Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1441 page)

   A positive IgG result coupled with a positive IgM result indicates recent infection with VZV.

   A positive IgG result coupled with a negative IgM result indicates previous exposure to VZV and immunity.

   A negative IgG result coupled with a negative IgM result indicates the absence of prior exposure to VZV and no immunity. However, a negative result does not rule out a VZV infection. Negative results in suspected early VZV infections should be followed by testing a new serum specimen in 2–3 weeks.

   Equivocal results should be followed up with testing a new serum specimen within 10–14 days.

   Limitations

   Test for VZV IgG antibodies is of use when clinical symptoms are present or infection suspected. Screening of the general population leads to no appreciable diagnostic advantage. Results from immunosuppressed patients should be interpreted with caution.

   Many different antibody tests are available with a wide range of performance standards. The fluorescent antibody to membrane antibody (FAMA) is the most extensively validated assay and correlates best with susceptibility to and protection against varicella. However, this test is not widely used because it is labor intensive and requires expert interpretation.

   Many commercially available ELISAs are available that are considered generally less sensitive than FAMA, although specificities are comparable.

   Commercial ELISA assays are suitable for screening for VZV susceptibility among health care workers. The rationale for this is that the risk of vaccinating an adult with a false-negative test result is much lower than the risk of natural infection in an individual falsely identified as seropositive.

   Routine screening for varicella in individuals born in the United States before 1980, who are not health care workers, is not recommended because of extremely high rates of seropositivity in this population.

VIBRIO
CULTURE OF STOOL (RULE OUT)

   Definition

   
Vibrio
species are an uncommon cause of bacterial enteric infections in this country, but endemic infections occur in many countries. Epidemic outbreaks are well described, generally associated with inadequately treated sewage or contaminated water.
Vibrio
species are halophilic, and brackish water and shellfish serve as an important reservoir for organisms. Although infection may be relatively mild and self-limited, some patients develop cholera: severe disease with vomiting and profuse watery diarrhea (rice water stools). The severe diarrhea may rapidly lead to life-threatening dehydration and electrolyte imbalance. Stool culture to rule out
Vibrio
species should be considered for patients who develop diarrhea, especially severe watery diarrhea, after travel to an endemic area, ingestion of contaminated sea food, or exposure to brackish water.