Acyclovir Does Not Significantly Affect Swab Test Results for Shingles
Ideally, obtain the swab specimen before initiating acyclovir therapy, but if the patient has already started treatment, proceed with testing as nucleic acid amplification tests (NAATs) remain highly sensitive even after antiviral therapy has begun. 1
Diagnostic Testing Considerations
Optimal Testing Approach
VZV can readily be detected in blister material scraped or swabbed from a disrupted skin lesion, and nucleic acid amplification technology tests (such as PCR) are both specific and sensitive, with both approaching 100% accuracy 1
NAATs can detect VZV DNA even in crusted lesions that are no longer infectious, indicating these tests maintain diagnostic utility across different stages of lesion development 1
Clinical Management Algorithm
When shingles is suspected:
Initiate antiviral therapy immediately without waiting for test results, as suspected varicella or zoster warrants prompt action 1
Obtain swab specimen concurrently with treatment initiation by scraping or swabbing material from a disrupted skin lesion 1
Use NAAT/PCR testing as the preferred diagnostic method due to superior sensitivity and specificity compared to older technologies 1
Important Caveats
Older diagnostic technologies are more problematic: Rapid antigen detection, Tzanck smear, and electron microscopy are more reliant on the quality and timing of the sample, making them less reliable if acyclovir has already been started 1
Serology is not useful for diagnosis of chickenpox or shingles, so antibody testing should not be used for acute diagnosis 1
The key advantage of modern PCR/NAAT testing is that it detects viral DNA rather than viable virus, meaning it remains positive even after antiviral therapy has begun to suppress viral replication 1
Practical Recommendation
Do not delay acyclovir treatment to obtain diagnostic specimens. The high sensitivity of modern molecular testing means that starting treatment first will not meaningfully compromise diagnostic accuracy, and the clinical benefit of early antiviral therapy (within 72 hours of rash onset) outweighs any theoretical reduction in test sensitivity 1, 2