My serum levels have never been above 600, usually hover around 400 after Dx with deficiency in 1995. I've been noncompliant with shots for a year but have been taking sublingual methylcobalamin 5000mcg. I took one the morning of my test and serum measured 1968!! High levels can be a marker for cancer, but was my test skewed? I plan to repeat, but how soon? I haven't taken sublingual in at least a week.