Should a low level of serum igm (26) for an adult that is asymptomatic be followed up with other diagnostic tests and if so, which ones?

IgM: Normal ranges for IgM can be challenging since this is the "first" antibody that appears, but also one that fades quickly. Neoplasms that make IgM are a bigger concern in an otherwise healthy adult. So, to answer your question, No, I would not recommend you either worry about a below "normal" result nor that you follow it up. Be grateful that it is not high.