Binding Antibody Unit (BAU): a new WHO unit concept - Dynamic NPU Manual

Binding Antibody Unit (BAU): a new WHO unit concept

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    Due to the rise of COVID 19 pandemic, huge efforts have been made to diagnose and monitor COVID 19, including an assessment of gained immunity. The latter has demanded the need of assessing a kind-of-concentration of SARS-COV-2 antibodies toward spikeprotein. WHO has established a certified reference material for this, and assigned two unit concepts, International Unit (IU) and Binding Antibody Unit (BAU), for a neutralising antibody assay and binding assay, respectively (Kristiansen PA, Page M, Bernasconi V, et al. WHO International Standard for anti-SARS-CoV-2 immunoglobulin. Lancet. 2021;397(10282):1347-1348. doi:10.1016/S0140-6736(21)00527-4).
    The latter is a novel and non-conventional approach. An International approved unit has never been directly related to it’s component (analyte) so the component type is included in the unit expression.
    -NPU28309 B—Haemoglobin; mass c. = ? g/L
    -NPU54578 B—Ethanol; mass c. = ? g/L
    -NPU54177 P—Choriogonadotropin+beta-chain; arb.subst.c.(IS 07/364; proc.) = ? IU/L
    -NPU54249 P—Antithrombin; arb.subst.c.(imm.; IS 08/258; proc.) = ? × 10³ IU/L

    The need to distinguish between measurands is there, but should it be done at the unit level?

    Young Bae


    I conquer that NPU should adopt the unconventional unit ”BAU”. The term “binding” belongs to the component; SARS CoV 2 (Spike glycoprotein) binding antibody(IgG) and the unit is an International Unit as defined by WHO. As NIBSC describes the unitage per mL, the unit as usual for International Units from WHO also has to be multiplied by a factor 10^3.

    Best regards


    I agree. The distinction should be on the component level, not on the level of unit/kind-of-property.

    In practice, having a mix of correct and incorrect unit terms is a problem that creates confusion in health care sectors. Especially, when an authority as WHO publishes an incorrect unit term. What to do?

    Another issue to discuss is how to term the antibodies with that specific property. By definition all antibodies bind to antigens which makes addition of “binding” to antibody a redundant information, like the term “total”.
    As I understand it, the specific antibodies block interaction between vira and host cells by binding to some of the virus’ parts.
    “Neutralising” is a very broad concept, and virus may be neutralised other ways than blocking.
    I suggest that the specific antibodies can be termed:
    -SARS CoV 2 (Spike glycoprotein) antibody(IgG, blocking)
    -SARS CoV 2 (Spike glycoprotein) blocking antibody(IgG)

    Young Bae

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