True statement about Antistreptolysin ‘O’ titre is –
**Core Concept**
Antistreptolysin 'O' (ASO) titre is a measure of the body's immune response to the streptolysin O toxin produced by Group A beta-hemolytic streptococci (GABHS). It is used as a diagnostic tool to confirm a recent streptococcal infection, particularly in cases of acute rheumatic fever.
**Why the Correct Answer is Right**
The ASO titre is elevated in individuals who have been exposed to the streptolysin O toxin, indicating an immune response. This occurs when the body produces antibodies against the toxin, which are then measured in the blood. An elevated ASO titre is often seen in patients with acute rheumatic fever, which is a complication of streptococcal infections. The ASO titre typically peaks 2-3 weeks after the onset of symptoms and returns to normal within 3-6 months.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the ASO titre measures the immune response to streptolysin O toxin, not the toxin itself.
**Option B:** This option is incorrect because while the ASO titre can be used to diagnose rheumatic fever, it is not the only diagnostic criterion and should be used in conjunction with other clinical and laboratory findings.
**Option C:** This option is incorrect because the ASO titre is not a direct measure of the severity of the infection, but rather an indicator of the body's immune response.
**Clinical Pearl / High-Yield Fact**
A high ASO titre can also be seen in patients with post-streptococcal glomerulonephritis, another complication of streptococcal infections. This is an important consideration in patients presenting with renal symptoms following a streptococcal infection.
**Correct Answer:** D.