Regarding ASO titre all are seen except –
ASO can be increased in school children
May be negative in post streptococcal glomerulonephritis
ASO titre included in major criteria in jones criteria
May not be elevated in 20% cases of carditis
Evidence of antecedent group 'A' streptococcal infection 

Positive throat cultures or rapid streptococcal antigen tests for group A streptococcoci are less reliable for antecedent infection capable of producing rheumatic fever because they do not distinguish between recent infection and chronic pharyngeal carriage (as many people are carrier of this bacteria).
Antibody tests are the most reliable laboratory evidence of antecedent streptococcal infection capable of producing        g acute rheumatic fever. The onset of clinical manifestations of acute rheumatic fever coincides with the peak of the streptococcal antibody response.
The antibodies used commonly for serological tests are antistreptolysin 0 (ASO), antideoxyribonuclease (Anti-DNAse) and antihyaluronidase.
ASO titre is well standardized and therefore is most widely used test. It has following features.


Elevated in 80% of patients with acute rheumatic fever (So, 20% patients do not show elevated titre) Sensitivity is 80%.
It is elevated in 20% of normal individuals perticularly in healthy school children of elementary school age (20% false positive).
ASO titers of at least 333 Todd units in children and 250 Todd units in adults are considered elevated.
It is included in minor criteria of jones criteria (not in major criteria).


Anti DNAse B titers of 240 Todd units or greater in children and 1200 Todd units or greater in adults are considered elevated.
If three antibody tests (ASO, anti-DNAse and anti-hyaluronidase) are used simultaneously, a titer for at least one antibody test is elevated in 95% of cases —> Sensitivity of combined three antibody tests is 95%.
The streptozyme test is a simple slide agglutination test for extra-cellular streptococcal antigen absorbed to red blood cells (passive hemagglutination) test. It is almost 100% sensitive but specificity is very low and it is less standardized and less reproducible than the other antibody tests. Therefore, it should not be used as a diagnostic test for evidence of antecedent group A streptococcal infection.

About option 'b'

ASO titre may not be elevated in PSGN if PSGN develops secondary to skin infection.