Oral fluid tests

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Oral fluid samples are very easy to take - this can be done by the patient or their parents.

Oral fluid samples can be used for:

  • "Serology" (not strictly serum, but antibody levels in oral fluid mirror those in serum)
  • Checking for the presence of viruses, usually using molecular methods such as PCR

Antibody tests may be routinely offered via Public Health England (or equivalent bodies in other administrations) for notified cases (and cases should be notified on suspicion) of certain diseases, including pertussis, measles, mumps, and rubella.

When checking for these diseases, an early sample (taken within a few days of onset of disease) may be tested, using PCR for evidence of the presence of the virus; or a sample taken 10-20 days after onset can be checked for the presence of antibodies, with the presence of IgM indicating recent infection or vaccination, the presence of IgG indicating previous infection or vaccination, and the absence of either suggesting no recent infection with the virus in question.

Note that the "oral fluid" required is the exudate from the mucous membranes. It is NOT saliva. Patients should NOT be encouraged to salivate prior to collecting the sample (this dilutes the oral fluid).

Oral fluid testscan be used for two purposes:

  1. To confirm infection during the acute phase. This is seldom required for the purpose of treating the initial patient (or "index case"; but it may be recommended for public health purposes, in order to clarify whether public health measures of prophylactic treatment of contacts is required. A polymerase chain reaction (PCR) test can be performed using the saliva kit sent out by the local health protection unit (or equivalent); or a dry swab or a viral medium swab can be used. (Swabs intended for bacteriology cannot be used for this test.) The test is most likely to be positive early after the onset of symptoms, but may still be positive up to a week after onset.
  2. To confirm the diagnosis retrospectively, in order to evaluate the efficacy of vaccination programmes, and to know when public health action - such as additional MMR campaigns - is required. If disease-specific IgM is identified, this indicates that there has been a recent infection. IgG at this stage usually indicates prior infection or vaccine-induced immunity.
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