Proteinuria (from protein an urine) means the presence of an excess of serum proteins in the urine. The protein in the urine often causes the urine to become foamy, although foamy urine may also be caused by bilirubin in the urine (bilirubinuria),retrograde ejaculation, pneumaturia (air bubbles in the urine) due to a fistula, or drugs such as pyridium.