Is it common for a blood test to miss lyme disease? also can lyme disease lead to weight loss?

Yes: Are difficult to detect for lyme disease unless it is an acute infection with a classic target lesion fever and malaise and the chronic state you have to use serology this is very nonspecific and sometimes it is difficult to detect the lymes antibody. Lymes disease can lead to weight loss especially the chronic phase.

Yes to both: It is fairly common for lab tests for lyme disease to be a false negative. The cdc criteria for a positive test is so strict that it might miss up to 50% of patients who have lyme disease.