Non-malignant CSF lymphocytosis in a patient with acute lymphoblastic leukemia

Patients with haematological malignancy are often profoundly immune suppressed, and more so if they require more than one line of therapy. Infection should always be considered when they become unwell. We discuss the differential diagnoses of a young man with multiply-relapsed Philadelphia chromosome-positive B-cell acute lymphoblastic leukaemia who presented with neurological symptoms and cerebrospinal fluid lymphocytosis. The diagnostic approach needs to be rapid and structured, and may require microbiology and neurology support.