++
Interpretation of tests for the diagnosis of T. gondii infection
is highly dependant on the clinical stage of suspected infection
and on the host.59 Clinical serological diagnostic tests
are used routinely, while nucleic acid amplification tests are reserved
for detecting the parasite in tissues or fluid.
++
Several caveats to diagnostic testing for toxoplasmosis are worth emphasizing.
In all cases, more than one type of serology will be required. In
most cases, paired sera taken 4 to 6 weeks apart are helpful for
determining the timing of infection. Other supporting tests include
histological studies of tissues and demonstration of the parasite
or nucleic acid in tissues. Isolation of the organism is indicative
of active infection.
++
Toxoplasma-specific IgG antibodies usually appear
within weeks of infection, peak at 1 to 2 months, and decline but
remain positive for life. In the absence of exogenous immunoglobulin,
the presence of IgG to T. gondii indicates past
infection or transplacental transfer of immunoglobulin from a mother
to the newborn. Avidity tests based upon the binding of serum antibodies
to fixed antigens that increase over time are more sensitive and
specific for differentiating recent from past infection, particularly
with only a single serum sample. High avidity is indicative of remote
infection, while low avidity is observed with more recent infection.
++
Toxoplasma-specific IgM antibodies usually appear
within the first weeks after an acute infection and become negative
within a few months. Therefore, its presence is usually indicative
of acute infection. However, in a significant proportion of people,
persistence of IgM beyond a few months does occur. This is problematic
because it can give the false impression of a recently acquired
infection.63 In addition, testing for IgM has revealed
a relatively high proportion of false-positive results because of cross-reactions
with other IgM antibodies, rheumatoid factor, or antinuclear antibodies,
thereby impairing its reliability as a marker of recent infection.64,65
Thus, a negative IgM has a good negative predictive value for a
recent infection, but a positive IgM is not always indicative of
a recent infection, and further testing is warranted in cases where
timing of infection is important. Toxoplasma-specific
levels of IgG, IgM, IgA, and IgE antibodies combined with avidity
tests improves sensitivity and specificity regarding the presence
and timing of infection. These are generally available in reference
laboratories.60,66 Test combinations—including Sabin-Feldman
dye test for IgG, IgM ELISA, IgA ELISA, IgE ELISA, and avidity testing—are more
sensitive and specific than single tests alone in determining the
timing of infection; these should be sought in most situations,
since determining the timing and establishing the diagnosis will impact
treatment decisions.60,65
++
Isolation of or histological visualization of replicating parasites
will confirm the presence of the infection but cannot be relied
upon to date the timing of infections. However, amplification of Toxoplasma DNA
using polymerase chain reaction (PCR) or the demonstration of tachyzoites
in fluid or tissues such as amniotic fluid, placenta, brain fluid,
or cerebrospinal fluid is indicative of active infection.67,68
+++
Evaluation of Children for Toxoplasma Infection
++
Serological tests are generally used to document infection in
immunocompetent persons. If only IgG antibodies to Toxoplasma are
present, then past infection with T. gondii has
occurred. The finding of T. gondii–specific
antibodies for IgM, IgA, or IgE suggests a recent infection in a
single specimen; however, IgM may remain positive in older children
and adults for years.60,66Initial titers of IgG may be
negative or low if the infection is very recent, but a fourfold
rise in titers of IgG from serial blood samples taken 3 weeks apart
indicates a recent infection. Presence of this serological profile
with a compatible clinical scenario is usually sufficient to confirm
the diagnosis. Lymph node biopsy specimens in immunocompetent individuals may
show characteristic histopathologic findings.
++
Immunocompromised children who have Toxoplasma-specific
IgG antibodies may have active recent infection or may be at risk
for reactivation. In clinical situations where the diagnosis is
not certain, tissue sampling for histopathology and identification
of the organisms and/or its nucleic acid may be required.
Particularly in this population, it is critical for diagnosis to
be made early in order that therapy be instituted.
+++
Evaluation of
Pregnant Women for Toxoplasma Infection
++
Universal screening for infection in pregnancy is routinely practiced
in France and Austria. In other parts of the world, prenatal screening is
performed sporadically depending on local epidemiology. More often,
evaluation takes place if there is a clinical suspicion or if there
are epidemiological factors to suggest infection. Since women rarely
transmit infection to the fetus unless acutely infected, diagnosis
is aimed at determining if acute infection took place during pregnancy. If
serological testing reveals no Toxoplasma-specific
antibodies, then the woman has not been previously infected and
is considered to be at risk for acquiring infection. Remote infection
in pregnant women is characterized by the presence of Toxoplasma-specific
IgG and the absence of Toxoplasma-specific IgM,
IgA, or IgE. In this case, there is no risk of infecting the fetus
unless the mother is immunocompromised.
++
If Toxoplasma-specific IgM with or without IgG antibodies
is detected, establishing if infection has occurred during gestation
is important so the mother and fetus can potentially be treated.
A clinical examination of the mother, with special attention to
the presence of lymphadenopathy, should be done. Since T.
gondii–specific IgM alone may persist for years
following primary infection, its presence is not specific for recent
infection. Serology should be repeated in 2 to 3 weeks to determine
IgG seroconversion or a rise in T. gondii–specific
IgG. Rising titers of IgG (twofold dilutions of specimens tested
in parallel) or seroconversion from negative to positive in serial
serum specimens is indicative of acute infection.60 In
order to assess time of acquisition of infection, avidity testing and
differential agglutination (AC/AS) testing, in addition
to T. gondii–specific IgA and IgE, should
be requested. A high-avidity test result indicates acquisition of
infection more than 12 to 16 weeks earlier.
++
Testing should be done in a reference laboratory. Consultation
with clinicians expert in interpreting serology is recommended.62 In
addition, fetal ultrasounds should be performed to assess growth
retardation, hydrocephalus, ascites, or calcifications.
++
If acute infection is diagnosed or suspected in the mother, attempts to
diagnose fetal infection are warranted.69 Currently, demonstration
of Toxoplasma DNA from amniotic fluid is the preferred
method for diagnosing fetal congenital infection. This technique
has supplanted fetal blood sampling using cordocentesis because
of decreased risk to the fetus and higher sensitivity.69,70 False
negatives may occur if the infection is acquired very early (4–16
weeks gestation) or late (greater than 22 weeks) in gestation.71 Since
transmission to the fetus must have occurred prior to the testing,
infections in the mother that have not had sufficient time to transmit
to the fetus will result in negative PCR testing of the fetal-placental
unit. The sensitivity of prenatal diagnosis with PCR is estimated
at 42.9% between 4 to 16 weeks gestation, 92.9% between 17
to 21 weeks gestation, and 61.7% at 22 weeks or more of
gestation. The negative predictive value up to 21 weeks gestation
is between 90% and 100% and then falls to 77% after
22 weeks gestation and to 14.3% after 31 weeks gestation.
It is, however, highly specific with a positive predictive value
of 100%.71 Therefore, a negative PCR in amniotic
fluid cannot completely rule out congenital toxoplasmosis, especially
after 22 weeks, but a positive result in a reference laboratory
is highly specific for diagnosis. Serial ultrasounds of the fetus
should be performed to detect hydrocephalus, microcephaly, or any
other clinical indication of congenital infection and should be performed
at monthly intervals on all infected pregnant women.69
+++
Evaluation of
the Newborn with Suspected or Confirmed Toxoplasma Infection
++
The clinical manifestations of congenital toxoplasmosis range
from nonexistent to very subtle or overt and may be nonspecific. Proper
diagnosis includes careful clinical evaluation and laboratory testing. Evaluation
should include a history and physical examination with additional specific
neurological and ophthalmological examinations. Baseline complete
blood counts, liver function tests, total IgM, cerebrospinal fluid analysis
(protein, glucose, cell count, T. gondii–specific
IgM, IgA, and IgG, and PCR for T. gondii), computed
tomography of the brain without contrast, and tests of hearing function
should be obtained.42 Abnormal cerebrospinal fluid (high
protein content and pleocytosis) and abnormalities of brain imaging
suggest infection.6,47,72
++
Specific serological evaluation is also critical for diagnosis
in the newborn period.42 In addition to IgG, the presence
of Toxoplasma-specific IgA, IgM, and IgE should
be sought in the sera of all newborns suspected of having Toxoplasma infection
and should be performed in a reference laboratory. If the diagnosis
of acute infection in the mother during gestation has not been established,
serum specimens from the mother should be sent in parallel and tested
for IgG, IgM, and IgA in a differential agglutination test (AC/HS)
with an avidity assay. False positives can occur if there has been
contamination in the first few days of life by maternal IgM and
IgA, depending on the type of assay used.73
++
A positive T. gondii–specific IgM in
a newborn indicates primary infection, as IgM does not cross the
placenta. IgA-specific antibodies to T. gondii are
also indicative of primary infection in infants infected congenitally
and after postnatal primary infection. Similar to IgM, however,
they can also persist for prolonged periods of time and cannot be
relied upon for timing in many instances. The sensitivity of T
gondii–specific IgA is higher than IgM for the
occurrence of primary infection in a newborn.74 The level
of IgG antibodies in the infant will fluctuate according to age,
depending on the rate of loss of maternal antibody and the infant’s
production of IgG antibody. Thus, a congenitally infected infant
may initially have a high titer that decreases as they lose maternal
IgG. The titer of IgG antibody may then increase as the child produces
his or her own IgG. In infants whose mothers have been treated with
pyrimethamine and sulfadiazine during pregnancy, serological titers
may also vary depending on the timing and effectiveness of therapy.
++
Interpretation of such tests should be performed in conjunction
with a reference laboratory experienced in interpreting Toxoplasma serology.
Measuring antibody levels in cerebrospinal fluid (CSF) is not accurate
if the mother’s IgG in the dye test is above 300 IU/L. However,
the presence of T. gondii–specific IgM
in the CSF is indicative of acute infection.
++
The placenta, peripheral blood clots, and CSF specimens should
be conserved to determine the presence of Toxoplasma organisms.
Isolation of T. gondii or detection of DNA in a
reference laboratory from any of these sites is indicative of acute
infection in the infant.75