Conversion of plaque-area measurements to plaque index scores - An assessment of variation and discriminatory power

Plaque areas recorded graphically or photographically provide a permanent r ecord of plaque accumulations on teeth at a moment in time. As such, these records could be re-evaluated and converted into other index scores. The pu rpose of this study was to determine the reproducibility of scoring a plaqu e index from previously recorded plaque areas and to compare such scores wi th the original scores of the same index. A randomised blind, crossover stu dy comparing 5 treatments for plaque inhibition scored by plaque area and i ndex was chosen. 2 examiners, the original scorer PRH and another, NC, 2x s cored the plaque area tooth charts according to the criteria of the plaque index system used in the original study. Standard deviations of the differe nces showed intra-examiner repeatability to be high particularly for the or iginal examiner. Inter-examiner reproducibility for the original index scor es was considered good but less than for intra-examiner repeatability. Corr elation coefficients were complimentary to the differences analysis, being very high within examiners and less high for between examiners and original and rescored index. Separation between distributions of plaque area measur ements for consecutive values of the index were particular good for scores 2 versus 3 and 3 versus 4 and less good for 1 versus 2 and 4 versus 5. Rean alysis of the study for treatment differences using rescored data revealed a similar level of significance as using the original data. Rescored index had similar discriminatory power for the study as plaque area and original plaque index when both were derived from the same buccal tooth surfaces. Ho wever, discriminatory power was less by comparison with original plaque ind ex derived from the buccal surfaces of all teeth. It is concluded that plaq ue area provides a permanent record of plaque distribution which can be con verted into index data at a later date. Such data collection could make pos sible comparisons between studies using different indices.