The shared SQL code will be added to a public repository to allow review of the underlying code directly and feedback. @alopez will update the post here when this repository is available.
Concrete domains and decimals can be used in other areas outside of medicinal products, so feedback on use in other hierarchies/use cases is welcome here also.
I like the proposal. The introduction of guard digits is new to me, and I think solves one of the issues I’ve been struggling with for a long time.
But first. The important thing here is to identify why we need these rounding rules in the first place, and what problem the rounding rules are solving. I’m not sure that’s actually specified anywhere? So, here’s my contribution.
We end up with non-terminating values when we have a product like “Posaconazole 300 mg/16.7 mL injection, vial”
In AMT we currently normalise this to have a concentration “17.964072 mg/mL” (we use a composite unit “mg/mL”).
The international model could model this with a Strength Numerator + Strength Denominator - with no need for thinking about rounding (but that’s not how it’s done).
In AMT we normalise the units so that we can do comparative arithmetic - such as identify that “Posaconazole 300 mg/16.7 mL injection, vial” and “Posaconazole 600mg/33.4 mL injection, vial” have the same concentration “17.964072 mg/mL”. And enable dose calculations by implementations.
However, these normalised “concentrations” are not always clinically intuitive, the descriptions use strength representations that reflect the registered labels. Depending on the product, this may be normalised (mg/mL) or not - mg/3mL.
What does the denominator value mean? Usually, it’s the size of the vial or syringe. Back to the “Posaconazole 300 mg/16.7 mL injection, vial” example - that’s 300mg of Posaconazole , in a 16.7 mL vial".
Clinically, it’s the 300mg that’s probably more useful. In AMT we call this the “total quantity” in the vial. For many (if not most) products - the vial represents a single dose.
So, to calculate the “total quantity” delivered by the vial = concentration x vial size
In my example - 17.964072 mg/mL x 16.7 mL = 300mg (after rounding).
It’s this round-trip calculation where the rounding errors creep in.
So while it’s helpful for us to have these rounding rules. It’s also really important to provide adequate implementation guidance for software developers to use the concrete domain modelling to implement the relevant calculations also.
And it’s typically for pack/unit sizes like 3mL that generate these recurring concentrations. And the patches - though I think the INT model has since been resolved how they’re modelled.
I think there is a fundamental tension between two different goals here, consistent with a point raised earlier by Michael:
defining deterministic strength properties to support concept definition, classification, and organisation within the terminology, and
conveying measurement precision or uncertainty about the strength of a medicinal product.
Trying to combine both goals into a single numeric property risks ambiguity, because they serve different purposes and carry different semantics.
By way of context, AMT uses three strength representation patterns:
Discrete unit products (e.g. paracetamol 500 mg tablet):
total quantity only (500 mg).
Continuous products without a stated size (e.g. amoxicillin 125 mg/5 mL):
concentration strength only (normalised, e.g. 25 mg/mL).
Continuous products in a known size (e.g. adalimumab 80 mg/0.8 mL, 0.8 mL syringe):
total quantity (80 mg)
size (0.8 mL)
calculated concentration (100 mg/mL).
Where concentration strength calculation produces a non-terminating decimal, AMT applies a deterministic rounding approach (6 decimal places, trailing zeros removed). This normalisation exists to support classification and comparison, not to convey measurement accuracy. Units are likewise normalised so values can be compared directly, and these properties are primarily for machine consumption.
Importantly these all have the same concentration strength of 100 mg/mL
adalimumab 80 mg/0.8 mL, 0.8 mL
adalimumab 40 mg/0.4 mL, 0.4 mL
adalimumab 20 mg/0.2 mL, 0.2 mL
Human-facing representations of strength and size are carried in the terms (FSN, PT, synonyms), which prioritise intuitive display and regulatory phrasing.
From this perspective, applying significant-figure and guard-digit rules to normalised concentration strengths introduces a semantic mismatch - significant figures are about measurement precision, whereas these values are intended for conceptual normalisation. Encoding precision implicitly via rounding risks overloading the property with multiple meanings.
A cleaner separation is to represent regulatory information directly using presentation strength numerator and denominator (e.g. 1.1 g in 30 mL)
presentation strength numerator = 1.1
presentation strength numerator unit = g
presentation strength denominator = 30
presentation strength denominator unit = mL
and include a separately calculated, predictably normalised concentration strength for classification. For AMT that would be 36.666667 mg/mL in this case matching the concentration for classification purposes for a product that was 110 mg in 3 mL.