Comparison of latex and haemolysin tests for determination of anti-streptolysin O (ASO) antibodies

Group A streptococcal infections and tic disorders in an Italian pediatric population

BACKGROUND

The relationship between childhood tic disorders and group A streptococcal (GAS) infections has been recently investigated by several research groups, but no systematic evaluation of laboratory indicators of GAS infections has been provided.

OBJECTIVE

The aim of our study was to seek clinical and laboratory evidence of GAS infections in a large population of children affected with tic disorders.

STUDY DESIGN

This investigation was a case-control study: 150 consecutive children presenting with tics were investigated for clinical and laboratory signs of streptococcal infections on the occasion of their first neuropsychiatric consultation. To compare the rate of exposition to GAS in a similar population, a control group of 150 children without tic disorders was examined during the same period.

RESULTS

In children with tics, mean antistreptolysin O (ASO) titer (434 +/- 338 IU) was significantly higher compared with that of control subjects (155 +/- 126 IU); 38% of the children with tics compared with 2% of the control subjects (P <.001) had ASO titers > or =500 IU. Twenty-six children with tics (17%) had throat cultures positive for GAS, but in all cases, only few colonies per plate were isolated. Among the strains isolated, no prevalent T pattern or M type was observed. In children with tics a positive correlation between ASO titers and severity of tic disorder (measured by the Yale Global Tic Severity Scale) was found.

CONCLUSION

Our results suggest that children with tic disorder could be a unique population in which GAS infection, or at least the exposure to streptococcal antigens, is correlated to the neurobehavioral disorder. Moreover, our data indicate a relationship between the severity of tic disorder and the magnitude of the serologic response to GAS antigens.

Improvement of the indirect hemagglutination test for the detection of antibodies to Streptococcus pyogenes

An indirect hemagglutination test for a seroepidemiological survey of Streptococcus pyogenes infection was standardized. This is an improved modification of the indirect hemagglutination test which utilizes an unstable reagent prepared with fresh blood cells. Two types of bacterial antigens represented by extracellular products and purified streptolysin O were assayed, but only the former antigen gave good results. Pretreatment of the bacterial antigen with 0.15 M NaOH and neutralization to pH 5.5, as well as postfixation of sensitized red cells with 0.1% glutaraldehyde at 56 degrees C for 30 min were found to be essential to give long stability to the reagent in liquid suspension, at least 9 months at 4 degrees C. A total of 564 serum samples with high, moderate and low anti-streptolysin O antibodies as determined by the neutralization assay were studied by the indirect hemagglutination test using the new reagent. The sensitivity, specificity, efficiency, positive predictive value and negative predictive value of the test in relation to the neutralization assay were 0.950, 0.975, 0.963, 0.973, and 0.955, respectively. The kappa agreement index between the two techniques was high (0.926) and ranked as "almost perfect". Antibody levels detected by both techniques also presented a high positive correlation (rs = 0.726). Five reagent batches successively produced proved to be reproducible. Thus, the improved indirect hemagglutination test seems to be useful for public health laboratories.

Evaluation of a new latex agglutination test for detection of streptolysin O antibodies

Acute- and convalescent-phase serum specimens were collected from 50 patients with group A streptococcal pharyngitis. The anti-streptolysin O (ASO) titer for each serum specimen was determined by using both the standard neutralization assay and the latex agglutination (LA) test (Rheumagen ASO; Biokit Inc., New Britain, Conn.). When the ASO titers derived by the two methods were compared, the correlation coefficient was 0.93. When the ability of the LA test to demonstrate a significant ASO titer rise (greater than or equal to 2 dilutions) was compared with that of the standard neutralization assay, the LA test had a sensitivity of 91%, a specificity of 86%, a positive predictive value of 83%, and a negative predictive value of 92%. Triplicate LA test determinations were performed on a subset of 31 serum specimens, and for 29 (94%), the repeated ASO titers were all within 1 dilution of each other; the width of the 95% confidence interval for the triplicate measurements of each serum specimen was +/- 32.8 IU. We found the Rheumagen ASO to be a simple, rapid LA procedure for measuring ASO titers that produces results that are highly reproducible, show little lot-to-lot variability, and are comparable to the ASO titers obtained with the standard neutralization assay.

A rapid turbidimetric immunoassay for serum antistreptolysin-O

We designed a rapid, quantitative turbidimetric immunoassay for serum antistreptolysin-O (ASO), based on the immunoprecipitating velocity with purified streptolysin-O (SLO), which can be measured turbidimetrically at 340 nm. The assay system involves polyoxyethylene mono-p-nonylphenyl ether to accelerate and enhance the immunoprecipitation reaction. No antibody excess phenomenon is observed, up to 1,700 U/ml. Grossly icteric sera can be assayed. Patient sera gave acceptable results. Free cholesterol offers no interference. The standard curve is linear for values up to 800 U/ml. Results measured by the proposed method and those by the conventional microtitration method correlated well. Analytical recovery averaged 90.1% (80.3%-102.5%). Within-run and day-to-day CVs ranged from 2.1% to 3.5% and from 2.5% to 6.9%, respectively. Working SLO is stable for at least 15 days at 4 degrees C. The proposed method has proven to be superior to other available methods for measuring serum ASO.