What does the antistreptolysin O (ASO) test measure in the patient's serum?

The antistreptolysin O (ASO) test specifically measures the presence of antibodies against streptolysin O, a toxin produced by the Group A Streptococcus bacteria. When a patient is infected with this type of bacteria, the immune system responds by producing antibodies to neutralize the streptolysin O toxin. The ASO test is commonly used to assess a recent streptococcal infection, particularly in cases of rheumatic fever or post-streptococcal glomerulonephritis.

By measuring the levels of these antibodies in the serum, healthcare providers can evaluate the immune response to a streptococcal infection. Increased levels of ASO can indicate a recent infection, making it a useful diagnostic tool.

The other options listed do not relate to the ASO test. One option refers to the measurement of an antigen, which would not be appropriate for this test as it quantifies antibodies instead. Additionally, a complete blood count and serum electrolytes are distinct laboratory tests used for different clinical purposes, unrelated to the detection of streptococcal infections.