Blindsided by the Monospot test.

A 20-year-old man developed bilateral forearm paresthesias after propping his elbows on the desk studying. He was diagnosed with ulnar neuropraxia and instructed to follow up with a neurologist. The symptoms continued and the patient was admitted for a formal workup of his neuritis. A Monospot test was positive. The patient was discharged with a diagnosis of infectious mononucleosis. A comment on his complete blood count, showing absolute lymphocytosis with atypical lymphocytes and rare blasts with flow cytometry recommended, was missed, possibly due to the fact that it was not highlighted red on the electronic medical record indicating an abnormal result. A month later, an outpatient diagnosis of B-lymphoblastic leukemia/lymphoma was made on flow cytometry. This case highlights cognitive errors in diagnosis, including premature closure and failing to 'see' key data, as well as vulnerabilities created by data display limitations of the electronic medical record.