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Request for: Mc Farland standards
McFarland Standards are used to standardize the approximate number of bacteria in a liquid suspension by comparing the turbidity of the test suspension with that of the McFarland Standard. A McFarland Standard is a chemical solution of barium chloride and sulfuric acid; the reaction between these two chemicals results in the production of a fine precipitate, barium sulfate. When shaken well, the turbidity of a McFarland Standard is visually comparable to a bacterial suspension of known concentration as indicated below.
https://www.dalynn.com/dyn/ck_assets/files/tech/TM53.pdf https://en.wikipedia.org/wiki/McFarland_standards https://microbenotes.com/mcfarland-standards/
Me: The UCUM Board added "Nephelometric Turbidity Unit" and "Formazin Turbidity Unit" as units of measure to UCUM in 2024. How would this information influence the pros and cons of adding the “McFarland Standard” as a UCUM unit of measure?
ChatGPT 5.0's Response: The UCUM Board’s decision in 2024 to add Nephelometric Turbidity Unit (NTU) and Formazin Turbidity Unit (FTU) is highly relevant, because it shows that the Board is willing to admit arbitrary but widely used turbidity scales into UCUM when they are entrenched in laboratory practice and supported by international or national standards.
Let’s examine the influence of that precedent on the “McFarland Standard” case.
1. Why NTU/FTU Were Accepted
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NTU and FTU are standardized turbidity scales:
- NTU is defined via light scattering at 90° from a specific wavelength using formazin suspensions.
- FTU is essentially the same but tied to ISO 7027 (infrared light).
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They are internationally standardized (ISO, EPA, WHO), and their definitions are method-based but reproducible across laboratories.
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Despite not being SI units, they are widely accepted “practical units” in water quality testing and have established legal and regulatory definitions.
2. Comparison to McFarland Standards
Similarities:
- Like NTU and FTU, McFarland Standards are turbidity reference systems.
- They are method-defined: turbidity created by a specific precipitate (barium sulfate).
- They are widely entrenched in clinical microbiology, especially in antimicrobial susceptibility testing (AST), with global guidelines (CLSI, EUCAST).
- Their numeric scale (e.g., 0.5 McFarland) is routinely used in laboratory communication and standard operating procedures.
Differences:
- Lack of international standardization: McFarland standards are prepared chemically but not harmonized under ISO or NIST as a formal turbidity unit.
- Less reproducible: A “0.5 McFarland” suspension may approximate 1.5 × 10⁸ CFU/mL, but this varies by organism and laboratory technique. In contrast, NTU/FTU are tied to reproducible optical properties.
- Ordinal vs. quantitative: McFarland values are more like a calibration ladder than a true continuous measurement unit. NTU/FTU can be measured directly by calibrated instruments.
3. How the NTU/FTU Precedent Strengthens the “Pro” Case
- Consistency with UCUM’s evolving scope: If UCUM admits method-defined, non-SI, arbitrary turbidity scales (NTU, FTU), then rejecting McFarland solely because it is method-based seems inconsistent.
- Widespread clinical use: McFarland values are arguably as entrenched in microbiology as NTU is in water quality. A clinician or lab tech reading “0.5 McFarland” expects a recognized unit.
- Improved interoperability: Adding “McFarland” would avoid workarounds (e.g., free-text or custom extensions) in LOINC/FHIR messaging.
4. How the NTU/FTU Precedent Strengthens the “Con” Case
- Slippery slope: Admitting McFarland could open the door to many other laboratory reference scales (e.g., “McIlvaine buffer pH units,” “Bates Turbidity Standards”), fragmenting UCUM with method-defined pseudo-units.
- Weaker metrological basis: Unlike NTU/FTU, which are anchored in ISO standards, McFarland remains a historical, informal method with batch-to-batch variation and no strict international definition.
- Clinical specificity: McFarland standards are largely confined to microbiology. NTU/FTU apply across water, environmental, and industrial sectors, making them more universal.
5. Balanced Assessment
- If UCUM wants to stay tightly tied to international standards bodies (ISO, NIST, WHO), then McFarland does not qualify, because it lacks formal codification.
- If UCUM’s practice is pragmatic, reflecting entrenched usage in clinical and laboratory practice, then McFarland might be justified, provided it is clearly labeled as a method-based turbidity reference scale (like NTU and FTU).
✅ Conclusion: The addition of NTU and FTU in 2024 weakens the argument that McFarland should be excluded simply because it is a method-defined, arbitrary scale. It creates a precedent: UCUM may include widely used, clinically essential, method-based turbidity standards.
However, NTU/FTU’s acceptance was bolstered by international standardization, while McFarland lacks that backing. The UCUM Board would need to decide whether clinical entrenchment alone is sufficient grounds for inclusion, or whether they should wait until McFarland is formally codified by ISO/CLSI/WHO.
