Request for an unit: 50% Chicken Infective Dose

[AAndics]

• unit: 50% Chicken Infective Dose
• symbol: CID50

According to the EU SPOR RMS Team, the symbol is not available in UCUM.

Unfortunately, NONE of the current 50% infective dose methods are appropriate for our case:

Determination, > Symbol, > Method, > Tests system
50% cell culture infectious dose (chicken), > CCID50, > Titration is performed by inoculating cell culture, > Cell culture,
50% tissue culture infectious dose (chicken), > TCID50, > Titration is performed by inoculating tissue culture, > Tissue culture,
50% embryo infectious dose (chicken), > EID50, > Titration is performed by inoculating embryonated eggs, > Embryonated egg,

Our veterinary company use a scientifically published titration method that is clearly distinguished from the above methods:

Determination, > Symbol, > Method, > Tests system,
50% Chicken infectious dose, > CID50, > Titration is performed by inoculating SPF chickens, > Chicken,

Example publications:

Ferreira et al. (2019)
Suzuki et al. (2010)
WOAH Terrestial Manual 2024, Ch 3.3.12 Infectious Bursal Disease (IBD), page 16-17, 19.

We kindly request to add unit "50% Chicken Infective Dose" and its symbol "CID50" to the UCUM.

Kind regards
Andrea Andics

WOAH_3.3.12_IBD.pdf

Ferreira et al_2019.pdf

Suzuki et al_2010.pdf

[SRKid]

Here is ChatGPT 5.0's analysis:

Short answer: this is a reasonable and defensible addition to UCUM as an arbitrary infectivity unit, provided it is named and scoped unambiguously and follows UCUM’s existing pattern for ID50-style “bioactivity” atoms (e.g., [CCID_50], [TCID_50], [EID_50]). Below are my findings and a concrete recommendation you can take back to the UCUM issue.

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What UCUM already covers (precedent)

• UCUM already defines multiple 50% infective-dose style units as atomic arbitrary units (isolated from conversions) for virology/vaccinology, notably:

  • [CCID_50] — 50% cell culture infectious dose (print symbol CCID50). ([reference.opcfoundation.org]1)
  • [TCID_50] — 50% tissue culture infectious dose (print symbol TCID50). ([reference.opcfoundation.org]1)
  • [EID_50] — 50% embryo infectious dose (print symbol EID50). ([reference.opcfoundation.org]1)

  FDA’s SPL “Units of Measure” page also lists these UCUM codes, reinforcing their status in regulated health data exchange. ([U.S. Food and Drug Administration]2)

• In UCUM, such units are “customary/arbitrary” atoms that are enclosed in square brackets and not convertible to other units—exactly the right treatment for assay-defined bioactivity titers. UCUM explicitly allows these as terminal symbols; full names/print symbols are out of scope (only the c/s and c/i codes are normative). ([ucum.org]3)

Implication: A species-specific, in-vivo infective-dose atom is consistent with UCUM practice if there is clear, unambiguous usage in the scientific/standards literature.

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Evidence that “CID50” (chicken infectious dose 50%) is an established term

Authoritative standards & protocols:

• WOAH (OIE) Terrestrial Manual, 2024 – Infectious Bursal Disease (IBD): Challenge by “approximately 100 CID50 (50% chicken infective dose).” The manual’s methods section uses “CID50” explicitly in the sense requested (live SPF chickens). ([WOAH]4)

Peer-reviewed literature using CID50 for chickens:

• Journal of Virology (ASM) – methods sections refer to “median (50%) infective dose for chickens (CID50).” ([ASM Journals]5)
• Veterinary Research (BMC) – oral inoculation of SPF chickens with “10³ CID50 of IBDV,” alongside TCID50 for a different virus in the same experiment; this shows concurrent, differentiated use of CID50 vs. TCID50. ([BioMed Central]6)

Additional contextual literature confirms the ecosystem of ID50 measures in avian species (EID50, LD50, etc.), underscoring the need to distinguish in vivo chicken from culture-based systems. ([FAOHome]7)

Implication: There is sufficient, sustained usage in standards (WOAH) and refereed literature to merit a UCUM atom.

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Ambiguity risk and how to mitigate it in UCUM

“CID50” is overloaded in the broader literature:

• Chimpanzee infectious dose 50% (hepatitis C vaccine/challenge literature). ([PMC]8)
• Cellular/cell infectious dose 50% (some virology contexts and public health modeling abbreviate “CID50” as a culture-based 50% dose). ([PMC]9)

UCUM helps by using distinct atomic codes; print symbols are out of scope. To avoid confusion:

• The UCUM code should be [CID_50] with an explicit full name “50% chicken infectious dose.”
• The kind of quantity should follow existing entries: “biologic activity (infectivity) of an infectious agent preparation.” ([reference.opcfoundation.org]1)
• The definition text in the UCUM table (non-normative) should explicitly say “determined by inoculating SPF chickens (in vivo), per established methods such as Reed–Muench/Spearman–Kärber” (method details themselves belong in the test definition/LOINC or protocol, not the unit). For background on TCID50 calculation conventions, see Lei et al. 2020. ([PMC]10)

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Why the existing UCUM atoms do not cover this use case

• [CCID_50] / [TCID_50]: Culture-based (cell or tissue), not live animals.
• [EID_50]: Embryonated eggs (in ovo), not live post-hatch chickens.
  Authoritative method manuals and papers differentiate these systems precisely because ID50 values are not interchangeable across systems; UCUM therefore distinguishes them with separate atoms. ([reference.opcfoundation.org]1)

The requestor’s use case—in vivo titration in SPF chickens—matches WOAH method language and is not covered by the three existing atoms. ([WOAH]4)

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Recommendation to UCUM editors

1. Add a new atomic unit (arbitrary):

   • Code (c/s): [CID_50]
   • Code (c/i): [cid_50]
   • Full name: 50% chicken infectious dose (or “50% chicken infective dose,” matching WOAH wording; UCUM delegates print names, but aligning to WOAH helps) (WOAH)
   • Print symbol (non-normative): CID50
   • Kind of quantity: biologic activity (infectivity) (consistent with [CCID_50]/[TCID_50]/[EID_50]). ([reference.opcfoundation.org]1)
   • Notes (non-normative description): “In-vivo median infectious dose producing infection in 50% of SPF chickens, as determined by established methods (e.g., Reed–Muench, Spearman–Kärber). Distinct from [CCID_50], [TCID_50], [EID_50].” ([PMC]10)

2. Document the rationale (in the change request/issue):

   • Widespread, unambiguous usage in veterinary standards (WOAH IBD chapter) and refereed studies. ([WOAH]4)
   • Clear semantic gap not covered by existing UCUM atoms. ([reference.opcfoundation.org]1)
   • Interoperability need in regulatory/industry data exchange (FDA SPL already relies on UCUM for these families of units). ([U.S. Food and Drug Administration]2)

3. Ambiguity management:

   • State explicitly in the UCUM narrative that [CID_50] refers to chicken (Gallus gallus domesticus) in vivo challenges. This guards against confusion with historical uses of “CID50” for “chimpanzee infectious dose” or “cell(ular) infectious dose.” ([PMC]8)

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Minor editorial considerations

• Spelling (“infective” vs “infectious”): WOAH uses “infective dose” in the IBD chapter. UCUM can list “full name” as “50% chicken infective dose” and permit “infectious” as a non-normative synonym in documentation. ([WOAH]4)
• Consistency with existing atoms: Use the underscore before “50” (i.e., [CID_50]) to match [CCID_50] / [TCID_50] / [EID_50] in UCUM c/s codes; display/print symbols omit the underscore (CID50), per existing mappings (e.g., OPC UA tables, FDA SPL). ([reference.opcfoundation.org]1)
• Method belongs to the test definition, not the unit: As with TCID50, UCUM should not encode specific titration methods or challenge routes in the unit itself; those belong in the observation definition (e.g., LOINC component/method or a protocol reference). UCUM’s spec explicitly separates the code from print names and external definitions. ([ucum.org]3)

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Bottom line

Given:

• established, standardized usage in WOAH guidance,
• repeated appearance in refereed veterinary/virology literature, and
• a clear semantic distinction from existing UCUM atoms,

adding [CID_50] (print symbol “CID50”) as an arbitrary UCUM unit is appropriate. It will improve semantic clarity and interoperability for veterinary vaccine potency and challenge titer reporting without over-encoding method details in the unit. ([WOAH]4)

If you’d like, I can draft a succinct change-request paragraph for the GitHub issue, with the exact wording and citations formatted the way UCUM likes to see them.

Here is Gemini's analysis of ChatGPT 5.0's analysis:

Yes, I concur with ChatGPT 5.0's response. The reasoning is sound, well-supported by evidence, and aligns with the established practices of the Unified Code for Units of Measure (UCUM).

Here are the reasons for concurrence:

1. Consistency with UCUM Precedent

The most compelling argument is the established pattern within UCUM for handling infectious dose units.

• Existing Arbitrary Units: UCUM already includes similar 50% infective dose units like [CCID_50] (Cell Culture), [TCID_50] (Tissue Culture), and [EID_50] (Embryo). These are defined as arbitrary atomic units enclosed in square brackets, meaning they are non-convertible and are defined by the assay itself—the correct treatment for bioactivity titers.
• Alignment: Adding [CID_50] for Chicken Infectious Dose follows this exact naming and scoping convention, making it a logical and consistent extension of the existing ID50 family of units.

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2. Clear Scientific and Regulatory Need

The request successfully demonstrates a significant gap in the current UCUM set that is necessary for clear communication in the veterinary field.

• Semantic Gap: The existing units ([CCID_50], [TCID_50], [EID_50]) all represent in vitro or in ovo systems. The proposed CID50 represents an in vivo titration using live SPF chickens, which is fundamentally different and yields non-interchangeable results. UCUM's purpose is to ensure semantic clarity, and this distinction is crucial.
• Authoritative Usage: The term CID50 is documented and actively used in authoritative veterinary standards, specifically the WOAH (World Organisation for Animal Health) Terrestrial Manual. This provides the necessary foundation of established, non-ambiguous use in a regulated context.

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3. Effective Mitigation of Ambiguity

ChatGPT 5.0 correctly identifies the risk of "CID50" being an overloaded acronym (e.g., Chimpanzee or Cell Infectious Dose 50%). Its recommended solution is the standard UCUM approach to managing such risk:

• Normative Code over Print Symbol: By defining the UCUM code as [CID_50] (with the underscore), the ambiguity is managed in the machine-readable, normative part of the standard.
• Explicit Definition: The recommendation to include a full name ("50% chicken infectious dose") and an explicit non-normative note defining it as in vivo in chickens further clarifies the intent and prevents confusion with other uses.

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Summary of Concurrence

The request is appropriate because it addresses a verifiable need for standardized veterinary reporting, uses an established term from authoritative sources, and can be integrated into UCUM in a manner that is completely consistent with the standard's existing structure and conventions for bioactivity units.

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Minor Points of Agreement

I also agree with the minor editorial considerations:

• Using [CID_50] for the code to match the existing pattern (e.g., [TCID_50]).
• Acknowledging the flexibility in "infective" versus "infectious" but prioritizing alignment with WOAH's "infective dose."
• Reinforcing the principle that UCUM defines the unit, but the specific titration method (e.g., Reed–Muench) belongs in the test definition (e.g., LOINC code or protocol), not the unit symbol itself.

[AAndics]

@SRKid
Dear William A. Hess,

Thank you for your supportive comments. We agree with it.

Kind regards
Andrea