How is Listeria infection diagnosed?
Listeria infection is diagnosed through blood or stool cultures. Spinal fluid can also be tested for Listeria.
Because there are few symptoms that are unique to listeriosis, doctors must consider a variety of potential causes for infection, including viral infections (like flu), and other bacterial infections that may cause sepsis or meningitis. [4, 18, 19]
Early diagnosis and treatment of listeriosis in high-risk patients is critical, since the outcome of untreated infection can be devastating. This is especially true for pregnant women because of the increased risk of spontaneous abortion and preterm delivery. Depending on the risk group, rates of death from listeriosis range from 10% to 50%, with the highest rate among newborns in the first week of life. 
Methods typically used to identify diarrhea-causing bacteria in stool cultures interfere or limit the growth of Listeria, making it less likely to be identified and isolated for further testing.  On the other hand, routine methods are effective for isolating Listeria from spinal fluid, blood, and joint fluid. [4, 18] Magnetic-resonance imaging (MRI) is used to confirm or rule out brain or brain stem involvement.