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Quality of the Evidence for Sameness of Two Infectious Isolates

Description

An outbreak of an infectous disease requires spread of the same organism to 2 or more individuals. There are many types of evidence that can be used to determine if the same organism is present, but the quality of the evidence varies. The Centers for Disease Control and Prevention (CDC) and National Healthcare Safety Network (NHSN) have developed surveillance definitions for health care-associated infection (HCAI) and specific types of infections in health care settings.


 

Evidence

Comments

common exposure (patient group, location, etc)

circumstantial

same genus

often circumstantial

same species

 

same Gram stain appearance

a unique morphology may be specific

same gross culture appearance

a unique feature may be specific (mucoid strain, odor, product)

same biochemical pattern

 

same antibiotic susceptibility pattern

 

same antigens or other protein markers

helpful with influenza and enteric bacteria like Salmonella

molecular fingerprint showing identical genetics (genotype, multiple probes)

gold standard if performed correctly

 

Factors affecting phenotypic markers:

(1) variation between technologists in interpreting results

(2) variation between wild and domesticated strains of the organism

(3) variation in reagents or culture conditions

(4) intermediate enzyme activity (may be read as positive one day and negative the next)

(5) antibiotic exposure

(6) other environmental factors within the patient (oxygen tension, presence of other organisms, etc)

 

The same issues affect interpretation of 2 isolates from the same patient following a course of therapy.

 


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Specialty

ICD-10