Antibody Strep Infection Testing for PANDAS
Dealing with a condition like PANDAS (pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections) is stressful enough for a parent, without having to decipher the technical terms involved in laboratory testing.
One term that comes up relates to testing for the presence of a strep infection. It’s called an anti-streptococcal antibody titer. The National Institutes of Health addresses common questions on this titer, or level of antibodies in the bloodstream.
The anti-streptococcal antibody titer determines whether there is immunologic evidence of a previous strep infection. Two different strep tests are commercially available: the antistrepolysin O (ASO) titer, which rises 3-6 weeks after a strep infection, and the antistreptococcal DNAase B (AntiDNAse-B) titer, which rises 6-8 weeks after a strep infection.
What does an elevated anti-streptococcal antibody titer mean? Is this bad for my child?
An elevated anti-strep titer (such as ASO or AntiDNAse-B) means the child has had a strep infection sometime within the past few months, and his body created antibodies to fight the strep bacteria. Some children create lots of antibodies and have very high titers (up to 2,000), while others have more modest elevations. The height of the titer elevation doesn’t matter. Further, elevated titers are not a bad thing. They are measuring a normal, healthy response – the production of antibodies to fight off an infection. The antibodies stay in the body for some time after the infection is gone, but the amount of time that the antibodies persist varies greatly between different individuals. Some children have “positive” antibody titers for many months after a single infection.
When is a strep titer considered to be abnormal, or “elevated”?
The lab at NIH considers strep titers between 0-400 to be normal. Other labs set the upper limit at 150 or 200. Since each lab measures titers in different ways, it is important to know the range used by the laboratory where the test was done – just ask where they draw the line between negative or positive titers.
It is important to note that some grade-school aged children have chronically “elevated” titers. These may actually be in the normal range for that child, as there is a lot of individual variability in titer values. Because of this variability, doctors will often draw a titer when the child is sick, or shortly thereafter, and then draw another titer several weeks later to see if the titer is “rising” – if so, this is strong evidence that the illness was due to strep. (Of course, a less expensive way to make this determination is to take a throat culture at the time that the child is ill.)
Should an elevated strep titer be treated with antibiotics?
No. Elevated titers indicate that a patient has had a past strep exposure but the titers can not tell you precisely when the strep infection occurred. Children may have “positive” titers for many months after one infection. Since these elevated titers are merely a marker of a prior infection and not proof of an ongoing infection it is not appropriate to give antibiotics for elevated titers. Antibiotics are recommended only when a child has a positive rapid strep test or positive strep throat culture.
Reprinted with permission from the National Institutes of Health
