The Antistreptolysin O (ASO) Titer test measures antibodies produced in response to a toxin released by Group A Streptococcus (GAS) bacteria. GAS causes common infections like strep throat and impetigo. The immune system creates Antistreptolysin O antibodies to fight the specific toxin, streptolysin O. The ASO titer test detects evidence of a prior or recent GAS infection, not an active one. A high result indicates the body has mounted a strong immune response, which may lead to serious health complications.
Interpreting an Elevated ASO Titer
A high ASO titer signifies the body has produced a significant amount of antibodies, confirming a recent Group A Streptococcus infection. This indicates a streptococcal infection occurred, even if the individual did not experience symptoms or the infection was unrecognized. ASO levels typically begin to rise about one week after the infection starts, peaking between three and six weeks later.
A high titer confirms a past exposure, not necessarily an active infection requiring immediate antibiotics. The antibodies can remain detectable for several months after the bacteria are gone. For adults, a titer above 200 International Units per milliliter (IU/mL) is generally considered elevated, though this can vary by age and laboratory.
How the Immune Response Triggers Complications
The connection between a high ASO titer and serious health issues stems from molecular mimicry. When the immune system creates antibodies to attack Group A Streptococcus, it targets bacterial proteins. Some of these bacterial proteins, such as the M protein, share a structure remarkably similar to proteins found in human tissues.
Due to this structural similarity, the antibodies mistakenly recognize and attack the body’s own cells. This leads to an autoimmune response where the body’s defenses turn against its own tissues, particularly connective tissue in the heart, joints, and kidneys. This cross-reaction links a strep infection to complex, non-infectious conditions like rheumatic fever and post-streptococcal glomerulonephritis.
Focus on Rheumatic Fever and Heart Damage
Acute Rheumatic Fever (ARF) is one of the most serious consequences of a streptococcal infection, triggered by the autoimmune response. ARF typically appears two to six weeks after a strep throat infection. Symptoms include fever, painful swelling in multiple joints, and small, painless nodules under the skin.
The most concerning manifestation is carditis, which is inflammation of the heart tissue. While other ARF symptoms often resolve completely, heart damage can persist, leading to chronic Rheumatic Heart Disease (RHD). RHD primarily affects the heart valves, such as the mitral and aortic valves. The chronic inflammation and scarring prevent the valves from opening and closing properly.
This valve damage can lead to heart failure, sometimes months or years after the initial ARF episode. For patients with a high ASO titer and suspected ARF, monitoring is necessary to detect progressive heart damage. Recurrent episodes of ARF due to subsequent strep infections can cause cumulative and more severe damage, emphasizing the importance of prevention and long-term care.
Focus on Post-Streptococcal Kidney Damage
A high ASO titer can also be associated with Post-streptococcal Glomerulonephritis (PSGN). This complication results from the immune system’s response affecting the glomeruli, the small filtering units within the kidney. PSGN typically develops about ten days after a strep throat or scarlet fever infection, or three weeks after a skin infection like impetigo.
Glomeruli inflammation affects the kidney’s ability to filter waste and excess fluid, leading to common symptoms. Patients often notice dark, reddish-brown urine due to the presence of blood. Swelling (edema) is also frequent, particularly around the eyes, face, hands, and feet. High blood pressure and decreased urine output can also occur.
PSGN is a complication of the immune reaction elsewhere in the body, not an infection of the kidney itself. The prognosis for PSGN is generally good, especially in children, with over 90% making a full recovery within a few weeks to months. This condition is less likely to result in long-term, chronic organ damage compared to the heart damage seen in Rheumatic Heart Disease.