Box 2 shows the laboratory criteria for diagnosing mumps.
Routine blood testing may show a leucopenia with a relative lymphocytosis or sometimes a neutrophil leucocytosis. Raised C reactive protein and erythrocyte sedimentation rate may also be seen.2
Box 2: Laboratory diagnostic criteria
- Isolation of mumps virus or nucleic acid from clinical specimen, or
- Significant rise in mumps specific immunoglobulin titres between acute and convalescent serum samples by any standard serological assay, or
- Positive serological test for mumps IgM in serum or saliva
Only 10% of suspected mumps cases were laboratory confirmed in England and Wales between 1989 and 1999,w1 highlighting the weakness of clinical diagnosis during a period of low transmission. In the absence of clinically apparent parotitis, a laboratory diagnosis is required even in the context of an epidemic.
Mumps specific IgM is detectable in serum as early as 11 days after exposure and is almost always present by the time of clinical illness. In the UK, mumps can conveniently be confirmed by detection of specific IgM antibody in salivary samples. This method is acceptable to children and parents and forms the basis for case confirmation by the Health Protection Agency.11
The specificity, as determined by analysis of blood donor samples, is greater than 98%.12
In the first week of illness the sensitivity is 75%, increasing to 100% after the first week.12
When salivary IgM is negative, reverse transcription-polymerase chain reaction can be used to detect mumps RNA in saliva.13
This sensitive and rapid method facilitates sequencing and subsequent genotyping for epidemiological purposes and is largely superseding viral isolation in animal cell lines. Viral culture or nucleic acid detection from urine (owing to replication in the kidney) is also possible for up to two weeks after symptoms begin.8
Where mumps is a cause of meningoencephalitis, cerebrospinal fluid typically shows a lymphocytosis averaging 250 cells/mm3
, raised protein in 60-70% of cases, and a normal opening pressure.8
The glucose in cerebrospinal fluid is less than 50% of the serum value in up to a quarter of patients.8
Mumps virus nucleic acid can be detected in the cerebrospinal fluid of up to 96% of cases of mumps meningoencephalitis by reverse transcription-polymerase chain reaction and has been found up to two years later.14
Mumps specific IgM can also be found in cerebrospinal fluid in half of cases and IgG in a third,15
sometimes persisting for a year or more with pleocytosis. Such persistent abnormalities of cerebrospinal fluid have not been correlated with long term sequelae.