Toxoplasma infection

Testing algorithm

Not immune Early Acute phase Acute phase Chronic phase or Reactivation of latent infection
Description No symptoms Flu-like symptoms or asymptomatic Symptomatic or asymptomatic
Toxo IgM - + + + -
Toxo IgG - - + + +
Toxo IgG Avidity* n.a. Low* Low* High/Low* n.a./High

n.a.: not applicable

* A low avidity result cannot confirm a recent infection, since persistent low avidity results have been commonly reported in serum from patients with past infections of months or even years, whatever the technique used. Nevertheless some authors suggest that a very low avidity result could be highly suggestive of a recent infection. Instead, presence of high-avidity IgG can rule out an acute primary infection.

Note: The detection of Toxo IgM antibodies in a single sample is not sufficient to prove an acute toxoplasma infection since elevated IgM antibody levels may persist even for years after initial infection. Further tests or a combination of test methods should be performed for clarification.5,6

Adapted from:

  1. Villard O, et al. Serological diagnosis of Toxoplasma gondii infection: Recommendations from the French National Reference Center for Toxoplasmosis. Diagn Microbiol Infect Dis. 2016;84:22-33.
  2. Dhakal R, et al. Significance of a positive Toxoplasma immunoglobulin M test result in the United States. J Clin Microbiol. 2015;53:3601–3605.
  3. Robert-Gangneux F and Darde ML. Epidemiology of and diagnostic strategies for toxoplasmosis. Clin Microbiol Rev. 2012;25:264-296.
  4. Tekkesin N. Diagnosis of toxoplasmosis in pregnancy: a review. HOAJ Biology. 2012;1. http://dx.doi.org/10.7243/2050-0874-1-9.
  5. Lappalainen M and Hedman K. Serodiagnosis of toxoplasmosis. The impact of measurement of IgG avidity. Ann Ist Super Sanita. 2004;40:81-88.
  6. Montoya JG and Liesenfeld O. Toxoplasmosis. Lancet. 2004;363:1965-1976.
Product Description Tests Product page
Elecsys® Toxo IgM a) Electrochemiluminescence immunoassay (ECLIA) for the qualitative detection of IgM class antibodies to Toxoplasma gondii in human serum and plasma 100
Elecsys® Toxo IgM b) 300
Elecsys® Toxo IgG a) Electrochemiluminescence immunoassay (ECLIA) for the quantitative detection of IgG class antibodies to Toxoplasma gondii in human serum and plasma 100
Elecsys® Toxo IgG b) 300
Elecsys® Toxo IgG Avidity a) Electrochemiluminescence immunoassay (ECLIA) for the qualitative detection of Toxoplasma gondii IgG avidity in human serum and plasma 50
Elecsys® Toxo IgG Avidity b) 50

a) for use on the cobas® e 411 analyzer and the cobas® e 601 / 602 modules
b) for use on the cobas® e 402 and cobas® e 801 analytical units

Toxo IgM Toxo IgG Toxo IgG Avidity Toxo DNA Interpretation
1st sample
- - n.a. n.a. Patient is not immune and susceptible to infection. Pregnant women should take preventive measures and be closely monitored during pregnancy.
- + n.a. n.a. Immunity to toxoplasmosis.
+ - n.a. n.a. Very early stage of infection or false positive IgM (unspecific IgM). Perform follow-up test incl. IgG Avidity (when IgG is reactive) after 2 – 3 weeks to confirm either result.
+ + n.a. n.a.
2nd sample
+ + low + Acute infection confirmed.
+ + low n.a. Recently acquired infection not excluded. Test follow-up sample after 3 weeks. PCR on amniotic fluid is recommended.
+ + high n.a. or - Acute infection excluded.

* except infants
n.a.: not available or not tested

Adapted from:

  1. Villard O, et al. Serological diagnosis of Toxoplasma gondii infection: Recommendations from the French National Reference Center for Toxoplasmosis. Diagn Microbiol Infect Dis. 2016;84:22-33.
  2. Robert-Gangneux F and Darde ML. Epidemiology of and diagnostic strategies for toxoplasmosis. Clin Microbiol Rev. 2012;25:264-296.
  3. Remington JS, et al.. Chapter 31: Toxoplasmosis. In (Ed.) Infectious Diseases of the Fetus and Newborn Infant (7th Edition) (ed., pp. 918-1041). Philadelphia: W.B. Saunders; 2011.
  4. Montoya JG and Liesenfeld O. Toxoplasmosis. Lancet. 2004;363:1965-1976.
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Step 1

Perform Toxo IgM and IgG tests in parallel

IgM test
IgG test
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Step 2

Perform Toxo IgM and IgG tests in parallel in
1st and 2nd sample

IgM test
IgG test
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Step 2

Perform Toxo IgM and Toxo IgG tests
in parallel in 2nd sample

IgM test
IgG test
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Step 3

Perform Toxo IgG Avidity test

Toxo IgG Avidity
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Step 3

Test amniotic fluid for Toxo DNA

Toxo DNA
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Step 4

Test amniotic fluid for Toxo DNA

Toxo DNA
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Summary

Not immune and susceptible to infection.
Pregnant women should take preventive measures and be closely monitored during pregnancy. Follow up (monthly) until delivery may be done depending on the country's policy.

In case a very recent infection is suspected, retest a second sample in 2-3 weeks.

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Summary

Immune to toxoplasmosis.
In some cases, Toxo IgM antibodies may be transient or even negative. In case a very recent infection is suspected, perform a second serology test 3 weeks later to check for any potential increase in IgG levels.

Stable Toxo IgG levels indicate a diagnosis of chronic toxoplasmosis. Increased IgG amounts are required to perform a Toxo IgG Avidity test to date the infection.

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Summary

IgM reaction is likely a false positive.
Continue with serological screening (according to country-specific guidance) and follow advice for avoiding infection.

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Summary

IgM reaction is likely a false positive or possible analytical error.
Continue with serological screening (according to country-specific guidance) and follow advice for avoiding infection.

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Summary

Acute infection or implausible results.
Investigate for possible analytical errors. Consider referring the patient to an expert Toxo laboratory, to perform additional tests.

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Summary

Results are implausible.
Investigate for possible analytical errors.

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Summary

Acute infection within the last 4 months is excluded.
Monthly serological screening can be stopped (provided stable antibody titers are found in a 2nd serum sample taken 3 weeks later).

Note: a high Toxo IgG avidity result obtained in a serum sample taken >4 months after the beginning of pregnancy does not exclude an infection during pregnancy.

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Summary

Fetal infection not confirmed
Start anti-parasitic treatment for prevention of horizontal transmission.

Continue pregnancy monitoring.

Thorough evaluation of newborn by postnatal diagnosis recommended.

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Summary

Fetal infection is not confirmed.
Start anti-parasitic treatment for prevention of horizontal transmission.

Continue pregnancy monitoring.

Thorough evaluation of the newborn by postnatal diagnosis is recommended.

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Summary

Fetal infection confirmed
Start anti-parasitic treatment.

Discuss options for possible pregnancy outcome.

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Summary

Fetal infection is confirmed.
Start anti-parasitic treatment.

Discuss options for possible pregnancy outcomes.

Back
Next Restart simulation
Step
Result
Interpretation
1
IgM test = ...
IgG test = ...
waiting for input
1
IgM test = negative
IgG test = negative
Not immune and susceptible to infection.
1
IgM test = negative
IgG test = positive

Immune to toxoplasmosis.

1
IgM test = positive
IgG test = negative

Possible early acute infection or false‑positive IgM result.

Obtain a new sample 2 weeks later for parallel IgG and IgM testing. If available use a confirmatory test for IgM.

Perform Toxo IgM and Toxo IgG tests in parallel in 2nd sample.

1
IgM test = positive
IgG test = positive

Possible acute infection (or false-positive IgM result or persisting IgM).

Obtain a new specimen and repeat IgG and IgM testing. Perform IgG Avidity testing if available.

Perform Toxo IgM and Toxo IgG tests in parallel in 2nd sample.

2
IgM test = ...
IgG test = ...
waiting for input
2
IgM test = negative
IgG test = negative

IgM reaction is likely a false positive or possible analytical error.

2
IgM test = negative
IgG test = positive

Acute infection or implausible results.
Investigate for possible analytical errors.

2
IgM test = positive
IgG test = negative

IgM reaction is likely a false positive.

Results for the 2nd sample remain the same (IgM levels remain stable, no IgG appearance in a no-treatment context)

2
IgM test = positive
IgG test = positive

Acute infection is strongly suggested due to seroconversion.

Management and treatment vary with the duration of gestation. Usually, test amniotic fluid samples for Toxo DNA after 16 weeks of gestation and at least 4 weeks after maternal infection.

Test amniotic fluid for Toxo DNA.

2
IgM test = ...
IgG test = ...
waiting for input
2
IgM test = negative
IgG test = negative
Results are implausible.
2
IgM test = negative
IgG test = positive
Possible acute infection or false-positive IgM result.

Perform Toxo IgG Avidity test.
2
IgM test = positive
IgG test = negative
Results are implausible.
2
IgM test = positive
IgG test = positive
Initial results are confirmed.

Perform Toxo IgG Avidity test.
3
IgG Avidity test = ...
waiting for input
3
IgG Avidity test = high
Acute infection within the last 4 months excluded.
3
IgG Avidity test = low

Acute infection is not excluded.

Test a second sample 3 weeks later:

  • If IgG titers are stable, infection occurred more than 2-3 months before the date of first sample taking.
  • If IgG titers are increasing, infection occurred within 2-3 months before first sample taking.

Consider referring patient to an expert toxoplasmosis laboratory.

Test amniotic fluid sample for Toxo DNA after 1-6 weeks of gestation and at least 4 weeks after maternal infection.

3
Toxo DNA test = ...
waiting for input
3
Toxo DNA test = positive
Fetal infection confirmed.
3
Toxo DNA test = negative
Fetal infection not confirmed.
4
Toxo DNA test = ...
waiting for input
4
Toxo DNA test = positive
Fetal infection confirmed.
4
Toxo DNA test = negative
Fetal infection not confirmed.
End of test sequence