Tests for Herpes

There are three common types of laboratory tests for genital herpes and several others that are less widely used.

Herpes laboratory tests are usually based on two categories:

• Sensitivity
• Specificity

Sensitivity

– Refers to the likelihood of the test correctly diagnosing herpes.

Specificity

– The probability of a test correctly determining that a patient does not have herpes.

For patients who have sores or other outward signs of infection, the leading method is to take a Viral Culture or sample of the infected tissue. When no symptoms are present or if they have already healed a blood test may be required. Speed and cost are also taken into account.

Viral culture

This is where the virus is grown in material known as a culture medium. A viral culture looks for the presence of the virus in the lesion.

Viral culture testing is very specific, this method:

– Does not frequently give a positive result when something else is the culprit
– Can also be very sensitive if the specimen is adequate
– Provides a way to tell whether the infection is caused by HSV-1 or HSV-2

Sometimes more than one test will be needed

A poor sample may cause sensitivity to drop. Even if herpes lesions are present, there may be very little active virus left in the lesions. In this case, the culture will come back as a false negative (the test says there is no herpes even when the patient actually has genital herpes).

Approximately twenty percent of culture tests produce a “false negative” when a patient has a first episode of herpes. With recurrent episodes, when less of the virus is present, the rate of false negatives goes up to 50 percent.

It can take any time from two to seven days to get viral culture results back from the laboratory, which may mean several visits to your Doctor before a diagnosis is confirmed.

Lots of people need to be tested more than once. The viral culture often misses herpes even when it is present. Often a patient who has received a negative culture result will be asked to come back again when a new genital lesion appears so the culture can be tried a second or third time.

Blood Tests (serologic tests)

Herpes blood tests take a very different approach. These tests work by measuring the levels of Herpes simplex antibodies inside your body.

Antibodies are substances produced by your immune system to fight off infections (such as herpes). Blood tests detect herpes by looking for antibodies in the blood or serum.

When can a blood test be performed?

Blood tests can be performed even when there are no symptoms present. If the herpes antibodies are found in the blood, it means that herpes simplex is latent in the body.

This method of testing has the advantage that it can be done even when a person has no symptoms, so it is a very effective way to detect an established herpes infection. Blood tests do not require swabbing a lesion, so they can be done long after symptoms have faded.

Type-specific blood tests are ideal for those who have had a history of genital symptoms but have never had a successful confirmatory test.

How effective is a blood test?

The sensitivity and specificity of blood tests is better than culture or antigen tests, but there are two important factors to consider:

Timing

– if this is the first exposure to herpes, a person may take several weeks to develop the antibodies that blood tests look for. This means that if you perform the test too early the antibodies that the tests look for may not show up and you may receive a false-negative diagnosis.

Conflicting Results

– some blood tests cannot tell the difference between the two types of herpes simplex, HSV-1 and HSV-2 (the cold sore and genital herpes viruses). For this reason, anyone seeking an accurate diagnosis of their herpes type should request a type-specific test, which can accurately distinguish HSV-2 from HSV-1 antibodies.

Most commercially available kit assays currently cannot make this distinction despite their claims.

Type-specificity in a blood test does not depend on the antibody class being detected (either IgM or IgG), but on what the antibodies are directed against (referred to as their antigens).

In herpes, most type specific antibodies are directed against a protein called glycoprotein G (gG). The gG in HSV-1 (called, unimaginatively, gG1) differs substantially from the corresponding gG in HVS-2 (called gG2).

Therefore, herpes blood tests which are based around specific detection of antibodies to either gG1 or to gG2 will be type-specific. People who are getting a type-specific test for genital herpes should ensure that the test they receive detects antibodies to gG2 since there are still blood tests out there which claim to be type-specific but which are not.

While a blood test may reveal infection with HSV at a time when no genital symptoms are present, confirmation of the genital HSV infection is still essential.

If the blood test is specific for detection of antibodies to HSV type 2, the likelihood of genital HSV infection is increased, but still not proven. The doctor may ask you to re-visit for a swab test when genital symptoms or discomfort appear.

Usually, it takes between two weeks to three months after exposure to herpes for antibodies to appear in the blood.

Some blood tests detect antibodies sooner than others. However, once antibodies are found they remain in the body for life.

If you have never had symptoms before but want to be tested for herpes, a type-specific blood test is the only way to find out your status. Getting tested may be relevant if you:

• Are in a relationship where your partner has herpes but you have had no symptoms and have never been tested
• Have been with an infected partner and feel you should be tested
• Have had several sexual partners and want to be tested for the most prevalent STDs

What is the difference between ‘Type-Specific’ and ‘Non Type-Specific’ tests?

The technology behind the type-specific test is relatively new. The “gold standard” of type-specific blood tests is known as the Western Blot Serology.

A number of similar tests are produced for research purposes and commercial products are gradually becoming more available. These tests detect antibodies (known as “IgG”) that differ between HSV-1 and HSV-2. It is now recommended that clinicians avoid serum tests other than type-specific tests.

What can I do to make sure I am getting a Type-Specific Test?

There are two ways to ensure that you are getting a truly type-specific test:

• There are several research labs that can perform type-specific tests, but the most reliable and widely used of these research tests is the Western blot.
• A finger prick test for herpes called the POCkit HSV-2 Rapid Test has also been approved

Antigen Detection Tests

Where components of the virus are specifically identified.

The third type of test, the antigen test, is used less frequently and, similar to the culture method, also looks for the presence of the herpes virus in the lesion.

However, unlike the culture method, this test does not require growing the virus but rather seeks to identify the herpes by the presence of antigens (fragments of the virus that are known to stimulate the immune response).

A swab is taken similar to the method used for viral culture. In general, antigen detection assays are less sensitive than viral culture. Antigen-detection is usually done only in research laboratories or large reference laboratories.

Antigen tests can produce results more quickly than cultures and are sometimes less expensive. However, better samples are needed than for cultures and many do not determine whether the sample is infected with HSV-1 or HSV-2.

Comparison table of Herpes Tests

The three different methods of herpes diagnosis:

Why isn’t everybody tested if Herpes is so widespread?

General population screening for herpes is normally not recommended. Guidelines for antibody testing are still being developed.

In the following situations, serologic tests (blood tests) may be required to:

• Confirm an infection in a person who has previously had symptoms of herpes but never had a positive confirmation through a test result
• Determine the risk of transmitting herpes to a sexual partner
• Detect herpes as part of a routine check for STDs
• Help determine a pregnant woman’s risk of passing herpes on to her newborn

Where to Now?

• How to Diagnose Herpes
• Cold Sore Information
• Genital Herpes Information