Infectious Disease · Medical Literature
[Articles] Evaluating the use of the first WHO International Standard for anti-SARS-CoV-2 immunoglobulin to harmonise the readouts between commercial SARS-CoV-2 binding antibody tests: the HARMONY multicentre calibration study
Abstract
These findings provide proof-of-concept evidence that for the biologically important SARS-CoV-2 spike protein, levels of anti-RBD, S1, or spike IgG binding antibodies can successfully be compared across a range of well validated commercial assays with different designs and formats through previous calibration and reporting in standardised units. The BAU/mL values assigned to the two secondary standards enable their use as alternative calibration reagents.
Clinical relevance
Can calibration of widely used commercial SARS-CoV-2 spike protein binding antibody assays against the first WHO International Standard (NIBSC code 20/136) and expression of results in Binding Antibody Units per milliliter (BAU/mL) achieve harmonisation of readouts across assays with different designs and formats? __SNAPSHOT__ {"studyDesign":"Multicentre calibration study (proof-of-concept laboratory methods study; not a randomised trial or clinical outcome study)","setting":"International multicentre; conducted through the WHO Solidarity II programme on SARS-CoV-2 seroepidemiology; led by the UK National Institute for Biological Standards and Control (NIBSC) and collaborating laboratories","sampleSize":"Not reported in the abstract; full sample size, number of participating laboratories, and number of assays evaluated are not available from the abstract alone","population":"Commercial SARS-CoV-2 binding antibody assays measuring anti-RBD, anti-S1, or anti-spike IgG; assays of different designs and formats; two secondary reference standards evaluated as alternative calibration reagents","intervention":"Calibration of commercial SARS-CoV-2 spike protein binding antibody assays against the first WHO International Standard for anti-SARS-CoV-2 immunoglobulin (NIBSC code 20/136), with results expressed in BAU/mL; assignment of BAU/mL values to two secondary standards","comparator":"Cross-assay comparison of IgG readouts in BAU/mL across assays measuring the same antigenic target (anti-RBD, anti-S1, or anti-spike), versus comparison in proprietary manufacturer units","primaryOutcome":"Harmonisation of IgG quantification in BAU/mL across different well-validated commercial assays measuring binding to the same SARS-CoV-2 spike antigenic target, following calibration against the WHO International Standard","followUp":"Not applicable (laboratory calibration study; no patient follow-up)","fundingSource":"Non-profit/government: conducted through the WHO Solidarity II programme, funded by the Swiss Agency for Development and Cooperation; all authors declared no competing interests"}
Key findings
The study reports proof-of-concept evidence that quantification of anti-RBD, anti-S1, or anti-spike IgG binding antibodies in BAU/mL — achieved through prior calibration of commercial assays against the first WHO International Standard (NIBSC 20/136) — enabled harmonisation of results across a range of well-validated commercial assays of differing designs and formats. Harmonisation was demonstrated for assays measuring binding to the same antigenic target; it was not shown to apply across assays directed at different SARS-CoV-2 antigens (e.g., anti-RBD cannot be harmonised with anti-nucleocapsid). Two secondary standards were assigned BAU/mL values traceable to the WHO International Standard, validating their use as alternative calibration reagents when the primary standard is unavailable. No clinical outcome data (e.g., correlates of protection, sensitivity/specificity for disease severity) were reported. No p-values, confidence intervals, effect sizes, or formal statistical comparisons between assay readouts are reported in the available abstract. The magnitude of residual inter-assay variability after calibration, and the number of assays or laboratories included, are not reported in the abstract.
Practice implications
This study established that calibration of different commercial anti-SARS-CoV-2 spike IgG binding assays to the first WHO International Standard, with results reported in BAU/mL, can harmonise quantitative readouts across platforms — but only when those assays target the same viral antigen (RBD, S1, or full spike) and detect the same immunoglobulin class (IgG). This is a laboratory methods and standards study, not a clinical efficacy or outcome trial. It does not establish clinical thresholds for protection, define correlates of immunity, or validate BAU/mL levels for individual patient management decisions. Patients not represented include those with Omicron-variant-specific antibody profiles, immunocompromised individuals, or those with infection-induced versus vaccine-induced antibody compositions, which prior literature has shown may exhibit differential harmonisation behaviour across platforms. The finding that secondary standards can serve as traceable calibration reagents is operationally important for laboratories that cannot access the limited-supply primary standard. Clinicians and laboratory directors should note that harmonisation does not equate to interchangeability: assays measuring different antigens or different immunoglobulin classes remain non-harmonisable by this method. Since only the abstract is available for this summary, quantitative data on the degree of harmonisation achieved, residual inter-assay variability, and study scale cannot be fully assessed. Practice change in laboratory standardisation policy may be supported by these findings, but clinical decision thresholds based on BAU/mL values have not been validated by this study. __LIMITATIONS__ 1. Only the abstract is available; full methodological details — including the number of participating laboratories, the number and identity of commercial assays tested, sample types and sources (vaccinated vs. convalescent vs. acutely infected), and the statistical approaches used — cannot be assessed. 2. The study is a proof-of-concept calibration exercise, not a prospective clinical study; no clinical outcomes or patient-level data are reported. 3. Harmonisation is demonstrated only for assays targeting the same antigen and immunoglobulin class; cross-antigen or cross-isotype harmonisation is explicitly not supported. 4. Prior literature (including national external quality assessment programmes) has documented persistent inter-assay variability in BAU/mL readouts even after WHO IS calibration, particularly in convalescent or acutely infected samples versus vaccinated samples; it is unclear how the HARMONY study addresses this concern. 5. The first WHO International Standard (NIBSC 20/136) was derived from convalescent plasma pooled from 11 individuals and may not fully represent the antibody repertoire induced by current SARS-CoV-2 variants or contemporary vaccines. 6. The clinical correlates of specific BAU/mL thresholds for protection remain incompletely defined, limiting the translational utility of harmonised readouts for individual patient risk assessment. 7. Generalisability to low- and middle-income country laboratory settings, which may lack access to well-validated commercial assays or WHO-calibrated secondary standards, is not addressed. __BOTTOM_LINE_CATEGORY__ potentially_practice_informing __BOTTOM_LINE__ The HARMONY multicentre calibration study provides proof-of-concept that commercial SARS-CoV-2 spike IgG binding assays with different designs can produce harmonised, cross-comparable readouts when calibrated against the first WHO International Standard and results are reported in BAU/mL — but only for assays measuring the same antigenic target. Two secondary standards were validated as alternative calibrants, supporting broader implementation when the primary standard supply is limited. This is a laboratory standardisation finding with significant public health and research implications; it does not establish clinical thresholds for immunity or change individual patient management, and confirmatory data on the degree of residual inter-assay variability and clinical correlates are still needed.