What is ASO?
Anti Streptolysin O (ASO) titer is a method to detect bacterial infection in the body caused by Streptococcus bacteria. This test measures the amount of antibodies produced against streptolysin O, a substance produced by group A Streptococcus bacteria. It is recommended in case of symptoms such as fever, joint pain, rashes and shortness of breath.
Early diagnosis is crucial as lack of symptoms or late diagnosis may lead to some complications such as rheumatic fever, glomerulonephritis (kidney disease) and bacterial endocarditis (heart inflammation).
A negative test during the initial weeks of infection does not always rule out the possibility of a streptococcal infection. If the symptoms persist, it is advisable to get tested again after 2 weeks.
Why is ASO done?
The ASO Titer Test is performed:
- To help determine if conditions like rheumatic fever or glomerulonephritis have occurred as complications of a Streptococcus infection
- To determine the presence of a Streptococcus infection in case the patient has received antibiotics
What does ASO Measure?
The ASO Titer Test detects and measures the level of ASO in blood to determine the presence of an infection by Group A Streptococcus bacteria. Streptococcus pyogenes or Group A Streptococcus is a bacteria which causes a number of infections like throat infections, skin infections like pyoderma, etc. These infections are easily recognizable and treated with the help of antibiotics. However, if the Streptococcus infection is left untreated or is treated improperly, it may lead to complications like rheumatic fever, glomerulonephritis, scarlet fever, heart and kidney damage, joint inflammation, swelling, etc.
Symptoms caused by an infection of Group A Streptococcus bacteria are caused mainly due to a toxin called Streptolysin O which is secreted by these bacteria. The body produces antibodies against Streptolysin O called Antistreptolysin O (ASO). Production of ASO antibodies by the body can begin anytime from a week to a month after an infection. Levels peak after about 3 to 5 weeks after the condition develops, after which they slowly taper off as the condition resolves. Increase in ASO levels is greater in case of throat infections than in skin infections.
Interpreting ASO results
Normal ASO level:
Children (up to 12 years of age): Below 150 IU/ml
Adults (above 12 years of age): Below 200 IU/ml
Higher than normal ASO levels indicate an infection of Streptococcus bacteria.
Low ASO titer indicates no recent infection. This is confirmed if the low levels persist in the subsequent test performed after 10 to 15 days. Other tests like Anti-Dnase B Test can also be performed.
Higher than normal ASO titer levels and continuously rising ASO titer in subsequent tests indicates a recent and ongoing infection.
High ASO levels which decline subsequently indicate a Streptococcus infection which is resolving or being cured.
A positive result in the ASO test (Higher than normal ASO levels) confirms a past Streptococcal infection. A positive result can thus help in the diagnosis of conditions like Poststreptococcal Glomerulonephritis (PSGN), Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcus (PANDAS), and rheumatic fever.
Note: ASO Test results may be negative (within normal range) in upto 20% of the patients suffering from Acute Rheumatic Fever (ARF). Other tests may be performed to confirm the condition in these cases.
Healths First, Lal Pathlab, Medcis Pathlab
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