As a rapid screening tool, the ratio of a patent’s white blood cell types differentiated acute purulent tonsillitis from infectious mononucleosis, researchers here reported.
A ratio of lymphocyte to white blood cell count (L/WCC) higher than 0.35 indicated that the patient would be correctly diagnosed as having mononucleosis 100% of the time,

The researchers then calculated that a ratio higher than 0.35 had a specificity of 100% and a sensitivity of 90% for the detection of mononucleosis. Thus an individual with a ratio this high would be correctly diagnosed as having mononucleosis 90% of the time, while a person with a ratio of 0.35 or lower would be correctly diagnosed as not having mononucleosis 100% of the time.