Antistreptolysin O titer

From Biology-Online Dictionary | Biology-Online Dictionary



A blood test used to assist in the diagnosis of a streptococcal infection or indicate a past exposure to streptococci by measuring the antibodies produced against streptolysin O


A titer is the empirical measure of the avidity of an antibody. Thus, the antistreptolysin O titer is a measure of the plasma level of antistreptolysin O in the blood. Antistreptolysin O is an antibody produced by certain immune cells of the host as a result of an exposure to the streptolysin O. The streptolysin O is an immunogenic hemolysin produced by the streptococci bacteria, e.g. groups A, C and G streptococci. This hemolysin can lyse animal cells, particularly WBCs and RBCs. It does so by binding to the membrane cholesterol, polymerizing it. This results in the formation of a transmembrane channel (about 30 nm in diameter), which could eventuate to cell lysis.

In the presence of this toxin, the immune cells produce antibodies, such as antistreptolysin O. The level of antistreptolysin O in blood plasma is a useful indication of a streptococcal infection. It is also measured for clinical diagnosis. An increase in the anti-streptolysin O in the blood could mean an increase in the risk of rheumatism. It is because this antibody could cross-react with other bodily components, such as collagen. It could attack the cellular matrix in heart, joints, etc. Thus, it is measured in order to determine the antistreptolysin O titer. A high titre, e.g. >200 units in adults or when the value is clinically considered beyond the acceptable range for age and condition, could indicate the need for antibiotic treatment, such as penicillin.

This test may be used to assist diagnosis of scarlet fever, rheumatic fever, and post-infectious glomerulonephritis.


  • ASOT
  • ASO titer
  • ASLO titer

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