Cycle threshold (Ct) values in nucleic acid amplification observables

Dear Observables Group,

Looking at microbiology reports I see a good few 'Ct' (cycle threshold) values for various organism RNA/DNA detections but I'm not seeing these described in SNOMED and I might assume a reason for this.  LOINC has these represented and uses LP188772-0 ThreshNum (threshold number) 

e.g. SARS-CoV-2 (COVID-19) RNA [Cycle Threshold #] in Respiratory specimen by NAA with probe detection

I would have just used 'number' as a property but oddly there isn't one!   In SNOMED we only have:

118589004 | Threshold (property) (qualifier value)

Which seems to be one half of 'threshold presence' property.

I cannot see a scalar number property, only vector ones.  260299005 | Number (qualifier value) seems directed at other uses and is thus not available as a property.  (I regret the use of a coherent quality/property as a mere header for numbers but that's an aside.)

To me this concept of Ct value for NAAT result is metrologically a number, 'threshold' just being added onto 'number' as extra qualification but maybe 118589004 | Threshold (property) (qualifier value) is OK? I cannot find a pure 'number' property except for 'entity' types.  This is almost a parameter setting perhaps?  

The LOINC concept seems to 'expect' a pure ordinal number but the notional 'amplification cycles' UoM seems to be included in the property? I think I'd expect not a 'pure' number but to see a UoM of '(amplification) cycles' if there is such a thing in IUPAC?  The only 'cycles' concept in SNOMED seems to be a pharmacy one.

Just for completeness, I did consider whether the Ct could be a flavour of presence, an adjunct to the pos/neg presence observable: e.g. Presence of x by NAA at Ct 25 cycles (obs entity).  It is worth considering but it would be highly combinatorial. I find in lists, the test Ct is a separate line so the information model could and should connect them IMHO. This seems more scalable anyways.

It would be useful to have a sense of the SNOMED editorial line on this topical issue (or a ruling on its in/out of scope-ness) if people think it's in order because, even in SNOMED expression terms, it is not obvious to me how this would be represented/modelled.  I'd be grateful for a view and would be happy to do a brief presentation in a group meeting if appropriate.

 Thanks.

Thanks Jim.  Whether reported or just recorded internally is a little moot here.  I've read the arguments for and against but what you write is fine.  I would still expect there to be a property that supports a simple number value as there must be other instances where this is needed (in machine parameters for example).  The only one I can see is 410681005 | Count of entities (property) (qualifier value).  Would this be the one to use, taking 'entity' in its broadest possible meaning? Daniel Karlsson?