I've tested positive for IgM deficiency twice. The tests were years apart too so not likely to be a lab error, I don't think.
Still doctors don't know what it means or what to do with it so they just shrug.
From.National Organization of Rate Disorders;
Selective IgM deficiency (SIgMD) is a rare immune disorder in which a person has no immunoglobulin M (IgM) antibodies, or too little IgM, with normal levels of IgG and IgA antibodies. IgM is the first antibody the immune system makes to fight a new infection.
Therefore, when a person does not have enough IgM, the body may have difficulty fighting infections. SIgMD can occur in infants, children, or adults. The disorder may occur as a primary disorder (on its own) or more commonly, as a secondary disorder (associated with another underlying disease or condition). SIgMD may occur in association with some cancers, autoimmune diseases, allergic diseases, and gastrointestinal diseases.
Symptoms of SIgMD may include repeated viral, bacterial, or fungal infections, such as ear infections, bronchitis, sinusitis, and pneumonia. Infections may be life-threatening. Repeated infections are common in infants with SIgMD. In some people, diarrhea or a skin rash is the first symptom. Others do not have symptoms specific to SIgMD and are diagnosed when being evaluated for another condition.
The diagnosis is made by blood tests showing low or absent IgM and normal levels of other antibodies, as well as diagnosing or ruling out underlying conditions that can cause low levels of IgM."