The Downey cell method for the diagnosis of infectious mononucleosis remains effective and cost-effective nearly 100 years after it was discovered:

"An 18-year-old woman visited her physician because she had a fever, a sore throat, and painful swellings in her neck for the past 5 days. A rapid antigen detection test for streptococcus was negative, and because of exudates on the patient's tonsils and very enlarged cervical lymph nodes, the physician tested for infectious mononucleosis or glandular fever. A rapid point-of-care test for heterophile antibodies—the Monospot test—was negative, a full blood count with autodifferential was normal, and a throat culture for group A streptococcus was negative. Epstein-Barr virus (EBV) antibodies—viral capsid antigen IgM and IgG, and EBV-associated nuclear antigen antibodies—were also negative."

After her visit to the physician the patient's condition deteriorated. When she went to the emergency room a manual differential was done, and it was loaded with with all three types of Downey cells. To confirm the diagnosis of infectious mononucleosis, they tested her plasma for viral DNA with the relatively expensive EBV PCR (Epstein Barr Virus Quantitative Polymerase Chain Reaction test), which gave a positive result. This confirmed the diagnosis.

(Thanks to Juan Weiss for this reference and its interpretation.)