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A longitudinal study of caries onset in initially caries-free children and baseline salivary mutans streptococci levels: a Kaplan-Meier survival analysis.

OBJECTIVES
To apply survival analysis to a longitudinal study of the relationship between salivary mutans streptococci (MS) levels at baseline in initially caries-free children and caries onset in deciduous, mixed, and permanent dentition.


METHODS
The Kaplan-Meier survival analysis method was used to compare survival times to caries onset for initially caries-free children with low levels of MS at baseline with survival times to caries onset for initially caries-free children with high levels of MS at baseline.


RESULTS
Data from a 6-year longitudinal study of caries risk in initially caries-free children in Rochester and the Finger Lakes Region of western New York were utilized for this study. Of 464 children analyzed, 327 had a low level of MS and 137 had a high level of MS at baseline. Survival analyses showed that children with a low level of MS at baseline remained caries-free for a longer period than children with a high level of MS at baseline. Statistically significant relationships [hazard ratios (HR)] with onset of caries in deciduous, mixed and permanent teeth were found with high and low levels of salivary MS.


CONCLUSIONS
Based on our analysis, we concluded that children who were caries-free at baseline and who had high salivary MS levels at baseline would be at greater risk, i.e. more susceptible to caries onset, at any given time than caries-free children who had low salivary MS levels at baseline. Survival functions for deciduous, mixed and permanent dentitions with their 95% confidence limits have been calculated. Survival analysis for the exploration of longitudinal caries studies has an advantage over traditional statistical methods, as it takes into account censored observations and incorporates the concept of risk over time. Hence, survival analysis is well suited for studying transitions from one health state to another, in this case, from a caries-free state to a caries-active state.