Toxoplasmosis - Diagnosing toxoplasmosis

If you're infected with the Toxoplasma gondii (T. gondii) parasite, your immune system will start producing antibodies to fight it.

Antibodies are proteins that neutralise or destroy invaders such as bacteria or parasites. If toxoplasmosis is suspected, you'll have a blood test to check for these antibodies.

Negative result

Early testing can sometimes produce a negative result when a person is actually infected. Known as a false-negative, it occurs when the body hasn't had time to start producing antibodies to the parasite, which usually takes up to 23 days after the initial infection. However, a person who has developed symptoms of toxoplasmosis will nearly always have a positive blood test.

If you're symptom-free but concerned, your GP may recommend another blood test two to three weeks later. If the result is still negative, it's unlikely you have the toxoplasmosis infection.

Positive result

A positive result doesn't necessarily mean you have an active toxoplasmosis infection. After having toxoplasmosis, the antibodies will remain in your blood for the rest of your life, protecting you from being infected again by the parasite. A positive result usually indicates you've been infected at some point in the past.

You will need more blood tests to find out whether you have a new infection. Checking the levels of antibodies in your blood a second time can help determine when the infection occurred. For example, if the levels of antibodies are:

• increasing – suggests a new active infection (perhaps a few weeks old)
• still the same – suggests an old infection and that you are now immune
• decreasing – suggests a recent infection that is no longer active (perhaps a few months old)

Specialised testing is important if you're pregnant or have a weakened immune system.

Pregnant women

If you are pregnant and tests confirm you have a recent toxoplasmosis infection, you'll 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 abdomen 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.

Congenital toxoplasmosis

Amniocentesis can confirm whether your baby has congenital toxoplasmosis. However, it can't determine whether the infection has caused any damage to your unborn baby or, if it has, how much.

If your unborn baby has congenital toxoplasmosis, treatment with medication will be started as soon as possible. After your baby is born, he or she will be examined for any signs of damage from the infection and treatment may continue.

If necessary, your baby will continue to have blood tests for up to a year, or possibly longer, until the test results for toxoplasmosis antibodies are negative. When results indicate there are no antibodies, your baby has been shown to be infection free.

Immune deficiencies

If you have a weakened immune system (because of chemotherapy, for example), 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 request the following tests:

• 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

The test results will reveal if you have any lesions (wounds) in your brain, which occur in some cases of toxoplasmosis.

New research

New research is looking at other ways of diagnosing toxoplasmosis – for example, by identifying the DNA of the T. gondii parasite in samples of cerebrospinal fluid (the fluid that surrounds your brain and spinal cord).

It's hoped that this may be a quicker and more reliable method of diagnosis, and will help confirm whether the damage to the brain has been caused by toxoplasmosis rather than by another condition.