Antistreptolysin O (ASO) is an antibody produced by the human immune system. These antibodies develop in response to specific bacterial components, indicating a recent bacterial infection.
Understanding Antistreptolysin O
The immune system generates antistreptolysin O antibodies when it encounters streptolysin O, a toxic enzyme secreted by Group A Streptococcus (GAS) bacteria. GAS causes various infections, including strep throat and scarlet fever. The presence of ASO antibodies in the bloodstream indicates a past or recent GAS infection.
Reasons for ASO Testing
Healthcare providers often order an ASO test to determine if a recent Group A Streptococcus infection has occurred. This is particularly relevant when a post-streptococcal complication is suspected, especially if the initial infection was mild or “silent.” Symptoms prompting an ASO test include unexplained joint pain, swelling, certain skin rashes, or signs of kidney or heart problems. These manifestations could indicate a complication arising from a prior GAS infection. The ASO test does not diagnose an active strep throat infection; instead, it helps confirm a previous infection linked to current symptoms.
Deciphering ASO Test Results
An ASO “titer” measures the amount of antistreptolysin O antibodies present in the blood. A normal or low titer generally suggests that there has been no recent strep infection, or that a past infection has fully resolved.
An elevated titer indicates a recent Group A Streptococcus infection, typically within the preceding weeks to months. A significant increase in ASO levels between two tests, taken a few weeks apart, strongly suggests a recent or ongoing strep infection. Conversely, a falling titer implies that the infection is resolving and antibody levels are declining.
Antistreptolysin O levels can remain elevated for several months after an infection, even after the bacteria are no longer present. A negative ASO test does not completely rule out a recent strep infection, as some individuals may not produce a strong antibody response, or the specific strep strain might not produce streptolysin O.
Diseases Linked to Elevated ASO
Elevated antistreptolysin O titers are a significant indicator in the diagnosis of specific post-streptococcal conditions.
Acute Rheumatic Fever (ARF)
Acute Rheumatic Fever (ARF) is a serious inflammatory disease that can develop after an untreated strep throat infection. This condition can affect various parts of the body, including the heart, joints, brain, and skin. An elevated ASO titer, when considered alongside the patient’s clinical symptoms, provides strong evidence to support an ARF diagnosis.
Post-streptococcal Glomerulonephritis (PSGN)
Post-streptococcal Glomerulonephritis (PSGN) is another kidney disorder that can manifest following a strep infection, which can originate from either throat or skin infections. Elevated ASO levels are used in the diagnosis of PSGN, often in conjunction with other laboratory tests that assess kidney function.
The ASO test result represents one piece of the diagnostic puzzle and must be interpreted in conjunction with a patient’s symptoms, medical history, and other relevant laboratory findings.