The heterophile antibody test (HAT) is commonly ordered at the same time as a complete blood cell (CBC) count in cases of suspected infectious mononucleosis. An absolute lymphocytosis with the presence of greater than 10% atypical lymphocytes is a good predictor of a positive HAT. We retrospectively studied patients who underwent HAT and had a simultaneous CBC count during an 8.5-year period. The ratio of the absolute number of lymphocytes to the absolute number of neutrophils (L/N) was

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... ed to see if this measure could be useful in distinguishing those most likely of having a negative HAT. A total of 5104 patient samples were analyzed of which 501 (9.8%) were HAT positive. The L/N ratio was less than 0.5 in 2214 patients (43.4%), and in this group, only 46 (2.1%) were HAT positive. In the group whose L/N ratio was greater than or equal to 0.5, the HAT positivity rate rose to 15.7% (455/2888), for a relative increase of 60%. In the HATpositive group, 153 (30%) did not demonstrate an absolute lymphocytosis. Restricting HAT to those having a CBC count with an L/N ratio greater than or equal to 0.5 will increase the HAT positivity rate and lead to a reduction of HAT performed. (Infect Dis Clin Pract 2008;16:154Y156) Original Article