Blood Work for Diagnosing Shingles
Blood work is generally not recommended for diagnosing shingles in immunocompetent patients with typical clinical presentation, as clinical diagnosis alone is usually sufficient. 1
When Laboratory Testing is Indicated
Laboratory confirmation should be pursued in specific circumstances rather than routinely:
- Immunocompromised patients with atypical clinical presentation require laboratory confirmation 2, 1
- Atypical presentations where clinical features overlap with other conditions warrant testing 3
- Zoster sine herpete (shingles without rash) requires laboratory diagnosis since clinical diagnosis is impossible 4
Preferred Diagnostic Tests (Not Blood-Based)
The gold standard for shingles diagnosis involves specimen collection from skin lesions, not blood work:
- PCR testing of vesicular fluid is the diagnostic method of choice, with nearly 100% sensitivity and specificity for detecting VZV DNA 1, 3, 5
- Collect material by scraping or swabbing the base of unroofed vesicles for optimal yield 3
- Specimens should be collected early when vesicles are present, though PCR can detect VZV DNA even in crusted lesions 3
Blood-Based Testing: Limited Utility
Serology (VZV IgG/IgM)
- Serology is not useful for diagnosing active shingles and should only be used to determine immunity status 3
- VZV IgM antibodies are detected in only 9% of acute sera and 38% of convalescent sera, making them insufficiently sensitive for diagnosis 6
- Paired acute and convalescent sera showing seroconversion or fourfold titer increase can confirm diagnosis retrospectively (77-85% sensitivity), but this requires waiting weeks and is not helpful for initiating timely treatment 6
Serum PCR for VZV DNA
- Serum PCR has poor sensitivity (28-40%) compared to PCR from skin lesions (88-100%) 7, 6
- VZV DNA is detectable in serum in only 40% of acute shingles cases 6
- Serum PCR may be useful when skin lesions are not accessible or in disseminated disease, but should not be the primary diagnostic approach 7
Clinical Diagnosis Approach
For typical presentations, initiate treatment immediately based on clinical suspicion without waiting for laboratory confirmation 1:
- Prodromal pain preceding rash by 24-72 hours in a dermatomal distribution 1
- Unilateral vesicular eruption strictly respecting the midline 1
- Lesion progression from erythematous macules to papules to vesicles that crust over 1
Key Pitfalls to Avoid
- Do not delay treatment while waiting for blood test results in typical cases—treatment is most effective within 72 hours of rash onset 1
- Do not rely on serology for acute diagnosis, as it lacks sensitivity in the acute phase and requires convalescent samples 6
- Do not use blood PCR as first-line testing when vesicular lesions are available for sampling 3, 7
- In immunocompromised patients, do not rely on clinical diagnosis alone even if presentation appears typical 2, 1