Traceable Value Assignment of a VLP-Derived HIV-1 p24 Antigen Material Using a Single-Point Calibration Framework with Dilution-Control Verification

Accurate quantification of HIV-1 p24 antigen depends on the availability of reference materials whose values are traceable to the International Unit (IU). The long-standing WHO International Standard 90/636 is now largely depleted, while access to more recent VLP-based standards remains limited, creating a practical need for locally produced materials supported by a robust and internationally traceable value-assignment procedure. We developed a streamlined, two-component framework for assigning p24 activity to a VLP-derived material using (i) single-point calibration against the WHO standard and (ii) verification through an internal dilution control prepared from the same material. A multi-laboratory study involving 16 laboratories and 37 fourth-generation Ag/Ab assays demonstrated that the approach yields stable relative-response measures and supports the use of robust statistics for activity estimation. Phylogenetic analysis confirmed that the VLP material corresponds to HIV-1 subtype B, ensuring biological alignment with existing WHO standards. The combined procedure produced an assigned value of 32 IU per vial, with dilution-control behaviour indicating correct preparation and consistent analytical parallelism across platforms. This methodology provides a reproducible, resource-efficient route for establishing traceable VLP-based p24 reference materials in settings where access to international standards or large collaborative studies is limited.