When is it infective endocarditis?

Case scenario

Peter is a 62-year-old accountant who presented after two and a half weeks of an ongoing flu-like illness. He reported headaches, arthralgia and general malaise initially, but over the most recent week, he had been experiencing frequent episodes of palpitations (suggestive of arrhythmia), night sweats and overwhelming fatigue. There was no history of travel or insect bites or extramarital sexual activity. He reported that he had a long-term heart murmur that had followed an attack of rheumatic fever as an adolescent.

Physical examination revealed only a loud systolic ejection murmur in the aortic area. Blood tests were not helpful, with the only abnormality reported being elevated levels of C-reactive protein (CRP; 70mg/L) and erythrocyte sedimentation rate. Blood cultures were negative.

At what stage should Peter be investigated or referred on for consideration of a diagnosis of infective endocarditis?