Hello, a protracted course may indicate either a viral infection (which won t respond to antibiotics), a bacterial infection that did not respond adequately to antibiotics or a complicated bacterial infection. Blood tests might be needed depending on the clinical findings. A complete blood count, C-reactive protein (quantitative), HIV and Epstein-Barr tests are some basic test to consider. Regarding treatment, since erythromycin did not work in the first place, it s doubtful it will now. In such cases I usually prefer clindamycin or amoxicillin-clavulanate (if infectious mononucleosis has been ruled out) but this is up to your doctor to decide. Your doctor know your medical history, allergies, previous treatment etc. Kind Regards!