Serology to Diagnose Mumps

Types of serologic tests

IgG serology testing

In addition to a positive rRT-PCR, other confirmatory tests include isolation of mumps virus and a significant rise in immunoglobulin G (IgG). These methods are not recommended though, because they are insensitive and take longer to obtain results.

A commercial, indirect EIA (non-quantitative) is used for detection of IgG.

IgM serologic testing

Detection of mumps immunoglobulin M (IgM) can aid in the diagnosis of mumps. However, a positive IgM result is only supportive laboratory evidence, based on the CSTE mumps case definition.

A capture IgM EIA (non-quantitative) that incorporates a recombinant mumps nucleocapsid protein as the antigen is used to detect mumps IgM.

IgM test methods and kits vary considerably in their sensitivity and specificity.

• Some indirect EIA and immunofluorescent assays detecting as few as 12%–15% of confirmed mumps cases.1
• IgM capture ELISA is the most sensitive serologic method detecting 46%–71% of rRT- PCR confirmed cases but has limited availability1.

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 more than 3 days post-parotitis onset are more likely to have a positive IgM result than those collected earlier.

Recommended serologic assays

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.

Interference with serologic assays

Certain etiologic agents are likely to interfere with serologic assays for mumps and produce false-positive results. 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.2

Testing results

Negative IgM & positive IgG result

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.

Positive IgM result

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.

Positive IgM & positive Monospot test

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 Epstein-Barr test should be considered less susceptible to a false-positive result with serum collected from a true case of mumps.

Positive IgG result

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.

Limitations of serologic tests

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.

Additional testing for negative lab results

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:

• 23% were positive for EBV
• 10% for HHV-6, 3% for HPIV2
• 1% for HPIV3
• 1% for human bocavirus

Parotitis has also been reported in persons with laboratory-confirmed influenza infections.

Serology in previously vaccinated people

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.

Demonstrating a rise in titer

It is difficult to demonstrate a rise in titer (seroconversion) from people with a history of vaccination. Collection of acute and convalescent phase serum specimen to demonstrate a four-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. IgG test results are typically positive and elevated at time of symptom onset and at the initial blood draw, making detection of a four-fold rise not possible or unlikely. Therefore, paired serum specimen from vaccinated persons, even if appropriately timed, may not show a four-fold rise in IgG titer.

A four-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 CDC's experience using a plaque neutralization assay, we have only detected a four-fold rise in neutralizing antibody titer between paired specimen in persons who were also IgM positive.