EBV is the sole causative agent of the acute illness in humans described either as infectious mononucleosis (IM), or glandular fever. IM, when not clinically silent, can present in patients with at least two of the classic triad of symptoms of fever, pharyngitis, and lymphadenopathy. Challenges for the clinician arise when atypical cases present. Early, accurate and informed laboratory test results are vital for diagnosis, appropriate treatment, and management. A key challenge for the practitioner, particularly in cases where the illness can present atypically, is distinguishing bacterial tonsillitis infections from early acute IM. The ability to draw on timely, clear, and insightful laboratory results to distinguish viral from bacterial infection is vital. Correct and prompt diagnosis of IM can help prevent the unnecessary administration of antibiotics and mitigate the need for other expensive exploratory tests in cases of IM that present with splenomegaly, lymphadenopathy, or suspect haematological conditions. Good communication between the requesting clinician and those carrying out the investigative process, and between the different laboratory departments involved, is good practice and would ultimately benefit the patient. This communication will comprehensively review the aetiology, clinical presentation, and laboratory findings in IM with a view to promoting further research and so derive a standard diagnostic algorithm of the condition.