Source: Ohio Department of Health
A virus sample on a nasal swab might spoil if it is not kept in a special liquid that keeps it fresh and viable during transport to a COVID-19 testing lab. The liquid is called viral transport medium (VTM) and supplies have run low during the pandemic.
The Centers for Disease Control and Prevention (CDC) has written instructions for professionals to make VTM themselves, and many labs and hospitals have used the document to increase their supplies. Harvard University included the instructions in a scientific study about producing mass quantities of VTM.