Parameters:
(1) number of enteric culture isolates for the Salmonella species
(2) number of non-enteric (extraintestinal) isolates for the Salmonella species
total number of isolates =
= (number of enteric isolates) + (number of non-enteric isolates)
invasion index = percent of all isolates that are non-enteric as a whole number =
= (number of non-enteric isolates) / (total number of isolates) * 100
Interpretation:
• minimum invasion index: 0
• maximum invasion index: 100
• A species commonly associated with extra-intestinal disease will have an invasion index >= 75. For example, the invasion index of S choleraesuis is 97.
• A species rarely associated with extra-intestinal disease will have an invasion index close to 0.
Limitations:
• How the isolates are obtained from individual patients is not specified. A single patient with extraintestinal disease could have a stool, blood and abscess isolate.
• This seems to be used as an epidemiologic tool evaluating all isolates at a hospital over a period of time such as a year.
