Dermatologic Manifestations of Rubella Workup

Updated: Aug 10, 2017
  • Author: Peter C Lombardo, MD; Chief Editor: Dirk M Elston, MD  more...
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Workup

Laboratory Studies

In a healthy child or adolescent, the diagnosis of rubella is made on a clinical basis, and a laboratory workup is not necessary.

A WBC count, if performed, may be lower than normal, as in many viral infections, with increased percentages in the lymphocyte count. In those very rare cases in which encephalitis is present, lymphocytes are present in the cerebrospinal fluid (CSF). If necessary, rubella virus can be cultured from the nasopharynx, blood, urine, and CSF. [10]

If the diagnosis is in doubt, a rising titer of immunoglobulin M (IgM) antibody over a 2-week period indicates a recent infection. Two specific antibodies are associated with Rubella. The first to appear is IgM antibody, which rises and peaks 7-10 after infection and then tapers off after several weeks. One exception to this rule is in the infected newborn, in whom IgM may be detected for months to a year. The immunoglobulin G (IgG) antibody develops more slowly but remains positive for life. It confers immunity against repeat infection. Therefore, the presence of IgM antibody indicates a recent infection, whereas IgG antibody indicates an old infection and immunity. IgG antibody may also indicate immunity caused by the measles, mumps, rubella (MMR) vaccine.

From a public health standpoint, attempting to confirm a rubella infection in a pregnant woman or a newborn infant is important; a pregnant woman exposed to rubella should be tested immediately for a rubella-specific antibody. The presence of rubella-specific IgG is evidence that the patient is immune. If the test result is negative, repeat the test again in 3-4 weeks, and repeat the test on the first specimen. If an antibody is present in the second test and not in the first test, an infection has occurred. If the second test result is negative, repeat the test in 6 weeks, and, again, test it with the first specimen. A negative test result in both specimens means an infection has not occurred, whereas if the last specimen is positive and the first specimen is negative, then an infection has occurred.

An infant with congenital rubella syndrome shows the IgG antibody from the mother, which disappears in a few months, and an elevated IgM antibody level because of antibody production by the infant. The presence of the IgM antibody usually indicates recent infection because IgM does not cross the placenta from the mother as does IgG. After 1 year, confirming the diagnosis of congenital rubella syndrome in an infant with serology alone is very difficult.

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Histologic Findings

The histopathologic features of the skin are those of the Togaviruses, namely, a light perivascular infiltrate of lymphocytes with mild endothelial swelling. If petechiae or purpura are present clinically, extravasation of erythrocytes may be observed.

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