According to the Mayo Clinic, results for the monospot test, a typical blood test for mononucleosis, come back within a day. The test checks for the presence of antibodies to the Epstein-Barr virus, which causes infectious mononucleosis, the CDC reports.
MedlinePlus reports that a negative monospot test usually indicates the patient does not have the disease; however, the test finds antibodies only after they have been active for a week or two. Therefore, if mononucleosis is just starting to develop, it is sometimes necessary to repeat the test. On rare occasions, people who are infected with mononucleosis never show positive test results.