For diagnosis You are interested in the following questions:
Diagnosis can be performed by (a combination of) several approaches:
Serological tests measure "antibodies"
Being infected with the parasite Toxoplasma gondii leads to some weeks of parasitaemia, that means, that there are many parasites in the blood (in that time they can be passed on to the unborn baby).
Then the body starts to produce "antibodies" (which can be found in the blood and be measured in serological tests).
They "fight" against the parasites and control the disease, no more parasites circle in the blood. (and the infection can not be passed on to the baby any longer)
There are some groups of antibodies.(=Immunoglobulines[Ig]: IgG, IgM, IgA, IgM, IgE).
For diagnosis of Toxoplasmosis usually IgG and IgM (in some cases: IgA) are measured.
Before there happened an infection no antibodies are found. IgG (and, of course, IgM, .. ) is "negative".
When an infection happens, IgG and IgM get "positive".
IgM are "positive" during acute infection and stay positive for a limited time (depending the methods of test You use). (Maybe 6 months to a year)
IgG-titers rise during an acute infection, sink slowly again, but stay positive, and protect against another parasitaemia (and protects so the unborn baby).
That means, if You find (stable) IgG and no IgM: there had been the infection longer time ago, and there is protection now (and in the future). ("latent infection")
If there had been negative testing for Toxoplasmosis earlier
in pregnancy and positive testing later on ("seroconversion"):
An acute infection occoured!
If there are IgG and IgM: the infection happened short time ago, further investigation can tell about when.
If the infection occurred during pregnancy:
anti-parasitic drugs should be taken until birth to reduce the risk of a fetal infection and
the fetus (unborn baby) should be tested
When a pregnant woman has got an infection, there is the possibility, that she passes the infection on to her baby. To know, if the fetus (unborn baby) is infected helps to
Amniocentesis is performed to look for the DNA of the
parasite by a PCR-technique.
"PCR-positive" means, that the fetus got the infection and should be treated: maternal treatment and treatment of the baby until the first birthday!
"PCR-negative" means, that the fetus is not yet infected. The mother should take Rovamycine until delivery to prevent fetal infection later in pregnancy.
Ultrasound shows some of the symptoms of the overt disease: hydrocephalus, intracranial calcifications, ...
Serological tests from umbilical cord samples have loosen much
of their values since the PCR-diagnosis has been established
Serological tests of a blood sample of the newborn baby
(or of an umbilical cord blood sample taken by the midwife) can
help to find the diagnosis of Congenital Toxoplasmosis. But the
interpretation of this test is ambiguous:
IgG are passed from the mother to the baby through the placenta and could be of maternal origin.
IgM cannot pass the placenta. If they are found infection of the baby is proofed. But: not all infected babies produce IgM, that mean, that the absence of IgM does not exclude Congenital Toxoplasmosis!
In these cases the serological diagnosis of Toxoplasmosis can be performed by a follow-up.
IgG-titers of maternal origin sink (half-life period about a month). Persisting (or rising) IgG proof congenital infection of the baby! Therefore serological testing should be repeated until IgG turns negative, exclusion of Congenital Toxoplasmosis cannot be done before that!
It is important to know if the baby has been infected to start
treatment as early as possible.
As serological tests are limited a prenatal PCR-diagnosis helps the pediatric care.
A clinical examination looks for signs of overt disease.
This includes cranial ultrasound examinations and fundoscopy.
Serological follow up decides if the baby suffers from Congenital
Clinical examinations (head-circumference, mental functioning, fundoscopy) should search for late-onset symptoms.