MICROBIOLOGY NOTES       ASO TEST ASO test is used to detect antibodies against Streptolysin O in serum of patients, as a retrospective serodiagnosis of acute rheumatic fever. Persons, who had suffered from pharyngeal infection due to Streptococcus pyogenes, develop antibodies against streptolysin O.  By the time patients develop acute rheumatic fever, bacteria are no longer detectable in the pharynx. Hence a retrospective diagnosis of pharyngeal infection due to Streptococcus pyogenes is made by this test. Principle of test: The conventional test is neutralization test. Patient serum is doubly diluted and to each tube standard purified streptolysin O is added. If the patient serum contains antibodies to streptolysin O, it would neutralize the added streptolysin O. Subsequently, to each tube sheep erythrocytes are added. If the streptolysin O were neutralized, there would not be any hemolysis. The highest dilution of serum that has inhibited the hemolysis is taken as the titre. Results were expressed in Todd units. A titre of 500 Todd units or more indicated recent or continuing streptococcal infection. A rise in titre in an interval of one week is more meaningful than single test. This test is cumbersome and difficult to setup, hence is no longer routinely performed. It has been replaced by easier latex agglutination test. Latex agglutination test:   This is a passive agglutination test where the reagent consists of latex particles coated with purified streptolysin O antigen. One drop of patient serum is mixed with one drop of reagent and rocked. Clumping of latex particles is taken as positive. The test can be performed qualitatively or quantitatively. Interpretation: A titre more than 200 IU is considered significant. In acute glomerulonephritis, ASO tests are not reliable. In such situations Anti-Dnase B (streptodornase), anti-hyaluronidase may be performed.