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
1. Is the difference between the acute and convalescent titers significant?

Answer 1

1. The ASO test is used to detect group A streptococcal infections, although it detects group C and group D infections as well. It detects the production of ASO antibodies produced in response to the streptolysin antigen expressed by these streptococci. SSO antibodies are detected by latex agglutination assay.

The antibody titer is the reciprocal of the highest dilution of the patient serum that was showing positive hemolysis in the latex hemagglutination assay. ASO test is positive only when the antibody titer is between 200-3500 mIU/mL. The acute and convalescent samples had 200 mIU/mL and 800 mIU/mL of antibodies respectively. Thus, there is streptococcal infection in both stages (acute and convalescence). In the acute sample, the positive dilution was 1: 1 while in convalescent sample, the highest positive dilution was 1:4. Hence, the rise in titer was four fold (4/1=4). An increase in patients ASO titer by two doubling dilutions or more or increase by fourfold rise in antibody titer is said to be significant change. As there was an increase by two doubling dilution or four fold increase in convalescent sample as compared to acute sample, there is a significant difference in the ASO titers of acute and convalescent sample. This significant difference indicates that the infection is recent and has not resolved.