Is a blood test, such as a type-specific Herpes Simplex Virus (HSV) serology test, an appropriate diagnostic alternative for a female patient with suspected genital herpes if swabs are not available?

Blood Testing for Genital Herpes When Swabs Are Unavailable

Yes, type-specific HSV serology (blood test) is an appropriate alternative when swabs cannot be obtained from a suspected genital herpes lesion, but it has important limitations you must understand before ordering. 1, 2

When Blood Testing Is the Right Choice

If no active lesions are present or swabs are unavailable, type-specific serological testing using glycoprotein G-based assays on venous blood samples is the CDC-recommended diagnostic approach, with approximately 97% sensitivity and 98% specificity for detecting HSV-2 antibodies. 3

• Blood testing identifies past HSV exposure and ongoing latent infection, with antibodies persisting indefinitely once acquired. 2
• Serology is particularly valuable for detecting the 58% of HSV-2 infected women who have never had recognized symptomatic episodes. 4
• This approach can identify asymptomatic carriers who may still shed virus and transmit infection to partners. 4

Critical Limitations You Must Know

The index value reported is crucial for interpretation—values between 1.1-2.9 have only 39.8% specificity, while values ≥3.0 have 78.6% specificity. 3

• For low positive results (index value <3.0), you must confirm with a second test using a different glycoprotein G antigen to avoid false-positive results. 2, 3
• A negative result within 12 weeks of potential exposure may represent the "window period" before antibodies develop—retesting should not occur until at least 12 weeks after exposure. 2, 5
• HSV-1 serologic assays lack sensitivity (only 70.2% in one study), which can result in false-negative diagnoses. 1

Why Swabs Remain Superior When Available

Swabs from active lesions tested by NAAT/PCR are first-line diagnostic tests, offering 11-71% superior sensitivity compared to viral culture and exceeding 90% sensitivity and specificity. 1, 2

• NAAT/PCR from lesions provides definitive diagnosis during active infection, while serology only indicates past exposure. 6, 7
• Viral culture from lesions has 77% sensitivity for confirming first episodes, though NAAT is preferred when available. 1, 4
• Do not obtain swabs in the absence of lesions to diagnose genital herpes—due to intermittent viral shedding, this approach lacks sensitivity. 1

Practical Algorithm for Your Patient

1. If active lesions are present: Collect swab for NAAT/PCR (preferred) or viral culture if NAAT unavailable. 1, 2

2. If no lesions present or swabs unavailable: Order type-specific HSV-2 IgG serology using glycoprotein G-based assay. 2, 3

3. If serology shows index value 1.1-2.9: Confirm with second test using different glycoprotein G antigen before making definitive diagnosis. 2, 3

4. If serology negative but high clinical suspicion: Consider retesting after 12-week window period if recent exposure suspected. 2, 5

5. If serology positive: Counsel about natural history, recurrence risk, asymptomatic shedding, and transmission prevention—even without symptoms, she can transmit infection. 2

Why HSV Type Matters

Always use assays that differentiate HSV-1 from HSV-2, as HSV-2 causes recurrences in 90% of patients at 12 months versus only 55% for HSV-1 genital infections. 1, 3

• This typing information directly impacts prognosis, counseling about recurrence frequency, and transmission risk discussions. 3
• HSV-1 can cause genital herpes through oral-genital contact, but behaves differently than HSV-2 genital infection. 5