Question

This 9-year-old boy was taken to the emergency department with a sore throat. On examination, he had redness of the throat and slightly swollen glands. The physician assistant ordered a throat culture and blood drawn for an antistreptolysin-O antibody (ASO). An antibiotic was prescribed for a 10-day period. His mother was told to make an appointment with his pediatrician for a follow-up. At the follow-up visit 2 weeks later, the results of the laboratory test revealed a throat culture with a few colonies of β-streptococci. The qualitative ASO test result was reported as positive. The acute serum was frozen at the time of testing. The pediatrician ordered a convalescent specimen to be tested semiquantitatively in parallel with the acute specimen for an ASO titer. The results of the parallel testing of the acute and convalescent specimens revealed the following: • Acute specimen positive, 1:1 dilution/titer (IU/mL 200) • Convalescent specimen positive, 1:4 dilution/titer (IU/mL 800)

Thinking Group Discussion Questions

2. What does a rise in titer mean?

Answer 1

2. Anti-streptolysin antibody test is used to test for the presence of antibodies against group A streptococci in recent infection. ASO test determines whether the antibody levels are increasing, decreasing or remaining the same during the infection. Acute and convalescent samples are tested for ASO titers for this purpose.

There was an increase in the titer of the anti-streptolysin antibodies (ASO) during convalescence by more than two dilutions. This indicates that the patient has a recent streptococcal infection and antibody levels are rising in response to the infection. If the antibiotics have worked, then the convalescent sample should show decrease in antibodies indicating that the acute infection is resolved. However, there was increase in titer of ASO antibodies. This indicates that there is still increase ASO antibody production. Hence, the streptococcal infection has not yet resolved. The immune system is still fighting thus infection in the patient despite use of antibiotics.