Does low serum globulin affect blood tests that assess immunoglobulin levels? for example if an illness caused proteinuria, could a disease actually cause the test for it to be falsely negative?

If I follow you: If you have low proteins and then get a disease that causes protein in your urine, could a test be falsely negative ? Well a smart doc would know to order quantitative protien and then it could be interpreted in light of known low immunoglobulins . Some people always spill small amounts of protein, most do not. Doc must know this info too. It's complicated.

Depends: If your immunoglobulin(s) is low especially when it involves igg, your immune system is likely impaired. It is not a "false-postive" result although the cause for the deficiency may vary and treated accordingly.