Rubella, commonly known as German measles, is a viral infection that is generally mild but can cause serious complications if contracted during pregnancy. For this reason, healthcare providers frequently screen for immunity, especially in women of childbearing age, using a blood test that measures antibodies. The goal is to determine if a person is protected against the virus through prior infection or vaccination. While many results are clear, some individuals receive a confusing outcome labeled “equivocal,” which indicates an indeterminate status. This result means the laboratory cannot definitively confirm protection, prompting the need for further evaluation.
The Spectrum of Rubella Test Results
The primary method for checking rubella protection is a serology test that looks for Immunoglobulin G (IgG) antibodies. These IgG antibodies are the long-lasting proteins the immune system creates after a successful vaccination or a past infection. Laboratories classify the results into three main categories based on the concentration of these antibodies in the blood.
A “positive” result indicates a concentration of IgG antibodies above a specific cut-off value, meaning the person is likely immune and protected against the virus. Conversely, a “negative” result shows antibody levels are below the required threshold, suggesting the person is non-immune and susceptible to infection. The third category, “equivocal,” is reserved for results that fall in the middle, creating an uncertain status.
Defining an Equivocal Rubella Result
An equivocal rubella result is a finding in the borderline or “gray area” of the antibody concentration spectrum. It signifies that the level of rubella-specific IgG antibodies is too low to be definitively considered protective but is not low enough to be classified as non-immune. Laboratories utilize specific concentration values, often measured in International Units per milliliter (IU/mL), to define these categories.
For example, a common laboratory cut-off for a positive result is \(10\) IU/mL or greater, while a negative result may be defined as \(7\) IU/mL or less. The equivocal range occupies the narrow band between these two thresholds, such as \(8\) to \(9\) IU/mL, or in some systems, \(10\) to \(15\) IU/mL. This borderline measurement means the testing method could not confidently assign the person to either the protected or the susceptible group.
Biological and Technical Reasons for Equivocal Readings
An equivocal reading can arise from several biological factors related to the body’s immune response. One common reason is that the person’s immunity may be waning, meaning antibody levels that were once protective have gradually declined over time. While the person still has some antibodies, the concentration is no longer above the laboratory’s strict protective threshold.
Alternatively, an equivocal result might occur if the test was performed too soon after a recent infection or vaccination. The immune system has begun producing IgG antibodies, but the levels have not yet reached the sustained concentration needed for a definitive positive result. Technical issues with the testing process can also contribute to an equivocal outcome, such as inherent limitations of the assay method or low-level interference from other substances in the blood.
Clinical Management and Next Steps
The primary course of action following an equivocal rubella result is to repeat the test to clarify immune status. For a non-pregnant individual, this retesting is often done two to four weeks after the initial sample. If the repeat test remains equivocal or is negative, the person is typically considered susceptible to rubella and vaccination is recommended.
If the repeat test shows a significant rise in antibody levels, this suggests the person may have had a very recent infection. However, a non-pregnant person with an equivocal result and no documented history of vaccination is often simply given a dose of the Measles, Mumps, and Rubella (MMR) vaccine. This approach ensures protection without the need for extensive additional testing.
Management is different and more urgent for pregnant individuals who receive an equivocal result. Vaccination with the MMR shot is not permitted during pregnancy due to the theoretical risk to the fetus. The healthcare provider will review the patient’s history and may opt to repeat the IgG test or use a different type of test, such as one for Immunoglobulin M (IgM), which indicates an acute, recent infection. If there is a known exposure to rubella, a specialist consultation is immediately necessary, and the MMR vaccine is administered shortly after delivery to protect future pregnancies.