How Shingles Is Diagnosed

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Shingles affects roughly one out of every three people in the United States, so chances are you probably already know someone who's had it. But even if you can spot the signs and symptoms of shingles yourself, it's still important to be seen by a healthcare provider for an official diagnosis and proper treatment plan.

For those with a classic case of the disease, doctors can typically diagnose shingles just by looking at the rash and asking a few questions about your medical history. But for those with more uncommon symptoms—such as a rash stretching completely across the body or no rash at all—lab tests can be helpful diagnostic tools.

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Physical Exam

Once the rash appears, the signs and symptoms of shingles—also called herpes zoster—are pretty distinctive and are often enough on their own for a doctor to make a diagnosis and recommend treatment.

During a physical exam, healthcare providers will ask you about your medical history, including whether you’ve had chickenpox or received the chickenpox vaccine. They’ll also take a look at the rash (if one is present) to see if it has all the telltale signs of a shingles rash, including being concentrated on one side or one area of the body, tingling, itching, or burning, or whether the rash is starting to or already has blistered.

Though it's not common, in some cases you can have pain or stinging associated with shingles without the rash—a condition called zoster sine herpete. If that's the case, doctors will likely seek confirmation of the diagnosis by ordering lab tests in addition to conducting a physical exam.

Labs and Tests

If you don’t have a rash, or if the rash stretches across the body or looks like it could be some other type of rash—like herpes simplex or contact dermatitis—your doctor might need lab tests in order to diagnose shingles.

Polymerase Chain Reaction (PCR)

Polymerase chain reaction (PCR) is a lab technique used for a wide variety of purposes, including to spot the DNA of varicella-zoster virus, the pathogen that causes shingles.

Commonly referred to as “molecular photocopying,” PCR uses swabs typically taken from the shingles blisters or scabs and then copies (amplifies) the virus DNA for easy detection. In some instances, a saliva swab can also be used, but it’s not as reliable as samples taken from blisters.

Once the swab is taken, the PCR process is almost entirely automated and fairly quick, typically providing results within one day. In addition to spotting the virus, PCR can also help determine whether the rash is caused by wild varicella or (in very rare cases) by a vaccine strain.

Other diagnostic methods, such as direct fluorescent antibody (DFA) or Tzanck smear testing, are not recommended because they aren’t as sensitive as PCR.

Serologic Methods

If you have an atypical rash or there aren’t any good swabs to use as a sample for PCR, serological testing can be used in addition to a physical exam to diagnose shingles, typically by looking for antibodies in your blood.

When you’re exposed to varicella-zoster virus, your body makes antibodies to defend itself. Serological testing can detect two types of these antibodies: IgM and IgG. IgM antibodies are short-term antibodies the body makes to immediately fight off a varicella infection—typically spiking within a week or two of getting chickenpox and then again when/if the virus is reactivated as herpes zoster. Over time, these antibodies can wane until they are undetectable but can resurface during shingles episodes.

IgG antibodies, on the other hand, are made a couple of weeks after first getting infected and remain in the body long-term. Levels are often detectable for the rest of your life. If test results detect both IgM and IgG, it could be an indication that you have shingles.

It’s important to note, however, that these tests aren’t foolproof. Individuals with weakened immune systems, for example, might not launch a very strong immune response to either chickenpox or shingle—in which case, serological testing could give a negative result even if they really do have shingles.

Similarly, even with a positive test result, in the absence of typical symptoms or a complete medical history, it can be difficult to know whether you have shingles or if it's really a primary infection of chickenpox. Your doctor will do their best to diagnose and choose an appropriate treatment.

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