Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1415 page)

RUBELLA SEROLOGY SCREEN (RUBELLA IgG AND IgM)

   Definition

   Rubella virus causes German measles, a mild subclinical infection with a characteristic exanthem that affects both children and adults. Rubella is transmitted directly by contact or by droplets from the nasopharynx of infected individuals and can cause significant birth defects if disease occurs early in fetal life. It has an incubation period of 14–21 days. Individuals may shed virus for up to 2 weeks prior to the outbreak of rash; therefore, patients are typically infectious for some time before the infection becomes clinically obvious. Virus shedding decreases significantly after the appearance of the rash, a period coinciding with the development of neutralizing antibodies. Rubella is no longer endemic in the United States as a result of an intensive vaccination campaign. Minor epidemics occurred in the United States every 5–7 years and major epidemics every 10–30 years.

   
Normal range:
Negative.

   Use

   Determination of rubella immune status assists in the diagnosis of rubella infection or determines susceptibility to rubella, particularly in pregnant women.

   Interpretation

   
Positive
(≥10 IU/mL): Indicative of past infection or vaccination

   
Equivocal
(≥5 to ≤10 IU/mL): Considered to be “indeterminate”

   
Negative
(<5 IU/mL): Does not preclude recent primary infection

   Limitations

   In general, 90% of the US population has been either vaccinated or exposed to rubella, with rubella IgG values of ≥10 IU/mL.

   The presence of IgG antibodies in a single specimen is not sufficient to distinguish between active infection and past infection. The results of the test must be taken within the context of the patient’s clinical history, symptomatology, and other laboratory findings.

   Patients suspected of having primary, active infection should be tested for the presence of IgM antibodies to rubella virus.