Laboratory investigation for toxoplasma infection in pregnancy aims to provide critical information to support appropriate and timely clinical management.  Essentially, there are three separate patient groups that need to be considered in the investigation of toxoplasma infection in pregnancy: the mother, the foetus, and the neonate.  The key information sought is summarised in the table below:

	Aim of laboratory investigation
Mother	Confirm or exclude risk to pregnancy by determining whether maternal infection was acquired before conception
Foetus	If risk to pregnancy is confirmed, determine whether foetal infection can be confirmed.
Neonate	If foetal infection is not confirmed, confirm or exclude congenital toxoplasma infection in the neonate.

Mother – Where acute toxoplasma infection is suspected, it is recommended that investigation be undertaken utilising separate IgG and IgM assays. Where a UK laboratory does not have access to separate IgG and IgM assays, samples screened with a combined IgG/IgM assay and yielding a result consistent with acute infection, may be sent to a Toxoplasma Reference Unit for further investigation. It should be emphasised that the more sensitive IgM immunosorbent agglutination assay (IgM-ISAGA) is not helpful in testing in pregnancy as it can detect IgM persisting for longer than one year after infection has been acquired, and therefore may detect IgM in a significant proportion of women whose pregnancy is not at risk from toxoplasma infection. 

A positive IgG result and negative IgM result indicates a non-acute infection.  The minimum duration of infection that can be inferred from the absence of IgM will depend upon the sensitivity of the IgM assay used.  Where infection can clearly be demonstrated to have occurred prior to conception, the pregnancy is considered to be not at risk from toxoplasma.

If the serum is found to be positive for IgM using an appropriate assay, it is recommended that further specialist investigation be considered, including IgG avidity testing and comparison of both IgG and IgM levels in sequential samples in order to gain insight into the likely duration of infection.  If infection acquired after conception cannot be excluded, the pregnancy is considered to be at risk from toxoplasma.

Foetus – Infection of the foetus is not an inevitable outcome of every maternal toxoplasma infection.  The risk of transmission from mother to foetus increases depending in which trimester maternal infection is acquired.  Based on a range of reported studies, the mean risk of transmission in the first trimester is estimated to be 10-15%, rising to 70-80% in the third trimester.  However, although foetal infection in the first trimester is less likely, the outcome is generally more severe (e.g. gross abnormality or spontaneous termination) compared to infection acquired in the third trimester which may result in more subtle neurological, ocular or systemic signs or may be sub-clinical, with the child born apparently normal.

The diagnosis of foetal infection is based upon the detection of the parasite and/or specific antibody responses in the foetus.  Direct detection of the parasite from foetal blood or amniotic fluid using the polymerase chain reaction (PCR) provides unequivocal evidence of infection.  Detection in amniotic fluid has been found to have as good a level of detection as the methods involved in cordocentesis and has fewer risks to the foetus.  However although PCR is a highly specific and sensitive technique it still has potential technical limitations and PCR results should not be interpreted in isolation from other tests.

Neonate - Since maternal IgG is transferred passively to the foetus in utero, detection of IgG in the neonate is of limited value unless levels are significantly elevated compared to maternal titres.  However, comparison of maternal and neonatal IgG by immunoblot may be helpful since detection of a neonatal immune response to any antigens not recognised by the maternal immune response would imply this IgG is unique to the neonate.

Detection of neonatal IgM and IgA by EIA and/or ISAGA are regarded as being diagnostic for neonatal infection but the possibility of contamination by maternal blood should be excluded.  If IgM and/or IgA are present in the mother, it is recommended that neonatal serum is collected 2-3 weeks later and re-tested to confirm the original findings.

It is important to note that IgM and IgA may only be present in 50-60% of congenitally infected children in the first month of life, but may appear subsequently.  It is therefore essential to monitor the child serologically throughout the first year of life by which time any passively-acquired maternal IgG antibodies will decline and disappear.  The disappearance of IgG within the first year of life excludes congenital infection.  Persistence of positive DT after 12 months confirms infection.  Treatment of an infected neonate may initially result in a reduction in antibody levels or even a complete disappearance.  In such cases antibody levels may increase after therapy is stopped.

A summary of helpful laboratory investigations is shown below:

Toxoplasma Test		Pregnant Women	Foetus			Neonate
		Blood		Blood	Amniotic Fluid		Cord Blood/Neonatal Blood	Amniotic Fluid
Dye Test (IgG + IgM)		+		+	-		+	-
IgG		+		+	-		+	-
IgM/IgA EIA		+		+	_		+	-
IgM/IgA ISAGA		-		+	-		+	-
IgG Avidity		+		-	-		-	-
IgG Immunoblot		+		-	-		+ a	-
PCR		-		+	+		+	+

(a) must be tested in parallel with maternal blood taken at birth for comparison