In order to decrease caries risk, it is encouraged for people to have less than 10,000 Streptococcus mutans per ml of saliva in their mouth.
Class 0-3 is referring to how many Streptococcus mutans reside in the mouth with class 0-1 acting as best case with good oral hygiene, while class 3 acts as the worst case.
class 0-1 <100,000/ml
class 2 >100,000/ml <1,000,000/ml
class 3 >1,000,000 /ml