test Details

Group A Streptococcal Pharyngitis, Rheumatic fever, Rheumatic heart disease, post-streptococcal glomerulonephritis.

Infections by group A Streptococci (GAS) are a significant cause of morbidity and mortality across the globe, particularly in developing countries. Group A streptococci cause pharyngitis, an upper respiratory infection, and skin infections.

An untreated infection may lead to acute rheumatic fever and develop rheumatic heart disease, glomerulonephritis, and nervous system disorders. There is a generalized inflammatory response in these conditions involving valves and layers of the heart, joints, skin, kidneys, and brain.

Glomerulonephritis is a disease of the kidneys that may cause low urine output, blood in the urine, swelling over the body, and high blood pressure. These conditions are of immense public health concern as they affect children and young adults primarily.

People living in overcrowded conditions and belonging to low socioeconomic groups have been more susceptible to these infections. The situation is worsened by the lack of awareness and delayed access to medical care.

The test for diagnosing streptococcal A-induced pharyngitis is cost-effective and straightforward. ASO test is the most commonly conducted test to diagnose group A streptococcal infections. This test measures the antibodies developed in response to the toxic enzyme released by the streptococcal A bacteria. Higher than normal levels of ASO are indicative of a recent infection.

ASO test helps in establishing the diagnosis of rheumatic fever or post-streptococcal glomerulonephritis. In addition, timely testing helps in initiating treatment and preventing progression to severe complications.

Acute pharyngitis is the most common illness of childhood, often causing recurrent episodes of throat infections. The Streptococcal group A pharyngitis contributes to 15-30% of all the episodes of pharyngitis a child suffers from in a year. All across India, many children remain undiagnosed and untreated for this infection, thereby contributing to an increase in the burden of rheumatic fever, rheumatic heart disease, and kidney disease.

The number of pharyngitis episodes per year was higher during the winter and rainy seasons. This statistic considers the populace in crowded places and younger age groups. Remote and hilly areas like Himachal Pradesh also showed a high incidence of rheumatic fever, possibly due to delay in the diagnosis and treatment, in addition to other contributory factors.

ASO test helps in detecting preceding group A streptococcal infections and establishing the diagnosis of rheumatic fever. ASO test is a cheap test, easy to perform and can be conducted even in rural settings.

Other names of ASO

• Streptolysin O titer
• ASO AB (Anti Streptolysin O Antibodies)

What does the ASO test detect/measure, and who is this prescribed for?

The ASO test measures the levels of anti-streptolysin O antibodies in the blood.

The doctor ordered this test when the symptoms suggestive of rheumatic fever develop following the recovery from a preceding throat or skin infection.

The ASO test does not measure the extent of damage after streptococcal A pharyngitis infection. However, post-infection the heart, kidneys, or nervous system could be affected.

Acute rheumatic fever presents as a group of symptoms involving joints, heart, skin, and brain. It develops as a sequel to streptococcal group A pharyngitis. It is considered that streptococci trigger an autoimmune response leading to rheumatic fever. It usually starts in the age group of 5 to 15 years.

Rheumatic fever presents with a sudden onset of symptoms. These symptoms depend upon the organs that are involved. The following symptoms indicate rheumatic fever:

• Fever, malaise, loss of appetite
• Pain and swelling in the joints
• Skin rash
• Painful hard swellings under the skin
• Sudden jerky movements
• Acute rheumatic fever may lead to damage to layers and valves of the heart. This condition is called rheumatic heart disease.

Streptococcal group A pharyngitis may also lead to kidney disease known as glomerulonephritis. Symptoms suggestive of it are:

• Decreased urinary output
• Blood in the urine
• Swelling over feet or body
• High blood pressure

ASO test is not required to be repeated once the diagnosis is established and treatment is started. Antibiotics and corticosteroids may decrease ASO antibody levels.

• In about 20% of Acute rheumatic fever patients, ASO may be below average. Other tests are indicated for confirming the condition.
• Levels of ASO may be affected by corticosteroids and antibiotics. It is essential to tell the doctor about the medicines that have been taken.
• The standard range value may vary with age, season, geographical area, socioeconomic status, and the population where the test is conducted.
• The ASO test value above the accepted standard in normal individuals shows uncomplicated past infection with group A streptococcal bacteria.
• Levels of ASO antibodies start rising 1-4 weeks after a person gets infected and reach peak level in 3-5 weeks before tapering off gradually as the recovery happens.
• Elevated levels of ASO antibodies are seen in about 80% of patients with rheumatic fever and 95% of patients with post-streptococcal kidney disease.
• ASO results may be below the cutoff limit in about 20% of persons presenting with symptoms suggestive of rheumatic fever or glomerulonephritis. In such a situation, the test can be repeated with a convalescent sample and run simultaneously with the acute phase sample to detect the rising titers.
• Antistreptolysin O antibodies are the most commonly tested antibodies employed to detect streptococcal group A infection. Other antibodies like anti-DNase B are needed to be tested if ASO results are not conclusive.

The results of the Anti-Streptolysin O test are based on the analysis of a blood sample.

Test Preparation

ASO test is a blood test requiring the usual blood sample collection method. Blood is drawn from the vein in the arm after cleaning the site and tying a tourniquet. Fasting is not required. Sample collection takes about five minutes.

Test inclusions: What parameters are included?

ASO test includes measuring the levels of antibodies to diagnose rheumatic fever and a preceding episode of throat infection. The immune system develops these antibodies to protect the body from the toxic effects of the streptolysin O enzyme.

This enzyme is released in the blood by the invading streptococcal A bacteria. In addition, several other antibodies are produced, out of which ASO and anti-DNase B are the most commonly tested antibodies.  

Since ASO levels become detectable 1-4 weeks after an acute infection, they cannot diagnose the disease in the critical stage. ASO is a cost-effective test and acts as a potent tool in the healthcare provider's hands to diagnose rheumatic fever and mitigate the severe sequelae of complications associated with it.

How frequently should you take this test?

The ASO test is done when the symptoms suggest rheumatic fever or kidney disease with a history of throat infection in the recent past or repeated episodes of throat infections.

The test is repeated two weeks after the first test. This repetition is done to check the levels of antibodies. The antibodies could be rising, falling, or remaining the same as in the first test.