Diagnostics Testing Preparedness Plan Act of 2025This bill requires the Department of Health and Human Services (HHS) to develop and publish a plan for the rapid development and distribution of diagnostic tests in response to a chemical, biological, radiological, or nuclear threat, including an emerging infectious disease, that causes or has significant potential to cause a declared public health emergency.Specifically, HHS must coordinate with any relevant public and private entities, such as government entities and device manufacturers, in creating the plan and facilitating its collaborative implementation. The plan must take into consideration certain factors specified in the bill, including domestic testing capacity, novel technologies, and medical supply needs. HHS must publish the plan within one year after enactment of the bill and then update the plan every three years.