What is the preferred method for diagnosing herpes simplex virus (HSV) infection, serum testing or swab testing?

Diagnostic Testing for HSV: Swab vs Serum

For patients with active genital lesions, nucleic acid amplification testing (NAAT/PCR) from a lesion swab is the preferred diagnostic method and should always be used over serology, as it provides 11-71% superior sensitivity compared to viral culture and >90% sensitivity and specificity for most FDA-approved assays. 1

When to Use Swab Testing (NAAT/PCR)

Swab-based NAAT/PCR is the test of choice for any patient presenting with active genital lesions, vesicles, or ulcers. 1, 2

Key advantages of swab testing:

• Provides definitive diagnosis of active infection with sensitivity and specificity exceeding 90% for most FDA-approved assays 1, 2
• Allows HSV-1 vs HSV-2 differentiation, which is critical for counseling about recurrence rates (HSV-2: 90% vs HSV-1: 55% at 12 months) 1
• Detects infection even in suboptimal specimens including non-ulcerative or healing lesions where culture would fail 1
• Less strict transport requirements compared to viral culture 1

Optimal sampling technique:

• Collect fluid from intact vesicles by opening with sterile needle 1
• For ulcers, swab the base of the lesion vigorously 1, 3
• For urethral/cervical involvement, insert swab 0.5-2 cm into the canal 1

Critical pitfall to avoid:

Never obtain HSV molecular assays in the absence of a genital ulcer or lesion - due to intermittent viral shedding, swabs from intact skin lack sensitivity and will produce false-negative results 1, 3

When to Use Serum Testing (Type-Specific Serology)

Serology should only be used when no active lesion is present - specifically for detecting asymptomatic infections, evaluating patients with atypical or healed lesions, or screening high-risk populations. 1, 2, 3

Appropriate indications for serology:

• Patient reports history of recurrent genital lesions but presents without active lesions 1, 2
• Pregnant women at risk of acquiring HSV near delivery 2, 3
• HIV-positive patients or men who have sex with men 2
• Partner of someone with confirmed HSV-2 infection 2

Major limitations of serologic testing:

HSV-2 serology has serious specificity problems - compared to Western blot gold standard, commercially available EIA/CLIA tests have specificity as low as 57.4%, with index values of 1.1-2.9 having only 39% specificity 1

HSV-1 serology lacks sensitivity - detecting antibodies in only 70.2% of confirmed infections 1

False-negatives occur in 12-30% of patients with proven recurrent HSV even months after infection 4

Window period creates false-negatives - antibodies may not develop for up to 12 weeks after exposure 2, 3

Critical interpretation guidelines:

• For HSV-2 index values <3.0, confirm with a second test using different glycoprotein G antigen to avoid false-positives 2
• Negative serology within 12 weeks of exposure may represent window period and should be repeated 2, 3
• Serology cannot determine if a current genital lesion is HSV - it only indicates past exposure 3, 5

Why Swab Testing Supersedes Serology for Active Lesions

The fundamental problem with using serology for active lesions is that positive results may reflect chronic infection rather than the current outbreak, while negative results may miss recent primary infection. 5

• Serology cannot distinguish between HSV-1 and HSV-2 unless type-specific glycoprotein G-based assays are used 1
• Direct immunofluorescence and Tzanck smear lack sensitivity and are not recommended 1, 3
• If NAAT is unavailable due to cost or laboratory limitations, viral culture is acceptable as second choice, though significantly less sensitive 1, 2, 3

Algorithm for HSV Diagnostic Testing

1. Patient presents WITH active genital lesion/vesicle/ulcer:

   • Order NAAT/PCR from lesion swab (first-line) 1, 2
   • If NAAT unavailable, use viral culture (second-line) 1, 2, 3
   • Ensure test differentiates HSV-1 from HSV-2 1

2. Patient presents WITHOUT active lesion but reports history of recurrent genital lesions:

   • Order type-specific HSV serology (glycoprotein G-based) 1, 2, 3
   • If index value 1.1-2.9, confirm with second test 2
   • If negative and exposure <12 weeks ago, repeat in 12 weeks 2, 3

3. Asymptomatic screening (only for high-risk populations):

   • Type-specific HSV-2 serology for pregnant women at risk, HIV-positive patients, MSM 2, 3
   • Do not screen general asymptomatic population 1, 2, 3

The CDC and IDSA explicitly state that widespread screening for HSV antibodies in asymptomatic low-risk populations is not recommended due to false-positive results and limited clinical utility. 1, 2, 3