logic definitions.
3.3.1 Reason for procedure
The reason a procedure is ordered should not be precoordinated with the procedure. SNOMED CT codes cannot constrain what the performer is allowed to report on. The reason that a procedure is ordered may influence the interpretation of the results, but it does (should) not affect the way the procedure is performed.