First let me preface this by saying that I am not a doctor and I am not giving out medical advice. These are questions that you really need to ask your doctor but I will give you a brief overview.
SIgM deficiency may occur as a primary or secondary condition. Secondary SIgM deficiency is much more common than primary SIgM deficiency and may be seen in association with malignancy, autoimmune disease, gastrointestinal disease, and immunosuppressive treatment.
Some patients are asymptomatic, whereas others (often infants and small children) develop serious infections. Patients may develop prolonged or life-threatening infections caused by both encapsulated bacteria and viruses, especially in infancy. In older children and adults, SIgM deficiency is usually discovered during the investigation of other conditions, such as autoimmune disease or malignancy.
What it is very important to remember is that Lupus, as with other AI Diseases, do not have to have the symptoms or blood results all at one time. They can be cumulative or add one onto another. If you have a positive ANA that is 1:640 speckled and then normal the next time but you have a low IgM you don't ignore the fact that the ANA was positive last time you just add the new IgM as another symptom.
Too often doctors want it all at one time but that is NOT how you diagnose AI Diseases. Just because you have a rash today but it is gone next week doesn't mean that you ignore the rash. Sometimes meds change our test results and sometimes our blood work just fluctuates. Keep track of your "out of range" results (I keep a sheet of paper with all mine on them to give to the doctor at each visit. I won't let them forget my symptoms). You have to be your best advocate and we are here to support you in any way that we can