What Are Serology ASO Quantitative Tests?

Streptococcus pyogenes, also known as Group A streptococcus, is a bacterium that causes infections, such as strep throat, scarlet fever, impetigo and cellulitis. Serious complications, specifically rheumatic fever and a type of kidney disease called glomerulonephritis, can develop when streptococcal infections go untreated or are misdiagnosed. Antistreptolysin O (ASO) blood tests help health-care professionals detect recent Group A streptococcal infections.

Background

• The immune system produces substances called antibodies that help protect the body from infection. Antibodies are proteins found in the blood that recognize foreign substances, called antigens, on the surfaces of bacteria, viruses and other cells. By attaching themselves to antigens, antibodies neutralize invaders and tag them for destruction by other parts of the immune system. A specific antibody is produced for each antigen the body encounters. The presence of antibodies in the blood indicates previous exposure to specific antigens. Qualitative tests detect the presence of antibodies to a particular antigen. Quantitative tests measure their amount.

Streptolysin O

• Streptolysin O is one of many antigens produced by S. pyogenes. It is inactivated by atmospheric oxygen, hence the letter "O" in its name. The toxic protein destroys cells within the body by disrupting the membranes that keep those cells intact. The immune system responds by producing anti-streptolysin O antibodies to neutralize the toxin's destructive action. Quantitative ASO tests measure anti-streptolysin O antibody levels in the blood.

Methods

• Scientists measure ASO antibodies by mixing commercially prepared streptomycin O-coated particles with the serum. If ASO antibodies are present, they bind to the particles and form clumps that will scatter light and cloud the specimen. Antibody quantity is determined by measuring either the intensity of light scattering, a technique known as nephelometry, or the degree of cloudiness, a method known as turbidimetry. Both methods determine antibody amounts by comparing patient specimens to standards of known antibody concentration.

Clinical Significance

• S. pyogenes infections are problematic, because they can be followed by serious complications, or sequelae. Latency, a period of time following the infection during which the patient is asymptomatic, precedes the complications. Rheumatic fever is a possible sequela of strep throat and can result in permanent heart valve damage. The latency period for rheumatic fever is approximately 20 days. Glomerulonephritis can follow throat or skin infections after a 10-day latency period. The condition causes kidney damage when antigen-antibody complexes accumulate in the blood-filtering vessels of the kidneys, called the glomeruli. When a patient exhibits symptoms of rheumatic fever or glomerulonephritis following a sore throat or skin infection, ASO tests confirm previous streptococcal infection.