Use of Serology to Aid in the Diagnosis of Mumps Infection

A buccal swab specimen collected <3 days after parotitis onset is the preferred specimen and rRT-PCR testing is the preferred method for laboratory confirmation of mumps. In addition to a positive rRT-PCR, other confirmatory tests include isolation of mumps virus and a significant rise in immunoglobulin G (IgG), though these methods are not recommended because they are insensitive and take longer to obtain results. Detection of mumps immunoglobulin M (IgM) can aid in the diagnosis of mumps, although a positive IgM result is only supportive laboratory evidence, based on the Council of State and Territorial Epidemiologists mumps case definition.

At the onset of a suspected mumps outbreak, it is necessary to adequately test suspect cases of mumps with confirmatory tests to rule out other possible etiologies. However, once a mumps outbreak is confirmed, jurisdictions should consider ways to ensure more efficient use of testing resources.

Vaccination status and timing of specimen collection can affect the ability to detect IgM in persons infected with mumps. In general, IgM detection is highest in unvaccinated persons, intermediate in one-dose vaccine recipients, and lowest in two-dose vaccine recipients. Specimens collected > 3 days post-parotitis onset are more likely to have a positive IgM result than those collected earlier.

IgM test methods and kits vary considerably in their sensitivity and specificity, with some indirect EIA and immunofluorescent assays detecting as few as 12–15% of confirmed mumps cases (Rota et al 2013). IgM capture ELISA is the most sensitive serologic method detecting 46 -71% of rRT- PCR confirmed cases but has limited availability (Rota et al. 2013).

Failure to detect mumps IgM in previously vaccinated persons who are infected with mumps has been well documented. People with a history of mumps vaccination may not have detectable mumps IgM antibody regardless of timing of specimen collection. IgG test results are typically positive and elevated at the initial blood draw, making detection of a 4-fold rise or seroconversion unlikely and therefore not recommended.

Q: What type of serologic assay is recommended for mumps IgM testing?

A: Both EIA and IFA assays can perform well for diagnosis of primary mumps infection. Acute-phase mumps specimens may contain significant levels of mumps IgG, especially among persons with a history of 1 or 2 doses of MMR. The IFA format is particularly susceptible to interference by high levels of mumps IgG. Treatment of serum with an agent to remove human IgG antibody, such as Gullsorb or a similar IgG inactivation reagent, is necessary to avoid false-positive IgM test results.

Q: What serologic tests are used at CDC to diagnose mumps?

A: A capture IgM EIA (non-quantitative) that incorporates a recombinant mumps nucleocapsid protein as the antigen is used to detect mumps IgM. A commercial, indirect EIA (non-quantitative) is used for detection of IgG.

Q: What does a positive mumps IgG test result mean?

A: A single serum sample tested for mumps-specific IgG is not useful for diagnosing acute mumps infections. The presence of mumps-specific IgG, as detected using a serologic assay (EIA or IFA), does not necessarily predict the presence of neutralizing antibodies or protection from mumps disease. See related questions regarding measurement of rise in IgG titer.

Q: What is the protective neutralizing antibody titer for mumps?

A: There is no known protective level of neutralizing antibody (antibody titer) for mumps, and there are no other immune parameters that correlate with protection from mumps disease.

Q: What etiologic agents are likely to interfere with serologic assays for mumps (i.e., produce false-positive results)?

A: Parainfluenza viruses (HPIV) 1, 2, and 3, Epstein-Barr virus (EBV), adenovirus, and human herpesvirus 6 (HHV-6) have all been noted to interfere with mumps serologic assays (Davidkin et al. 2005).

Q: What additional testing should be considered for sporadic cases that have negative laboratory results for mumps?

A: Consider testing for other etiologies such as influenza virus, EBV, adenovirus, HPIV types 1,2 and 3, or bacteria including Staphylococcus aureus and alpha hemolytic streptococcus.  During 2009-2011, 8 jurisdictions throughout the United States investigated sporadic cases of parotitis.  Labs tested 101 specimens for alternate etiologies, and 23% were positive for EBV, 10% for HHV-6, 3% for HPIV2, 1% for HPIV3 and 1% for human bocavirus. Parotitis has also been reported in persons with laboratory-confirmed influenza infections.

Q: My patient specimen was positive by the Monspot test (Epstein-Barr) but also gave a positive result for mumps IgM antibody. What is the explanation for this?

A: The initial immune response to Epstein-Barr produces a polyclonal B cell stimulation; the antibodies are broadly reactive and can result in a positive mumps IgM result. However, the Monospot test should be considered less susceptible to a false-positive result with serum collected from a true case of mumps.

Q: If the IgM result is negative and IgG is positive, can mumps be ruled out?

A: Absence of a mumps IgM response in a vaccinated or previously infected individual presenting with clinically compatible mumps does not rule out mumps as a diagnosis. A positive IgG result is expected among previously vaccinated persons. Older persons or foreign-born persons with no history of mumps illness or vaccination may have detectable mumps IgG due to a previous subclinical infection.

Q: Can serologic tests differentiate between a recent or prior exposure to mumps virus and a response to mumps vaccine?

A: The presence of mumps-specific IgG indicates a recent or a prior exposure to mumps virus or mumps vaccine. Serologic tests cannot differentiate between an exposure to vaccine and an exposure to wild-type mumps virus. See question regarding utility of viral specimen for genetic analysis.

Q: What does a positive IgM result mean?

A: Mumps diagnosis is supported by detecting mumps IgM antibody in serum specimen collected as soon as possible after symptom onset. A positive IgM test result indicates current or very recent infection or reinfection. A positive IgM test result may also be observed following mumps vaccination.

Q: Why is it difficult to demonstrate a rise in titer (seroconversion) from persons with a history of vaccination?

A: Collection of acute and convalescent phase serum specimen to demonstrate a 4-fold increase in IgG titer is not recommended. In vaccinated persons, the titer of existing IgG will begin to rise soon after exposure and infection. At the time of symptom onset and collection of the acute-phase serum sample, IgG may already be elevated to such high levels that detection of a 4-fold rise in titer when comparing acute- and convalescent-phase serum specimen is not possible. Therefore, paired serum specimen from vaccinated persons, even if appropriately timed may not show a 4-fold rise in IgG titer.

Q: What is the experience at CDC with paired serum specimen from previously vaccinated persons with mumps?

A: A 4-fold rise in IgG titer is rarely demonstrated between paired serum specimens from persons who have received one or two doses of MMR vaccine. In our experience using a plaque neutralization assay, we have only detected a 4-fold rise in neutralizing antibody titer between paired specimen in persons who were also IgM positive.