Background: The initial recognition of the 2026 Bundibugyo virus (BDBV) disease outbreak was delayed by adverse political circumstances and logistical constraints. The deployed test was regulatory cleared but intended for Ebola virus rather than BDBV, a distinct viral species. After this limitation had been recognized, development of BDBV-specific tests was complicated by the absence of a virus isolate representing the outbreak strain. Increased regulatory demands on diagnostics can improve quality and patient safety but pose challenges during outbreak response.<br><br>Methods: To compensate for the lack of a virus isolate, we generated an inactivated patient-derived genome reference standard (GRS; 2·4 × 106 copies/mL) for international distribution and conducted rapid multicentre validation of pilot diagnostic tests within academic and public health laboratory networks. Four commercial real-time RT-PCR kits, one CE-labelled and three research use-only (RUO) assays, were assessed for analytical sensitivity, run-to-run consistency, and specificity using limiting dilution/probit analysis, water controls, cultured pathogen materials (52–62 samples per test, representing 30 pathogens), and clinical leftover samples (120–172 samples per assay, representing 22 differential diagnostic aetiologies).<br><br>Findings: The GRS remained stable for 90 h at –80 °C, 25 °C, and 38 °C, including after 1:10 dilution. Estimated 95% LODs were 8·4 copies/µL (Altona RealStar Filovirus Screen), 2·4 copies/µL (Altona RealStar Bundibugyo), 11·9 copies/µL (Roche LightMix Modular Ebola), and 2·5 copies/µL (Roche LightMix Modular Bundibugyo). No cross-reactivity occurred in cultured or clinical panels. Bundibugyo virus-specific tests were more sensitive than broad-range Filovirus or Orthoebolavirus tests.<br><br>Interpretation: Decentralized rapid evaluation can facilitate emergency-use decisions, national authorisation, or regulatory review in ongoing outbreaks. This approach defines a modus operandi for laboratory networks to safeguard diagnostic medical countermeasures in collaboration with manufacturers. It bridges the interval between first outbreak recognition and availability of fully certified commercial diagnostic tests, especially for rare high-consequence pathogens for which virus isolates or clinical materials are scarce.