Herpes zoster is a viral infection caused by Varicella zoster virus. It is the very same virus which causes chicken pox. Chicken pox usually occurs in childhood and herpes zoster in adults. After an attack of chicken pox, the virus remains dormant in the nerves and gets activated with a decrease in immunity. Herpes zoster infection manifests in conditions like diabetes, major surgery, HIV/AIDS or when recovering from a major illness.
It is characterized by an initial stage in which there is severe pain. This is followed by the active stage wherein rashes occur, characterized by redness and clear fluid filled blisters localized to one side of the body.
Why are Diagnostic Tests of Herpes zoster Ordered?
Herpes zoster is usually a clinical diagnosis. But occasionally laboratory tests may be required to for final confirmation. The diagnostic tests of Herpes zoster are ordered when it:
- Occurs around the mouth or on the buttocks when it may be confused with Herpes simplex infection.
- There is severe pain and no rash and it is confused with pain of gallstones.
- Follows a non typical course such as Bell’s palsy or Ramsay Hunt syndrome.
- Affects the eye or causes fever and delirium.
The most commonly performed test is the immunofluorescence assay.
How is the Lab Test Done?
The immunofluorescense assay is done to detect antibodies to a specific virus. Varicella zoster virus specific IgM antibody is detected by this test. This antibody is detected only in active stage of herpes zoster and not when the virus is dormant.
In this process:
- Fluid is collected from the blisters caused by the virus.
- A preparation of cells is made from these blisters.
- Ultraviolet rays are applied to them.
- This preparation is observed under a microscope.
- The specific characteristics of the light seen helps to identify the specific IgM antibodies to Herpes zoster.
What Do the Results Mean?
A positive IgM result means that the patient is suffering from active herpes zoster. This test is also positive in case of chicken pox. But the clinical manifestations of chicken pox are completely different, aiding in definitive diagnosis of herpes zoster. Absence of IgM antibodies usually rules out herpes zoster. This test is negative when the virus is dormant in the body or when the active stage is over.
Other Tests Used to Diagnose Herpes zoster
The other diagnostic tests of Herpes zoster are:
- Tzanck smear – Although an easy and inexpensive test, it cannot differentiate between Herpes simplex and Varicella zoster virus.
- Viral culture – The earliest test for the virus, this test is slow, expensive as well as less sensitive than the immunofluorescense assay.
- Polymerase chain reaction – It is expensive but highly specific test. It can be used in unusual cases like infection of the central nervous system.
- Rook’s textbook of Dermatology, 7th ed.
- Harrison’s textbook of Medicine, 17th ed.
- MedlinePlus Medical Encyclopedia