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Read about the tests used to diagnose toxoplasmosis, including about testing in pregnancy.

A blood test can be carried out to see if you have been infected with the Toxoplasma gondii (T. gondii) parasite.

This blood test looks for specific antibodies produced by your immune system to fight the infection.

When might a blood test be recommended?

The blood test for toxoplasmosis may be carried out if there's a chance you have the condition and you have symptoms of the infection, or you're at risk of serious complications of toxoplasmosis. For example, the test may be recommended if you're diagnosed with HIV or are scheduled to have an organ transplant.

In the UK, testing for toxoplasmosis isn't routinely carried out during pregnancy because the risk of picking up the infection while you're pregnant is very small and there are some concerns about the reliability of the test in pregnant women. It's also not clear exactly how effective treatment during pregnancy is in stopping the infection spreading to the baby.

You may be tested during pregnancy if you develop symptoms of the infection, problems with your baby are picked up during a routine ultrasound scan, or you have a weak immune system. Discuss the possibility of having a blood test with your midwife, GP or obstetrician if you're concerned.

What the blood test result means

Negative result

Testing soon after an infection can sometimes produce a negative result when a person is actually infected. This is known as a false negative. It occurs when the body hasn't had time to start producing antibodies to the parasite, which can take up to three weeks after the initial infection.

If you don't have any symptoms but there's a possibility you may be infected, your doctor may recommend another blood test a few weeks later. If the result is still negative, it's unlikely you've had the toxoplasmosis infection.

However, you may be at risk of becoming infected at some point, so you might be advised to take steps to prevent toxoplasmosis if you're at risk of developing complications of the infection.

Positive result

A positive result means you've been infected with toxoplasmosis at some point, but doesn't necessarily mean you have an active infection.

Normally, two different types of antibody are tested for to help decide whether or not you have an active infection – these are called IgG and IgM antibiodies.

IgG antibodies will stay in your blood for life, protecting you from being infected again. IgM antibodies appear soon after you have been infected and then disappear, usually over a period of a few months.

If you have IgG antibodies, but no IgM antibodies, this indicates you have been infected at some time in the past, but you have not had a recent infection.

If you have IgM antbodies, this indicates you have been infected relatively recently (probably within the past year), although it still doesn't determine for certain whether any symptoms you have are caused by toxoplasmosis.

You may need more blood tests to check the levels of antibodies in your blood a second time to help determine whether the infection is currently active. For example, if the levels of antibodies are:

  • increasing – suggests a new active infection
  • still the same – suggests an old infection and you're now immune
  • decreasing – suggests a recent infection that is no longer active

Diagnosis during pregnancy

If you're pregnant and blood tests confirm you have a recent toxoplasmosis infection, you may need a further test to determine whether your unborn baby is also infected.

Amniocentesis is the most effective and commonly used test, and can be carried out any time from 15 weeks of pregnancy onwards. It involves inserting a fine needle through the mother's tummy to collect a sample of amniotic fluid (the fluid surrounding the foetus in the womb). The sample will be tested for toxoplasmosis.

Amniocentesis carries around a 1% risk of causing a miscarriage. The procedure usually takes 10 to 30 minutes, and you may find it slightly uncomfortable.

The procedure can confirm whether your baby has the infection, but it can't determine whether the infection has caused any serious damage.

Diagnosis after birth

After your baby is born, he or she will be examined for any signs of damage from the infection and treatment may continue. They may also have a blood test to help determine if they have the infection.

Treatment for the infection will continue for up to a year after birth, and regular blood tests may be carried out during this time.

Diagnosis in people with a weak immune system

If you have a weakened immune system, routine blood tests for antibodies can produce a false-negative result. This is because it's possible your immune system won't produce antibodies to fight the infection.

Your doctor may also request the following tests to see if you have physical signs of the infection in your brain:

  • computerised tomography (CT) scan – where several X-rays are taken at slightly different angles and assembled by a computer to produce a clear, three-dimensional image of the inside of your brain
  • magnetic resonance imaging (MRI) scan – where a strong magnetic field and radio waves are used to produce detailed images of the inside of your brain

You may also have an eye examination to check for signs of damage that could be caused by the infection.

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