All requests for laboratory investigation must give the following information to enable the results to be reported with the correct interpretation:

• Full demographic details
• Gestation of pregnancy (date of last menstrual period)
• Date of onset, clinical features, type and distribution of any rash illness
• Past history of rubella and any other antibody testing and/or rubella vaccine administration (and dates/places)
• Any known contacts with rash illness, and dates of contact

Antenatal sera should be retained for at least one year to assist diagnosis/exposure in later pregnancy and investigation of the neonate. This may include exposure to varicella-zoster virus and parvovirus B19, when the availability of such sera for testing can be invaluable in rapidly assessing susceptibility.

 

• Laboratory investigation of rubella
• Laboratory investigation of parvovirus B19
• Laboratory investigation of hydrops fetalis

 

Laboratory investigation of rubella

The routine antenatal testing for rubella antibody is for determining susceptibility and identifying those for whom vaccine is advised post delivery: it does not determine whether rubella may have occurred in the current pregnancy. If such investigation is required, the request form must clearly state that recent rubella is a possibility, and full clinical and epidemiological details given ( as detailed above).

The serological diagnosis of rubella is well-established ( 35). A serum at first presentation must be collected and sent for laboratory testing.

It is recommended that the laboratory investigates all cases of possible rubella by simultaneous testing for rubella-specific IgG (or total rubella antibody) and IgM.

When reporting the results of rubella serology, the laboratory must advise on any further sera/follow-up required, and give a definitive conclusion of their investigations, eg "No evidence of recent primary rubella".

Problems arise when investigation commences four weeks or more after the onset of rash illness. If rubella-specific IgG is detected, and specific IgM is not detected, rubella as a cause of the rash illness cannot be excluded serologically unless past sera can be tested to determine whether seroconversion has occurred recently. An assessment of probabilities has to be made based on recent epidemiology of rubella in the community, past history of vaccine and testing, characteristics of illness, etc.

Some women present significant problems in diagnosis, particularly in those who give a positive result for rubella-specific IgM. Although positive rubella IgM results which do not reflect recent rubella (primary or reinfection) ("false positive") are infrequent, the control of rubella in the UK means that most rubella-specific IgM positive results do not reflect recent rubella. No woman in the first 20 weeks of pregnancy should have rubella diagnosed on the basis of a positive rubella-specific IgM alone. Results must be interpreted in relation to full clinical and epidemiological information. Unless seroconversion has been shown, further testing by alternative rubella-specific IgM tests and measuring the strength of binding of specific IgG (avidity) ( 35) is advised. IgG avidity is low soon after a primary infection, but matures over a few weeks to become more strongly binding. If rubella-specific IgM positivity reflects a recent rubella episode (whether primary or reinfection), the degree of reactivity will usually change over the period of a few weeks, rather than persisting at a similar level.

Screening for rubella antibodies

Laboratory investigation of parvovirus B19

Recent parvovirus B19 infection can be confirmed or excluded by testing for parvovirus B19 specific IgM on the first serum obtained.

Failure to detect parvovirus B19 specific IgM excludes infection in the four weeks prior to collection of the serum. Hence infection cannot be excluded if investigation commences more than four weeks after onset of rash illness (vide supra, rubella).

If parvovirus B19 IgM is detected in the first 20 weeks of pregnancy, confirmation is required by alternative assay, eg M-capture RIA, IgM specific immunofluorescence or IgG seroconversion using an antenatal booking blood. Repeat testing will demonstrate a decline in IgM reactivity and provide an additional confirmation method.

Screening for parvovirus antibodies

Laboratory investigation of hydrops fetalis

In a pregnant woman presenting with hydrops fetalis without a rash history, the diagnosis of recent parvovirus B19 infection can only sometimes be achieved by testing for parvovirus B19 specific IgM as the acute infection was usually some weeks prior to presentation. Infection with parvovirus B19 as the cause of hydrops fetalis can be investigated by testing the antenatal booking sample in parallel with the sample at presentation for parvovirus-specific IgG to show seroconversion.

Following confirmation of parvovirus B19 in a pregnant woman presenting with hydrops fetalis, referral to a Regional Unit of Fetal Medicine is recommended if this has not already occurred. If a fetal blood sample is collected then examination by molecular methods (PCR or dot blot hybridisation) and/or electron microscopy for parvovirus B19 virus particles to confirm fetal infection can be arranged by Virology Laboratories.

Proven parvovirus B19 infection in the hydropic fetus will influence the management of the patient as it is important in establishing the aetiology of the hydrops and in excluding other causes so allowing appropriate counselling of the patient.

 

Laboratories

• Immunisation & Diagnosis Unit (IDU) (Rubella, Measles, Parvovirus B19, Hydrops fetalis)
• Meningococcal Reference Unit (MRU) 
• Meningococcal infection