In this section, we illustrate how description filters can be applied to expression constraints to further restrict the matching concepts.

Overview

Description filter constraints provide the ability to limit the set of concepts, that satisfy a given expression constraint, based on the descriptions associated with each concept. Only concepts that have at least one matching description for each filter criteria will be included in the set of matching concepts. Descriptions can be filtered based on their term, type, language, dialect, acceptability in a given dialect, module, effectiveTime, active status and description identifier.  Description filters are specified inside double curly braces, and optionally being with the letter "D". Any filter that does not specify its type is, by default, assumed to be a description filter.

In the following sections, we explain each type of description filter criteria.

Term Filter

Term filters enable an expression constraint to match on only those concepts with an associated description whose term matches the given search term. For example, the following expression constraint is satisfied by SNOMED CT concepts with a description matching the search terms "heart" and "att". This expression constraint works like a term search performed in a SNOMED CT browser. Please note that the "D" (either upper or lower case) at the start of the filter indicates that this is a description filter constraint, rather than a concept filter constraint (see 6.9 Concept Filters). If the type of a filter constraint is not specified (as in most of the examples below), then it is assumed that the constraint is a description constraint.

By default, term filters match using a word-prefix-any-order match technique. This means that each string value in the search term must match the start of a word in the concept's description term, but that these words may appear in any order. This word-prefix-any-order match technique can be explicitly specified in the term filter, using the keyword "match:" before the search term. For example, the following four expression constraints are equivalent, and are each satisfied only by diseases with a description term that includes both a word starting with "heart" and a word starting with "att" (in any order). To indicate that a matching description may match either one search term or another, a search term set may be used.

The example below matches only those diseases with a description term containing either a word starting with "heart" or a word starting with "card" (or both).

The other technique that may be used is a wildcard search.  This technique is specified using the keyword "wild:" and matches the search term in the expression constraint against the entire candidate description term (rather than just individual words).  An asterisk ('*') is used as a wildcard to indicate that any (zero to many) characters may appear in the given position.

For example, the expression constraint below will match only diseases with a description term starting with "cardi" and ending with "opathy" with any number of characters between. This term filter would therefore match on terms such as "cardiopathy", "cardiomyopathy" and "cardiac channelopathy", but would not match on terms like "atrial cardiopathy" or "Cardiomyopathy (disorder)".

Please note that to perform an exact string match on an entire term, a wildcard search without a wildcard can be used (e.g. term=wild:"cardiopathy"). For example, the following expression constraint will match only diseases with a description term that exactly matches the full string "cardiopathy". This expression constraint will therefore match the concept  56265001 | Heart disease (disorder)|  (with synonym "Cardiopathy"), but will not match the concept  870575001 | Disorder of cardiac atrium (disorder)|  (with synonym "Atrial cardiopathy") It is also possible to mix the match techniques in a search term set. For example, the expression constraint below will match those diseases with a description term that either contains a word starting with "gas", or ending with "itis" - e.g. "gastric flu", "gastritis", or "tonsillitis". If more than one filter is applied, then all filters (surrounded in double braces) must match at least one description of a concept, for that concept to satisfy the constraint. The descriptions that match each of the filters can either be the same description, or different descriptions on the same concept.

The expression constraint below matches those diseases which have both a description that contains a word starting "eye" and a description that ends with "itis". For example, this constraint would match the concept  9826008 | Conjunctivitis (disorder)| (with synonyms "Pink eye disease" and "Conjunctivitis") and the concept  15680481000119104 | Viral conjunctivitis of bilateral eyes (disorder)| (with synonyms "Bilateral viral conjunctivitis" and "Viral conjunctivitis of both eyes"), but would not match the concept  45261009 | Viral conjunctivitis (disorder)| (which does not have a synonym matching the word prefix "eye").

Language Filter

Language filters enable an expression constraint to match on only those concepts with a matching description in a specified language. Language filters use the keyword "language", followed by a comparison operator (e.g. "=" or "!="), and the ISO 639-1 two-character language code (in upper or lowercase).

The expression constraint below matches only those diseases with a Swedish description containing the word prefix "hjärt" - e.g.  41884003 | hjärtpolyp| from the Swedish Edition (20200531) 

The expression constraint below matches only those diseases with a Swedish description containing the word prefix "hjärt" and an English description containing the word prefix "heart" - e.g.  84114007 | hjärtsvikt| (with English synonym "Heart failure") from the Swedish Edition (20200531).

Description Type Filter

Type filters enable an expression constraint to match on only those concepts with a matching description of a specified type. Type filters may either use the keyword "type" with the values "fsn", "syn" or "def", or may use the keyword "typeId" with a concept value that is  <  900000000000446008 |Description type| . 

The following table lists the valid description type keywords in both the brief and full syntax, and their equivalent concept reference alternatives. Please note that the full syntax accepts both the brief and full syntax keywords. If additional description types are required, these must be specified in a filter using the 'typeId' keyword with the corresponding concept reference.

For example,  the expression constraint below matches all the subtypes of  | Heart disease| , that have a fully specified name containing the word prefix "heart".

The following two expression constraints are equivalent, and both match only the subtypes of  | Heart disease| , which have a Swedish synonym containing the word prefix "hjärt". The two equivalent expression constraints below match the subtypes of  | Heart disease| , which either have a synonym containing the word prefix "heart", or a fully specified name containing the word prefix "heart".

Dialect Filter

Dialect filters enable an expression constraint to match on only those concepts with a matching description in a specified language reference set. Dialect filters may either use the keyword "dialect" with a value that represents a valid alias for a specific language reference set,  or may use the keyword "dialectId" with a concept value that is <  900000000000506000 |Language type reference set| . Please refer to Appendix C - Dialect Aliases for a selection of valid dialect aliases for known language reference sets.

For example, the two equivalent expression constraints below will match all subtypes of  | Disease|  that have a description in the Australian English language reference set.

The expression constraint below matches all diseases with a description in the New Zealand English language reference set that has a word starting with "cardio". In some situations, multiple language reference sets need to be used together to identify an appropriate set of concepts. A filter constraint may include a list of dialects to specify that a matching description may belong to any of the given language reference sets. 

For example, the following expression constraint matches all diseases that have a description in either the en-nhs-clinical or en-nhs-pharmacy language reference sets, where that description contains a word starting with the prefix "card".

Acceptability Filter

Acceptability filters enable an expression constraint to match on only those concepts with a matching description that has the specified acceptability in the specified language reference set. Acceptability filters must always be applied to a specified dialect. As such, they are represented by placing the required acceptability in brackets after the value of the dialect filter. Acceptabilities can be indicated using either one of the keywords below, or using a concept value that is  <  900000000000511003 |Acceptability| . The following table lists the valid acceptability keywords in both the brief and full syntax, and their equivalent concept reference alternatives. Please note that the full syntax accepts both the brief and full syntax keywords.

For example, the following two expression constraints both match all descendants of disease with a description that matches the word prefix 'box', has the type 'synonym', and has an acceptability of 'preferred' in the en-us language reference set. In other words, this expression constraint matches diseases with a US English preferred term that uses the word prefix 'box'.

Multiple dialect filters may be used with different acceptabilities applied to each. For example, the expression constraint below matches on diseases, which have a synonym with word prefix "box" that is preferred in the en-nhs-clinical language reference set and is acceptable in the en-gb language reference set. To support alternative acceptabilities in more than one language reference set, a dialect set can be used. For example, the following two equivalent expression constraints match on diseases, which have a synonym with word prefix "box" that is either preferred in the en-gb language reference set or preferred in the en-nhs-clinical language reference set.

Filters with Negation

Filters can use negation in a number of ways. The simplest approach is to use the 'not equal to' comparison operator (e.g. "!=") before the value.

For example, the following expression constraint matches on subtypes of  | Fracture of bone|  that do not use the word prefix "fracture" in their US English preferred term.

If we remove the type and acceptability filters, as shown below, the remaining expression constraint matches on those subtypes of  | Fracture of bone|  which have any US English description that does not contain the word prefix "fracture". Concepts including  263171005 | Fractured nasal bones|  (with synonym "Broken nose") will match the constraint below. To find the set of concepts, for which all descriptions match some specified criteria, the expression constraint must use the MINUS operation to exclude concepts that have a non-matching description. For example, the expression constraint below matches all subtypes of  | Fracture of bone| , for which every description contains the word prefix "fracture". Please note that the filter only applies to the descendants of 125605004 | Fracture of bone|  (i.e. the subexpression directly proceeding the filter). This expression constraint can be simplified to the equivalent one below, using the wildcard character '*' (which represents any concept in the substrate). Using a similar principle, the expression constraint below matches all concepts that do not have a preferred term specified in the en-nz language reference set.

Module Filter

Description module filters enable an expression constraint to match on only those concepts with a matching description that belongs to a specified module. Module filters use the keyword "moduleId" with a concept reference that is <  900000000000443000 |Module| .

For example, the expression constraint below matches all subtypes of  195967001 | Asthma|  with a description that belongs to the US National Library of Medicine maintained module.

And the expression constraint below matches all subtypes of  404684003 | Clinical finding|  with a definition that belongs to the international core module.

Effective Time Filter

Description effective time filters enable an expression constraint to match on only those concepts with a description that has an effectiveTime matching the specified criteria. Effective time filters may use any of the date comparison operators shown below:

Please note that the value of an effective time filter (if present) must be a 8 digit date, formatted according to ISO 8601's basic calendar date format (i.e. YYYYMMDD). If the effectiveTime of the description in the substrate includes a time and/or time zone designator, these should be ignored when performing the comparison.

For example, the following expression constraint matches all subtypes of  125605004 | Fracture of bone|  with a description that has an effective time of 31st January 2021.

And the following expression constraint matches all subtypes of  125605004 | Fracture of bone|  with a description that has any effective time that is not 31st January 2021. Similarly, greater than, less than, greater than or equals and less than or equals operators may be used in an effectiveTime filter. For example, the following expression constraint matches all subtypes of  125605004 | Fracture of bone|  with a description that has an effectiveTime of 31st July 2019 or later (i.e. more recent). And the following expression constraint matches all subtypes of  125605004 | Fracture of bone|  with a description that has an effective time of 31st July 2019 or earlier. The effectiveTime filter can also use sets of effective times. For example, the following expression constraint matches all subtypes of  125605004 | Fracture of bone|  with a description that has an effectiveTime of either 31st January 2019, 31st July 2019, 31st January 2020, or 31st July 2020. And the expression constraint below matches all subtypes of  125605004 | Fracture of bone|  with a description, which does not have any of the following effective times: 31st January 2019, 31st July 2019, 31st January 2020 or 31st July 2020. To match concepts with unpublished descriptions, to which an effectiveTime has not been assigned, an effectiveTime value of "" can be used. For example, the following expression constraint matches all subtypes of  125605004 | Fracture of bone|  with a description to which an effectiveTime has not yet been assigned. Please note that description effectiveTime filters, which use the comparison operators "<" and ">", will not match any descriptions with an effectiveTime = "".

Active Filter

Description active filters enable an expression constraint to match on only those concepts with a description that has a matching active status. Descriptions are either active (i.e. active = 1 or active = "true") or inactive (i.e. active = 0 or active = "false"). By default, only active descriptions are included in the substrate.

For example, the following expression constraints return all concepts in the International Patient Summary reference set, which have an active description.

And the following expression constraints return all concepts in the International Patient Summary reference set, which have an inactive description.

Description Id Filter

Description id filters enable an expression constraint to match on only those concepts with a description that has a matching description identifier. For example, the following expression constraint matches any concept, which has an associated description with the identifier "3032638017". The only concept that matches this expression constraint is 707444001 | Uncomplicated asthma (disorder)| .

Description id filters can also be applied to other expression constraints, to check whether the concept with the matching description id is in a given set of concepts. For example, the following expression constraint will match any descendant of 195967001 | Asthma (disorder)| , which has a description with identifier "3032638017". This can be used to check if the concept with the given description id is a descendant of 195967001 | Asthma (disorder)| . Description id filters may also include a set of description identifiers. The following expression constraint will match any descendant of 195967001 | Asthma (disorder)| , with a description whose identifier is either "1208972017", "2674140012" or "3043971012". Please note that inactive concepts can have active descriptions, so the description id filter can be applied after inactive concepts are added to the query results via a history supplement. For example, the following expression constraint matches any descendant of the concept 195967001 | Asthma (disorder)| , or any inactive concept that is associated with a descendant of 195967001 | Asthma (disorder)| via an historical association, as long as the concept has a description with the identifier "264553015". The only concept that matches this expression constraint is the inactive concept  170644007 | Mild asthma| . For more information on history supplements, please refer to 6.11 History Supplements.