How is polymerase chain reaction testing performed for suspected herpes zoster?

How to Perform PCR Testing for Shingles (Herpes Zoster)

PCR testing for shingles is performed by collecting vesicular fluid from early lesions using a sterile needle to pierce vesicles, then swabbing with cotton-wool or Dacron swabs, which provides results within approximately 2 hours and is the preferred diagnostic method. 1

Optimal Sample Collection Technique

For Vesicular Lesions (Highest Yield)

• Pierce intact vesicles with a sterile needle 2
• Collect vesicular fluid using a cotton-wool or Dacron swab 2, 1
• Vigorously swab the base of opened vesicles to obtain epithelial cells, which increases detection rates 3
• Sample early vesicular lesions for highest yield, as detection rates drop dramatically as lesions progress to crusting stage 1

For Non-Vesicular Presentations

• Swab erythematous skin in early-stage infection before blister eruption, as VZV DNA can be detected even from pre-vesicular erythema 4
• Collect crusts if vesicles are no longer present, though yield is lower 3
• Obtain skin biopsy specimens if other methods are unavailable, though this is more invasive 3

Why PCR is the Test of Choice

PCR detects VZV DNA with 95% sensitivity in confirmed zoster cases, far superior to viral culture (20% sensitivity) and direct immunofluorescence (82% sensitivity) 5. The key advantages include:

• Results available within approximately 2 hours 1
• Less strict transportation requirements compared to viral culture, which requires strict refrigeration 1
• Can detect VZV DNA from multiple specimen types including Tzanck smear debris, vesicle fluid swabs, crusts, and fixed tissue 3
• Remains positive even in crusting lesions where culture fails 3

When to Obtain Laboratory Confirmation

Laboratory confirmation should be obtained when: 1

• The presentation is atypical
• The patient is immunocompromised
• There is diagnostic uncertainty
• The patient has darker skin pigmentation where rash may be difficult to recognize

Clinical diagnosis alone is typically sufficient for immunocompetent patients with characteristic unilateral vesicular eruption in dermatomal distribution preceded by 24-72 hours of prodromal pain 1

Sample Handling and Transport

• PCR samples tolerate less strict transportation conditions compared to viral culture 6
• No special refrigeration required during transport for PCR specimens 6
• Samples can be applied to microscope slides for immunofluorescence staining or introduced into transport media 2

Critical Pitfalls to Avoid

• Do not wait for crusted lesions - detection rates drop to 27% even with viral culture at the crusting stage 6
• Do not rely on Tzanck smears alone - while they can suggest herpesvirus infection with multinucleated giant cells, they cannot distinguish VZV from HSV and have low sensitivity 1
• Do not use serologic IgM testing for acute diagnosis - it is a poor indicator of recent infection 2, 6
• Avoid delayed transport to the laboratory, as this substantially affects testing outcomes even for culture 2

Alternative Testing Methods (When PCR Unavailable)

If PCR is not available: 1

• Viral culture has poor sensitivity (20%) and requires 24-72 hours for results with strict refrigeration during transport 1, 5
• Direct immunofluorescence has 82% sensitivity but 76% specificity 5
• Tzanck smear is the least reliable option and should only be used when no other methods are available 1