Implementation of molecular techniques for diagnosis of mumps

Dear Editor,

It was with great interest that we read the work by Sanz et al.¹ on the serological diagnosis of mumps and the associated detection of elevated specific IgG titres. The authors reveal that 73.4% of patients with mumps (with no primary infection marker such as IgM) presented significantly elevated IgG titres and therefore conclude that new studies are needed which use other serological methods to clarify the diagnostic performance of IgG quantitation and to define the concept of “elevated titres”. We would therefore like to illustrate our experience of the mumps epidemic that occurred in Valencia from January to November 2017, the incidence rate of which was 42.5 cases per 100,000 inhabitants (provisional data from the Public Health Directorate of the Valencian Community's Ministry of Health).

During this period, 140 saliva samples from suspected mumps cases were sent to our microbiology department (age range: 3–78 years; mean: 23.9; 80 males) for diagnosis through viral RNA detection with a Real-Time Polymerase Chain Reaction (RT-PCR) (BD MAX^®, Beckton Dickinson, USA), following the protocol available on the Centers for Disease Control (CDC) website, available at: http://www.cdc.gov/mumps/lab/qa-lab-test-infect.html#realtime-pcr.

Eighty-eight cases were successfully confirmed, 50 of which were simultaneously subjected to the RT-PCR technique, qualitative serological IgM determination and quantitative determination of specific IgG by chemiluminescence (LIAISON^®, Diasorin, Italy), with a measurement range of 5–300 arbitrary units (AU) and a positivity cut-off point of 11, according to the manufacturer. Only 16 cases tested positive for specific IgM antibodies, equating to 32% sensitivity. Of the 34 cases that presented no primary infection marker, 68.5% tested positive for elevated IgG antibodies (Ab) (>300 AU/ml). Likewise, of the 52 suspected cases that were not confirmed, since no viral RNA was detected in saliva, only 27 could undergo serological testing, with 25.9% presenting elevated IgG levels.

To analyse whether there is a link between the detection of viral RNA in saliva and the quantity of IgG Ab, Spearman's test was used, obtaining a correlation coefficient (rho = 0.573; p < 0.05). These results show that there is a direct and moderate relationship between both variables.

The results published by Sanz et al.,¹ as well as those detailed above, highlight the limited sensitivity of serology—24.7 and 32%, respectively—for the diagnosis of mumps in a largely vaccinated population. Similar findings were also published by Maillet et al., who obtained 45% sensitivity.² We found a moderate correlation between mumps cases confirmed by RT-PCR and elevated IgG levels. This phenomenon is similar to what may be observed in light of a secondary infection.³ Following vaccination, the immune system produces antibodies to vaccine strains (Rubini and/or Jeryl Lynn). However, these antibodies are not protective and when the patient comes into contact with a circulating wild-type strain, it generates a rapid and intense antibody response against recognised antigens.

To improve the diagnostic results for mumps at our hospital, we decided to perform RT-PCR to confirm suspected mumps cases, as recommended by other authors,^4,5 and following the recommendations of the CDC's Manual for the surveillance of vaccine-preventable diseases.⁶ As well as avoiding the diagnostic uncertainty generated by serology, the results considerably improved sensitivity, which reached 62.85% among the suspected cases versus 11.42% in the detection of IgM.

In our experience, the routine laboratory implementation of the RT-PCR technique on saliva samples is vital in order to provide a solid microbiological mumps diagnosis and relegates the usefulness of serology, which is far less sensitive, to an indicator of vaccination status.