A patient was referred to me in the paediatric cardiology clinic because of a risk that he may have had missed Kawasaki’s disease a couple of weeks earlier and was therefore at risk of having coronary artery aneurysms. The referring doctor had carried out an antistreptolysin O titer (ASOT) in case the symptoms of a red, sore mouth, rash and later peeling fingers had been secondary to a streptococcal infection rather than KD. The result came back as 400units/ml (normal is < 200units/ml). The child was very well when I saw him and had a normal echocardiogram. What should I do with the elevated ASOT result?
I needed a quick text box as a gap filler for the April edition of the Paediatric Pearls newsletter and thought ASOT results would be a suitable topic but, when I sat down to write it, I opened up a can of worms. No one really knows what to do with high ASOTs in a well child. In fact, authors can’t even agree on whether 400 is elevated in a young person.
My reading list is at the foot of this article. Salient points from these sources are summarised below.
- The ASOT is ordered primarily to determine whether a previous group A Streptococcus infection has caused a poststreptococcal complication, such as rheumatic fever or glomerulonephritis. So the start point should be on-going clinical symptoms of strep infection or the effect of a recent infection. If used in this way, it can be a useful pointer to a causative organism and will guide management. Rheumatic fever is treated with long term antibiotics. The ASO test does not predict whether complications will occur following a strep infection, nor does it predict the type or severity of the disease. If symptoms of rheumatic fever or glomerulonephritis are present, an elevated ASO level may be used to help confirm the diagnosis.
- ASO antibodies are produced a week to a month after an initial strep infection. The amount of ASO antibody (titer) peaks at 3 to 5 weeks after the illness and then tapers off but may remain detectable for several months after the strep infection has resolved.
- A negative ASO or ASO that is present at very low titers means the person tested most likely has not had a recent strep infection. This is especially true if a sample taken 10 to 14 days later is also negative (low titer of antibody) and if an anti-DNase B test is also negative (low titer of antibody). A small percentage of people with a complication related to a strep infection will not have an elevated ASO. This is especially true with glomerulonephritis that may develop after a skin strep infection.
- An elevated titer of antibody (positive ASO) or an ASO titer that is rising means that it is likely that the person tested has had a recent strep infection. ASO titers that are initially high and then decline suggest that an infection has occurred and may be resolving.
My conclusion at the end of reading about ASOT and the management of streptococcal infections and complications is that I should only do the ASOT if the child is symptomatic. If I think they have rheumatic fever, I should treat with antibiotics for a long time (up to 10 years in some cases). If they do not satisfy the Jones criteria for rheumatic fever and indeed are well now, I do not need to blindly treat an elevated ASOT but it may be prudent to repeat the test a couple of weeks later to ensure it is dropping.
Very good summary article on rheumatic fever: https://patient.info/doctor/rheumatic-fever-pro
Why treat sore throats at all? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1949249/
Cochrane on short term antibiotics: https://www.ncbi.nlm.nih.gov/pubmed/22895944