A single antibody titer 1:160 is interpreted as a presumptive positive result for diagnosing tularemia. Agglutination test is used to confirm the diagnosis of tularemia. A fourfold increase in titer between paired serum samples collected 2-3 weeks apa is considered diagnostic. Late in infection, titers of 1:20-1:80 may persist for years. Tularemia is caused by F. tularensis, a gram-negative, pleomorphic, nonmotile, non-spore-forming bacillus. Ref: Jacobs R.F., Schutze G.E. (2012). Chapter 158. Tularemia. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds),Harrison's Principles of Internal Medicine, 18e.