# Description Filters

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.

{% hint style="info" %}
**Note for ECL Core**

Description filters are not part of the ECL Core profile.

Some of this functionality, like term filtering and selection, is covered by the FHIR API and should be available in FHIR Terminology Servers.
{% endhint %}

## 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](https://snomed-international.gitbook.io/uat-snomed-international-docs/kdfQQ4zexrP8YsnhBh12/specifications/wip-snomed-ct-expression-constraint-language/behaviour-specification-with-examples/6.9-concept-filters "mention")). 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.

```
{{ D term = "heart att" }}
```

### Match Term Filter

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).

```
< 64572001 |Disease| {{ term = "heart att"}}
< 64572001 |Disease| {{ term = "heart", term = "att"}}
< 64572001 |Disease| {{ term = match:"heart att"}}
< 64572001 |Disease| {{ term = "att heart"}}
```

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).

```
< 64572001 |Disease| {{ term = ("heart" "card")}}
```

### Wildcard Term Filter

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. This filter is case-insensitive.

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)".

```
< 64572001 |Disease| {{ term = wild:"cardi*opathy"}}
```

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)|](http://snomed.info/id/56265001) (with synonym "Cardiopathy"), but will **not** match the concept [870575001 | Disorder of cardiac atrium (disorder)|](http://snomed.info/id/870575001) (with synonym "Atrial cardiopathy")

```
< 64572001 |Disease| {{ term = wild:"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".

```
< 64572001 |Disease| {{ term = (match:"gas" wild:"*itis")}}
```

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").

```
< 64572001 |Disease| {{ term = "eye"}} {{ term = wild:"*itis"}}
```

Wildcard search can be used to find compound words, in English or germanic languages like Danish.

```
< 64572001 |Disease| {{ D term = wild:"*pancrea*"}}
```

Or other partial string matching where perhaps a user doesn't recall the specific spelling of the whole word.

```
< 64572001 |Disease| {{ D term = wild:"*esoph*"}}
```

## 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).

```
< 64572001 |Disease| {{ term = "hjärt", language = sv }}
```

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).

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ term = "hjärt", language = sv }} {{ term = "heart", language = en }}
```

{% endcode %}

## 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.

<table><thead><tr><th width="343.98828125">Type Keyword</th><th>TypeIdConcept Reference</th></tr></thead><tbody><tr><td><strong>Brief Syntax</strong></td><td><strong>Full Syntax</strong></td></tr><tr><td>fsn</td><td>fullySpecifiedName</td></tr><tr><td>syn</td><td>synonym</td></tr><tr><td>def</td><td>definition</td></tr></tbody></table>

For example, the expression constraint below matches all the subtypes of `56265001 | Heart disease|` , that have a fully specified name containing the word prefix "heart".

```
< 56265001 |Heart disease| {{ term = "heart", type = fsn }}
```

The following two expression constraints are equivalent, and both match only the subtypes of `56265001 | Heart disease|` , which have a Swedish synonym containing the word prefix "hjärt".

```
< 56265001 |Heart disease| {{ term = "hjärt", language = SV, type = syn }}
```

{% code overflow="wrap" %}

```
< 56265001 |Heart disease| {{ term = "hjärta", language = sv, typeId = 900000000000013009 |synonym| }}
```

{% endcode %}

The two equivalent expression constraints below match the subtypes of `56265001 | Heart disease|` , which either have a synonym containing the word prefix "heart", or a fully specified name containing the word prefix "heart".

```
< 56265001 |Heart disease| {{ term = "heart", type = (syn fsn) }}
```

{% code overflow="wrap" %}

```
< 56265001 |Heart disease| {{ term = "heart", typeId = ( 900000000000013009 |Synonym| 900000000000003001 |Fully specified name| ) }}
```

{% endcode %}

## 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](https://snomed-international.gitbook.io/uat-snomed-international-docs/kdfQQ4zexrP8YsnhBh12/specifications/wip-snomed-ct-expression-constraint-language/appendices/appendix-c-dialect-aliases "mention") 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 `64572001 | Disease |` that have a description in the Australian English language reference set.

```
< 64572001 |Disease| {{ dialect = en-au }}
```

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ dialectId = 32570271000036106 |Australian English language reference set| }}
```

{% endcode %}

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".

```
< 64572001 |Disease| {{ term = "cardio", dialect = en-nz }}
```

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".

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ term = "card", dialect = ( en-nhs-clinical en-nhs-pharmacy ) }}
```

{% endcode %}

## 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.

| Acceptability Keyword | AcceptabilityIdConcept Reference |
| --------------------- | -------------------------------- |
| **Brief Syntax**      | **Full Syntax**                  |
| prefer                | preferred                        |
| accept                | acceptable                       |

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'.

```
< 64572001 |Disease| {{ term = "box", type = syn, dialect = en-us (prefer) }}
```

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ term = "box", typeId = 900000000000013009 |Synonym| , dialect = en-us ( 900000000000548007 |Preferred| ) }}
```

{% endcode %}

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.

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ term = "box", type = syn, dialect = en-nhs-clinical (prefer), dialect = en-gb (accept) }}
```

{% endcode %}

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.

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ term = "box", type = syn, dialect = ( en-gb (prefer) en-nhs-clinical (prefer) ) }}
```

{% endcode %}

{% code overflow="wrap" %}

```
< 64572001 |Disease| {{ term = "box", type = syn, dialect = ( en-gb en-nhs-clinical ) (prefer) }}
```

{% endcode %}

## 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 `125605004 | Fracture of bone|` that do not use the word prefix "fracture" in their US English preferred term.

{% code overflow="wrap" %}

```
< 125605004 |Fracture of bone| {{ term != "fracture", type = syn, dialect = en-us (prefer)}}
```

{% endcode %}

If we remove the type and acceptability filters, as shown below, the remaining expression constraint matches on those subtypes of `125605004 | 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.

{% code overflow="wrap" %}

```
< 125605004 |Fracture of bone| {{ term != "fracture", dialect = en-us}}
```

{% endcode %}

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 `125605004| 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).

{% code overflow="wrap" %}

```
< 125605004 |Fracture of bone| MINUS < 125605004 |Fracture of bone| {{ term != "fracture"}}
```

{% endcode %}

This expression constraint can be simplified to the equivalent one below, using the wildcard character '\*' (which represents any concept in the substrate).

{% code overflow="wrap" %}

```
< 125605004 |Fracture of bone| MINUS * {{ term != "fracture"}}
```

{% endcode %}

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.

```
* MINUS * {{ type = syn, dialect = en-nz (prefer) }}
```

## 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.

{% code overflow="wrap" %}

```
< 195967001 |Asthma| {{ D moduleId = 731000124108 |US National Library of Medicine maintained module| }}
```

{% endcode %}

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

{% code overflow="wrap" %}

```
< 404684003 |Clinical finding| {{ D type = def, moduleId = 900000000000207008 |SNOMED CT core module| }}
```

{% endcode %}

## 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:

| Operator | Name                        |
| -------- | --------------------------- |
| =        | Equals                      |
| !=       | Not equals                  |
| <        | Before the given date       |
| <=       | Before or on the given date |
| >        | After the given date        |
| >=       | After or on the given date  |

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.

```
< 125605004 |Fracture of bone| {{ D effectiveTime = "20210131" }}
```

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.

```
< 125605004 |Fracture of bone| {{ D effectiveTime != "20210131" }}
```

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).

```
< 125605004 |Fracture of bone| {{ D effectiveTime >= "20190731" }}
```

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.

```
< 125605004 |Fracture of bone| {{ D effectiveTime <= "20190731" }}
```

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.

{% code overflow="wrap" %}

```
< 125605004 |Fracture of bone| {{ D effectiveTime = ("20190131" "20190731" "20200131" "20200731" }}
```

{% endcode %}

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.

{% code overflow="wrap" %}

```
< 125605004 |Fracture of bone| {{ D effectiveTime != ("20190131" "20190731" "20200131" "20200731" }}
```

{% endcode %}

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.

```
< 125605004 |Fracture of bone| {{ D effectiveTime = "" }}
```

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.

```
^ 816080008 |International Patient Summary| {{ D active = 1 }}
```

```
^ 816080008 |International Patient Summary| {{ D active = true }}
```

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

```
^ 816080008 |International Patient Summary| {{ D active = 0 }}
```

```
^ 816080008 |International Patient Summary| {{ D active = false }}
```

## 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)|` .

```
*{{ D id = 3032638017 }}
```

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)|` .

```
< 195967001 |Asthma (disorder)| {{ D id = 3032638017 }}
```

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".

```
< 195967001 |Asthma (disorder)| {{ D id = (1208972017 2674140012 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| .

```
(< 195967001 |Asthma (disorder)| {{+HISTORY}}) {{ D id = 264553015 }}
```

For more information on history supplements, please refer to [6.11-history-supplements](https://snomed-international.gitbook.io/uat-snomed-international-docs/kdfQQ4zexrP8YsnhBh12/specifications/wip-snomed-ct-expression-constraint-language/behaviour-specification-with-examples/6.11-history-supplements "mention").

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