Herpes blood test (IgG): when it helps and when a swab is better

If you have a blister or sore that might be genital herpes, the NHS describes a swab test: a doctor or nurse at a sexual health clinic uses a small cotton bud to collect fluid from a blister or sore for testing. The British Association for Sexual Health and HIV (BASHH) guideline from 2024 calls testing a swab by NAAT, such as PCR, the preferred diagnostic method, and says confirming and typing the infection by directly detecting the virus in the lesions is essential.

A herpes blood test looks for antibodies rather than the virus itself. It is useful in a limited number of situations, which are described below, but it is not the first-line way to confirm a new episode, and it is not recommended as a general screening test for people without symptoms.

The two tests answer different questions. A swab asks whether virus is present in a lesion right now and which type it is. A blood test asks whether your immune system has made antibodies to the virus at some time in the past.

BASHH lists situations in which antibody testing is beneficial, and these are the circumstances in which a specialist is most likely to suggest it.

• Recurrent genital symptoms where the swab PCR is negative. Type-specific antibody tests can help rule out genital herpes in someone who has symptoms that suggest it.

• Counselling after a first episode that was confirmed by PCR, to help tell recent infection from established infection, particularly in pregnancy and especially if the first episode occurs in the third trimester, because the result can inform the mode of delivery.

• Investigating partners without symptoms of a person with genital herpes, including couples planning pregnancy, to identify partners who are not infected with the same type and so can be counselled about reducing risk.

BASHH sets out several reasons for caution. Type-specific antibody tests can take time to become positive after infection, and people with proven infection can lose their antibodies; the guideline says up to 12% of patients will subsequently lose their IgG antibodies. A negative result soon after a possible exposure therefore does not exclude infection.

False positives are also a concern. BASHH says false positive results are more common with low antibody index values, typically within three times the cut-off, and that low-prevalence populations have a lower positive predictive value. It gives an example in which an assay with 95% sensitivity and specificity has a positive predictive value of 86% in a sexually transmitted infection clinic population but 50% in a general antenatal population. A low-level positive may be retested using a different assay, or a repeat sample taken after at least 3 weeks.

Antibody results also do not distinguish how HSV-1 was acquired. BASHH explains that HSV-2 IgG implies anogenital infection, since nearly all HSV-2 infections are acquired sexually, but HSV-1 IgG can reflect oral infection acquired in childhood, so a positive HSV-1 result does not mean genital herpes.

The guideline also points out that the US Centers for Disease Control and Prevention does not advise routine antibody screening of everyone attending sexual health clinics, because diagnosing genital herpes in someone without symptoms has not been shown to change their sexual behaviour or stop the virus spreading.

Because the swab needs a lesion, timing matters. The NHS advises going to a sexual health clinic as soon as possible if you have symptoms, and says you should go even if you have not had sex for a long time, as blisters can take months or years to appear. It also says the test cannot be done if you do not have visible blisters or sores.

The NHS adds that if you have had symptoms for more than 5 days before attending, you can still be tested to find out the cause, although antiviral treatment for a first episode is intended to be started within 5 days of symptoms appearing. Many sexual health clinics offer walk-in services and often get results more quickly than GP practices.

A positive swab confirms current herpes in that lesion and its type. A positive antibody result shows past exposure to that virus type; with a positive HSV-1 result in particular, the infection may be oral rather than genital, The NHS notes that genital herpes can be caught if a cold sore touches your genitals.

The NHS describes genital herpes as a long-term infection with no cure, in which symptoms clear up by themselves but can come back, and recurrent outbreaks are usually milder than the first. Treatment from a sexual health clinic can help. Whatever the result, a clinician can explain what it means for you in context, including relationships and pregnancy.