La streptolysine O is a substance produced by streptococcal bacteria (group A) when they infect the body.

The presence of streptolysin triggers an immune response and the production of anti-streptolysin antibodies, which aim to neutralize the substance.

These antibodies are called antistreptolysins O (ASLO). 

This test can detect antistreptolysin O antibodies in the blood, which testify to the presence of a streptococcal infection (eg angina or pharyngitis, rheumatic fever).

The test is not routinely prescribed to detect streptococcal pharyngitis (a rapid test on a throat smear is used for this). It is reserved for other cases of suspected streptococcal infections, such as rheumatic fever or acute glomerulonephritis (kidney infection).

What results can we expect from an analysis of antistrptolysin O?

The examination is carried out by simple blood test, in a medical analysis laboratory.

There is no particular preparation. However, it may be recommended to take a second sample 2 to 4 weeks later in order to measure the evolution of the antibody level.

Normally, the level of antistreptolysin O should be less than 200 U / ml in children and 400 U / ml in adults.

If the result is negative (that is, within the norms), it means that the patient has not recently been infected with streptococcus. However, during a infection streptococcique, the marked rise in ASLO is usually not detectable until 1 to 3 weeks after infection. Therefore, it may be helpful to repeat the test if symptoms persist.

If the ASLO level is abnormally high, it is not enough to state without a doubt that there is a strep infection, but the likelihood is high. To confirm this, the dosage must show a clear increase (multiplication by four of the titre) on two samples spaced fifteen days apart.

The value of these antibodies returns to normal no later than 6 months after infection.