Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1379 page)

   Culture isolates may be typed, but clinical management decisions can generally be made without typing results. Typing is used mainly for epidemiologic purposes.

HERPES VIRUS (HSV OR VZV) DIRECT DETECTION DIRECT FLUORESCENT ANTIBODY (DFA)

   Definition and Use

   These tests may be ordered in patients presenting with a vesicular rash in whom specific and rapid diagnosis of HSV or VZV infection is important for therapy or management. Infection is diagnosed by detection of antigens in specimens by staining with fluorescently labeled virus-specific antibodies.

   Cells are collected from the base of wet ulcers or vesicles (after unroofing) using a swab or edge of a scalpel. Slides are prepared by gently rolling the swab or spreading cells collected by scalpel onto the slide surface.

   After fixation, the smear is stained with an HSV- or VZV-specific antibody reagent tagged with a fluorescent label. After washing away excess reagent, the slide is examined by fluorescence microscopy for the presence of cells showing specific staining.

   
Turnaround time:
<24 hours.

   Interpretation

   
Expected results:
Negative, with no cells showing fluorescent staining

   
Positive result:
The presence of any cells showing +2 or greater specific fluorescent staining

   Limitations

   The slide must be assessed to ensure cells are present in the smear. If no cells are present, the slide is uninterpretable.

   The number of staining cells drops with evolution of the skin lesion from vesicle to crusted/healing ulcer.

   Faint staining may be an indication of problems with the staining technique or reagents.