Request for feedback on Patches strength representation

Stuart Abbott I have just looked at hyoscine patch: Scopoderm 1.5mg Patch

https://www.medicines.org.uk/emc/product/13379/smpc

It is a flat, round reservoir patch approximately 1.8 cm in diameter. One side of the patch is tan; the other side is silver and is placed on an oversized clear hexagonal film. Each patch is a flat system of laminates, sealed around the edge, containing a clear oily filling. Each system has a contact surface area measuring 2.5cm2 and hyoscine content of 1.5mg. The average amount of hyoscine absorbed from each system in 72 hours is 1mg.

In the international edition we have 769509004 |Product containing precisely scopolamine 13.889 microgram/1 hour prolonged-release transdermal patch (clinical drug)|

I think to my mind this raises the issue of presentation strength (1.5 mg patch)  v concentration strength described as 1 mg per 72 hours and SCT have done the maths to equal 13.889 micrograms/1 hour (note: our Ed guidance for rounding is to 3 decimal places).

Linda Bird Matt Cordell Ian Spiers Julie M. James  and others - if you are able to document your thoughts here I am hoping to move forward with this speedily once we have consensus.

Hi Nicola Ingram ,

As discussed this week, I think a better approach is to consider the denominator as a single (value = 1) block of a prescribed amount of time. For example 1 by 24 hours, or 1 by 16 hours, where 24 hours or 16 hours are a single indivisible block of time.

This means keeping the denominator value of 1, and having unit of measure concepts for the required blocks of time - e.g. 8 hours, 16 hours, 24 hours, 72 hours etc.

The advantage of this approach is that the ratio strength doesn't appear divisible and comparable to other equivalent ratios, which for these products they are not.

For example a 24 mg 24 hour patch does not produce a linear 1 mg/h - the exact rate is complex and unknown. All that we know is that over 24 hours a 24 mg dose is delivered, probably with a higher rate at first that drops off over time.

Going back over 10 years in AMT we had this debate and extensive conversations with industry and our clinical safety team. The conclusions were that given the 24 mg 24 hour patch example we

• definitely couldn't represent this as 1 mg/h, because that is not correct.
• couldn't represent this as follows because this can be divided down and is equivalent to 8 mg / 8 hours or 1 mg / 1 hour which is incorrect
• numerator = 24
• numerator unit = mg
• denominator = 24
• denominator unit = hour

Instead we represented it as 24 mg / 24 hours where "24 hours" is a single unit of measure concept. This is effectively

• numerator = 24
• numerator unit = mg
• denominator = 1
• denominator unit = 24 hours

It is then not possible to equate this to 1 mg/h, 8 mg / 8 hours or any other ratio.

I'd provide some examples from AMT but I can't add files here so I can't include screen shots

So here's a link https://browser.ihtsdotools.org/?perspective=full&conceptId1=45044011000036104&edition=MAIN/SNOMEDCT-AU/2025-03-31&release=&languages=en and an example product registration data is at http://www.ebs.tga.gov.au/servlet/xmlmillr6?dbid=ebs/PublicHTML/pdfStore.nsf&docid=125489&agid=%28PrintDetailsPublic%29&actionid=1

The unit of measure "mg/24 hours" in this example is defined using the "24 hour" concept - you can find these at https://browser.ihtsdotools.org/?perspective=full&conceptId1=258836006&edition=MAIN/SNOMEDCT-AU/2025-03-31&release=&languages=en and https://browser.ihtsdotools.org/?perspective=full&conceptId1=258703001&edition=MAIN/SNOMEDCT-AU/2025-03-31&release=&languages=en

Note that for other patches that do quote mg/h in the registration data we model them that way. Here's an example https://browser.ihtsdotools.org/?perspective=full&conceptId1=933196301000036101&edition=MAIN/SNOMEDCT-AU/2025-03-31&release=&languages=en. You can find its registration data at http://www.ebs.tga.gov.au/servlet/xmlmillr6?dbid=ebs/PublicHTML/pdfStore.nsf&docid=163067&agid=%28PrintDetailsPublic%29&actionid=1

I hope that helps,

Dion

This is one of the problems or complexities with patches. 

Scopolamine (hyoscine in the UK) patches contain 1.5mg in the patch, and the amount absorbed on average is 1mg in 72hours. 

So what does SNOMED, or any drug dictionary, do about that. We could represent the total amount actually in the patch. but be aware that not all companies and patches will tell you that. 

Instead we generally represent the strength as the amount of drug that gets into the patient as stated by the regulatory information. As this is expressed differently between patches, we represent that even where the amount is not expressed as a per one hour. 

The scopolamine patch is interesting as at no point does the regulatory information imply an amount per hour. Only an average of 1mg over 72 hours. 

To divide that down may mean you create false equivalence between two patches that have the same per hour if you calculate it, but are not equivalent in usage.  

Response received from Canada from Linda Bird 

Canada is happy for the International Edition to proceed with the proposed change (as soon as possible).

We are happy for the strength to be represented using the “/1 hour”, “/16 hours” or “/24 hour” strength stated in the original product monograph, e.g. Estradiol 50 mcg/ 24 hour transdermal patch
In contrast to Australia’s position, we would prefer to keep the denominator value and units in separate attributes, which we believe is more consistent with the current international model and common national drug extension model. For example:
|Has concentration strength numerator value| = 50
|Has concentration strength numerator units| = |microgram|
|Has concentration strength denominator value| = 24
|Has concentration strength denominator unit| = |hour|
We would prefer that the |Has concentration strength…| attributes are used (rather than |Has presentation strength…|). The main reason for this is that we see the same issues with the presentation strength denominator value/units (e.g. “/1 capsule”, “/1 vial”) as Australia has raised, and would prefer to think of the presentation strength as equivalent to the “total amount of active ingredient” (as per the AMT’s approach). This helps us with correct subsumption when handling Powders for solution that are later reconstituted into a Solution. Happy to explain further.

Summary of feedback and discussion - proposed solution:

Patches will be remodelled as presentation strength. This approach aligns more closely with real-world pharmaceutical documentation.

For example, the strength of Nicotine 10 mg / 16 hours patch (product) would be represented as:

1142135004 |Has presentation strength numerator value (attribute)| = 10
732945000 |Has presentation strength numerator unit (attribute)| = mg
1142136003 |Has presentation strength denominator value (attribute)| = 16
732947008 |Has presentation strength denominator unit (attribute)| = hour

This approach omits the use of 733005001 |Patch (unit of presentation)| as the denominator unit value. However, currently, products such as 376491004 |Product containing precisely lidocaine 50 microgram/1 milligram conventional release cutaneous patch (clinical drug)| do not represent 'patch'. Instead, 733005001 |Patch (unit of presentation)| would be represented as the target value of 763032000 |Has unit of presentation (attribute)|. 

To (attempt to) support Nicki in coming to a conclusion:

The BN describes a first step in a process to address one of the major concerns shared by the DEUSG - the "inappropriate" use of decimal places to describe CD strengths, especially for patches, when that strength had been calculated on the basis of a "per hour" description, when clinically the products are described as "per 24 hours" etc.

So, in the example of the hyoscine/scopolamine patch, this would in future be described as having a strength of 1mg/72hours, which is its clinically relevant strength, the strength that will be used for posology calculation and the sensible prescribable name in RxNorm - not as currently in S-CT where we have 13.889 microgram/1 hour - which is clinically unhelpful and possibly not even accurate because, as discussed by contributors above, it is unlikely that in the first hour or so the patient will actually receive 13-14mcg of hyoscine....which is why the posology indicates that the patient should apply the patch 5-6 hours before the therapeutic effect (to counteract motion sickness) is required.

In terms of keeping the denominator value and units separate, to do anything else would compromise the existing model and would not appear to add value.  Implementations will need to parse composite units such as "per 24 hours" so that posologies can be at least estimated, even if not calculated exactly, especially in situations where multiple formulations are used (for example in palliative care).

For transdermal patches, the strength is truly neither a concentration strength nor a presentation strength; those of us who have been around a long time will remember that any strength described with a unit of time as a denominator is a rate strength.  However, it was considered not essential to have an explicit rate strength type for the transdermal patches and therefore to use one of the two existing strength types as a proxy for a rate strength.  It was originally suggested that this could be concentration strength, but over time, that has evolved to the current suggestion to use presentation strength as a proxy for rate strength, because in using a "per 24 hour"  approach, the description is of the "rate of the presentation of the medicine".  So neither presentation nor concentration strength is strictly correct, and the choice of which to use is not easy.  I note that Linda/Canada has a preference for concentration strength because of subsumption...and maintaining consistency with reconstitution of powders into solutions/suspensions etc.  I have to admit my brain is boggling somewhat at putting those two things different concepts together....A concentration strength can allow subsumption of appropriate presentation strengths CDs, when the presentation strength CD encloses or presents the continuous dose form in a unit of presentation (whether reconstituted or provided directly).  I am not clear how this pattern of uses would relate to a transdermal rate strength in a patch.  

Proposed Modelling Approach for Pharmaceutical Patches

Patches present a challenge, particularly where strength is expressed as a rate of delivery over time. No single approach perfectly satisfies all editorial, clinical, and technical perspectives, as evidenced by the varied feedback received. The following proposal represents, I think, a balanced and practical way forward for the International Edition, based on the collective input received to date.

Modelling Approach
1. Use of Has presentation strength 
Patch strength will be represented using Has presentation strength attributes when the label describes delivery over a defined period — e.g. 10 mg / 16 hours or 50 micrograms / 24 hours.

Example: Nicotine 10 mg / 16 hour patch

1142135004 |Has presentation strength numerator value| = 10  
732945000 |Has presentation strength numerator unit| = mg  
1142136003 |Has presentation strength denominator value| = 16  
732947008 |Has presentation strength denominator unit| = hour  
This reflects the labeled intent and avoids inappropriate conversions (e.g. to per-hour rates) that could introduce clinical risk or suggest misleading equivalence. Modeling based on 10 mg / 16 hours reflects the total amount delivered over a stated period, which is not necessarily consistently delivered per hour across that timeframe. This point will be addressed in editorial guidance.

2. Denominator values
While there is valid concern (notably from AMT) that expressions like 10 mg / 16 hours could be misinterpreted as mathematically equivalent to 0.625 mg/hour, the SNOMED CT data model requires a decimal number value for the denominator. Indivisible time blocks like "16 hour" cannot currently be used as units.
Editorial guidance will stress that use of non-unit denominators does not imply that mathematical conversion to unit-based rates is appropriate, and that only rates explicitly stated in the product label should be used.

3. Has unit of presentation = Patch will not be included
When the dose form already conveys “patch” (e.g. cutaneous patch, transdermal patch), the use of Has unit of presentation = Patch has been flagged as redundant and will not be assigned. This is not in line with current international editorial guidance, but reflects recent feedback. Also, if included, it would be an exception as the only product type modeled with presentation strength using Has unit of presentation (attribute) where that value would not be mirrored in the Has presentation strength denominator unit (attribute).

Application to Common Patch Types
To support implementation, common types of patches are listed below.

1. Prolonged-release transdermal patches
Example:
1231775000 |Product containing precisely nicotine 1.563 milligram/1 hour prolonged-release transdermal patch (clinical drug)|

Modelling approach:
Use presentation strength based on the total amount delivered over the specified period (e.g. 10 mg / 16 hr), aligned with the drug label.

2. Opioid patches with per-hour strengths
Example:
Fentanyl 25 microgram/hour patch

Modelling approach:
Where the per-hour rate is directly stated on the label, model using presentation strength with “per hour”.
If needed, further discussion can clarify whether this is a justifiable exception for using concentration strength, though I believe consistent use of presentation strength may be preferable for simplicity and alignment.

3. Concentration-like strength expressions (e.g. lidocaine)
Example:
376491004 |Product containing precisely lidocaine 50 microgram/1 milligram conventional release cutaneous patch (clinical drug)|

Modelling approach:
Express using presentation strength, preserving the numerator/denominator structure from the label.

4. Unit-based patches with a total amount per patch
(Only one in the SNOMED CT International Edition, so a kind of exception)
Example:
786029003 |Product containing precisely capsaicin 179 milligram/1 each conventional release cutaneous patch (clinical drug)|

Modelling approach:
Continue using presentation strength, with a denominator of 1.

Editorial Guidance
Guidance will be drafted to clarify:

• To use presentation strength for patches (no unit of presentation)
• That denominator values should match the product label
• Why per-hour conversions should be avoided unless explicitly stated

Existing concepts will be inactivated and replaced.

Sorry, I'm late to the discussion.

The proposed model looks reasonable to us in the Norwegian NRC, we've already adopted this approach based on the needs of our national drug catalogue, so we're happy to transition to the same solution internationally. Please get in touch if you'd like to have a list of our existing concepts in this format, Nicki, we'd be happy to provide them.

As a bonus, this approach seems to align with planned modeling of PhPIDs for IDMP as per the most recent business rules for construction of PhPID level 4-concepts:

https://gidwg.org/wp-content/uploads/2024/11/Business-Rules-for-PhPID-Construction-public-version.pdf

Page 16:

Hello everyone and many thanks for your feedback and to Hanne especially for confirming that:

this approach seems to align with planned modeling of PhPIDs for IDMP as per the most recent business rules for construction of PhPID level 4-concepts.

I have received feedback regarding the lack of consistency with other concepts modeled as presentation strength if unit of presentation is not included, and also the clinical usefulness of the attribute 763032000 |Has unit of presentation (attribute)| with value 733005001 |Patch (unit of presentation)|. Therefore it is included in the model. I have now drafted the informational Briefing Note that is being circulated to the Member Forum, CMAG and EAG and it can be viewed here and is attached also:

Briefing Note Patch Strength New Model

The changes are planned for Q3 2025

Shane Byrnes Alejandro Lopez Osornio 

For discussion at DEUSG Thursday 5th June 11:00 UTC:

The proposal is that all patches be modeled using presentation strength, which aligns with SPC documentation and typically reflects the amount of drug delivered to the patient over a specified time. A question has been raised about whether opioid patches like fentanyl should be treated as exceptions and modeled as concentration strengths. While fentanyl patches are expressed in micrograms per one hour, they are not normalized concentration strength forms; this representation is consistent with product labeling and regulatory expectations. The /1 hour expression does not imply that calculations can be derived in the way they can for true concentration strengths - no more than with other patch types. The hourly rate is used for clinical reasons: fentanyl’s high potency means even small dosing variations can have significant consequences. Regulatory guidance may mandate hourly expression to support safe titration and precise dose adjustment in pain management. This contrasts with other patches, such as hormone or nicotine replacement therapies, where strength is typically stated as a total over the patch’s duration or per day (e.g., 16- or 24-hour nicotine patches based on clinical use). IDMP introduces a third modeling option called Pattern C which uses concentration strength with a unit of presentation (e.g., per patch), acting as a proxy for rate. However, the actual product behavior is more consistent with presentation strength. IDMP further states that this pattern applies to a “product enclosed in a presentation where dose has a delivery rate,” placing emphasis on the presentation rather than on a derived concentration. This supports the position that all patches, including fentanyl, should be modeled using presentation strength. 

Shane Byrnes (Hanne Johansen re IDMP)

Following yesterday's DEUSG meeting and the Patches discussion (Agenda item 2) I have reviewed the Global IDMP Working Group document of 1st May 2025.posted on https://gidwg.org/. It is essentially the same as the previous version but presented in a different format. Patches are still assigned to “Pattern C”, defined as:

Pattern C: “The strength with delivery rate over time ‘Pattern C’ is when the strength of a substance is expressed as a delivery rate over time, such as mg/h.”

PhPID Value Denominator
"The denominator for the value number should always be 1 (e.g., mg/mL) except for patches and vaginal rings, where it may differ."

Example: 5 mg/16 hours → value number = 16.

Expression of Strength for Patches Delivery rates may be expressed in a variety of ways, such as: /h, /8h, /24h, /72h, etc. The strength is the delivery rate as stated in the SPC.

Example on Page 19 – Fentanyl
This example (expressed as per hour or /1 hour) and matches the BN proposal for /1 hour. For discussion of opioid patches see my comment above.

 https://gidwg.org/ differs in that GIDWG uses concentration strength as a proxy for rate, and we propose presentation strength. Thus for SNOMED CT concentration strength we would still be keeping the concentration strength denominator always as 1 (e.g. mg/1 mL). For our rationale for using presentation strength see comment above.

There were no objections raised to the Briefing Note during our DEUSG discussions, but please add feedback and comment here. This item will be open until the next DEUSG in a fortnight.

Thanks!

Having received no objections since the last DEUSG meeting two weeks ago including to the proposal to change opioid patches to use presentation strength I have begun work on the changes.

Opioid patches will be an exception to the usual inactivation and recreation process. Because their strengths are normalized to per 1 hour (/1 hour), they cannot be inactivated and re-created using the exact same terming. Therefore, the only changes that will be made to opioid patches are:

The addition of Unit of presentation

A change from concentration strength to presentation strength

For the remaining patches, the work is nearly complete. Following research, the new patch Clinical Drugs align with the SPC documentation, and the inactivation of existing concepts (as duplicates) and creation of new Clinical Drugs is almost finished.

All changes will be promoted together in time for the August 2025 release.

Hello DEUSG, scheduled for Q3 2025 in Early Visibility, and on course for 1st August release but an issue has arisen in the re-modelling and the release date will now be Q3 1st September. Apologies to all for the delay in this long awaited update. 

The work has been completed, but research has uncovered an issue with a change in strength representation of GTN patches in the US.

While other jurisdictions have /24 hours patch strengths for Glyceryl trinitrate patches the US differs:

NOW /1 HOUR RATE  - SEE PAGE 8 of pdf drug label information https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=5e15296c-5acd-4bcc-80a8-b49cb1ef3eb0

*Release rates were formerly described in terms of drug delivered per 24 hours. In these terms, the supplied nitroglycerin transdermal system would be rated at 2.5 mg/24 hours (0.1 mg/hour), 5 mg/24 hours (0.2 mg/hour),  10 mg/24 hours (0.4 mg/hour), and 15 mg/24 hours (0.6 mg/hour).

Nicholas McGraw Patrick McLaughlin further discussions will be required re RxNorm and impact. John Snyder 

Please comment regarding impact of a change to /24 hour presentation strength or any mitigation required.

All feedback welcome.

Nicki vacation: 4th July-16th July. Return to work 17th July.

Nitro-Dur when it was available in the UK was expressed as 'Nitro-Dur 0.2mg/hour transdermal patches'. We expressed that as a type of 'Glyceryl trinitrate 5mg/24hours transdermal patches'. 

As far as I can tell, all newer GTN patches are expressed as x per 24 hours in the licensing. 

For example: 'Deponit 5 mg/24 h transdermal patch' 

'One patch contains glyceryl trinitrate 18.7 mg
The average amount of glyceryl trinitrate absorbed from each patch in 24 hours is 5 mg. '

And it says later:

'Distribution
The steady-state concentration in the plasma depends on the patch dosage and the corresponding rate of absorption. At a rate of absorption of 0.4 mg/h, the steady-state concentration is about 0.2 µg/l on average. Plasma protein binding
is about 60%.'