This code system http://hl7.org/fhir/certainty-rating defines the following codes:

CodeDisplayDefinition
high High qualityHigh quality evidence.
moderate Moderate qualityModerate quality evidence.
low Low qualityLow quality evidence.
very-low Very low qualityVery low quality evidence.
no-concern no serious concernno serious concern.
serious-concern serious concernserious concern.
very-serious-concern very serious concernvery serious concern.
extremely-serious-concern extremely serious concernextremely serious concern.
present presentpossible reason for increasing quality rating was checked and found to be present.
absent absentpossible reason for increasing quality rating was checked and found to be absent.
no-change no change to ratingno change to quality rating.
downcode1 reduce rating: -1reduce quality rating by 1.
downcode2 reduce rating: -2reduce quality rating by 2.
downcode3 reduce rating: -3reduce quality rating by 3.
upcode1 increase rating: +1increase quality rating by 1.
upcode2 increase rating: +2increase quality rating by 2.
<status value="active"/><experimental value="false"/><date value="2020-12-28T16:55:11+11:00"/><publisher value="HL7 (FHIR Project)"/><contact><telecom><system value="url"/><value value="http://hl7.org/fhir"/></telecom><telecom><system value="email"/><value value="fhir@lists.hl7.org"/></telecom></contact><description value="The assessment of quality, confidence, or certainty."/><jurisdiction><coding><system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/><code value="001"/><display value="World"/></coding></jurisdiction><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/certainty-rating"/><content value="complete"/><concept><code value="high"/><display value="High quality"/><definition value="High quality evidence."/></concept><concept><code value="moderate"/><display value="Moderate quality"/><definition value="Moderate quality evidence."/></concept><concept><code value="low"/><display value="Low quality"/><definition value="Low quality evidence."/></concept><concept><code value="very-low"/><display value="Very low quality"/><definition value="Very low quality evidence."/></concept><concept><code value="no-concern"/><display value="no serious concern"/><definition value="no serious concern."/></concept><concept><code value="serious-concern"/><display value="serious concern"/><definition value="serious concern."/></concept><concept><code value="very-serious-concern"/><display value="very serious concern"/><definition value="very serious concern."/></concept><concept><code value="extremely-serious-concern"/><display value="extremely serious concern"/><definition value="extremely serious concern."/></concept><concept><code value="present"/><display value="present"/><definition value="possible reason for increasing quality rating was checked and found to be present."/></concept><concept><code value="absent"/><display value="absent"/><definition value="possible reason for increasing quality rating was checked and found to be absent."/></concept><concept><code value="no-change"/><display value="no change to rating"/><definition value="no change to quality rating."/></concept><concept><code value="downcode1"/><display value="reduce rating: -1"/><definition value="reduce quality rating by 1."/></concept><concept><code value="downcode2"/><display value="reduce rating: -2"/><definition value="reduce quality rating by 2."/></concept><concept><code value="downcode3"/><display value="reduce rating: -3"/><definition value="reduce quality rating by 3."/></concept><concept><code value="upcode1"/><display value="increase rating: +1"/><definition value="increase quality rating by 1."/></concept><concept><code value="upcode2"/><display value="increase rating: +2"/><definition value="increase quality rating by 2."/></concept></CodeSystem>