Diagnostic Testing for Shingles
Clinical diagnosis is usually sufficient in immunocompetent patients with typical presentation, but PCR testing of vesicle fluid is the gold standard when laboratory confirmation is needed, offering nearly 100% sensitivity and specificity. 1, 2, 3
When Laboratory Testing is Indicated
Laboratory confirmation should be obtained in specific clinical scenarios rather than routinely:
- Atypical presentations where the rash is nonspecific, localized, faint, evanescent, or difficult to recognize in darker skin pigmentation 1, 2
- Immunocompromised patients including transplant recipients, HIV-positive individuals, or those on immunosuppressive therapy 1, 2, 3
- Diagnostic uncertainty when clinical features overlap with other conditions like impetigo, herpes simplex, or enteroviral infections 1, 4
- Absence of characteristic pain which may delay recognition 1
Preferred Diagnostic Tests
PCR Testing (Gold Standard)
- Nucleic acid amplification testing (NAAT/PCR) of vesicular lesions is the preferred method with sensitivity and specificity approaching 100% 2, 3
- Collect specimens by scraping or swabbing the base of unroofed vesicles for optimal viral material 3
- PCR can detect VZV DNA even in crusted lesions that are no longer infectious 3
- Specimens should be placed in appropriate transport medium immediately after collection 3
- Sample early in the disease course when vesicles are present for highest yield 3
Alternative Testing Methods (Less Preferred)
- Tzanck preparation showing multinucleated giant cells can suggest herpesvirus infection but cannot differentiate between VZV and HSV and requires a skilled microscopist 1, 3
- Direct fluorescent antibody (DFA) testing can detect viral antigens but is less sensitive than PCR 3
- Viral culture is less sensitive than PCR and requires strict transport conditions 3
- Immunofluorescent viral antigen studies are acceptable alternatives when PCR is unavailable 1, 2
Tests That Should NOT Be Used
- Serology (VZV IgG/IgM) is not useful for diagnosis of active shingles and should only be used to determine immunity status 2, 3
- Blood culture has no role in diagnosing localized herpes zoster in immunocompetent patients 1
- Skin biopsy is not indicated for typical herpes zoster and is reserved for immunocompromised patients with atypical skin lesions 1
Critical Clinical Pitfall
Treatment should be initiated immediately upon clinical suspicion without waiting for laboratory confirmation, as delays can worsen outcomes, particularly regarding postherpetic neuralgia risk 2. Laboratory testing should not delay antiviral therapy initiation in patients with typical dermatomal vesicular rash and characteristic pain 2.