Quantitative light-induced fluorescence (QLF):: a method for assessment of incipient caries lesions

Objectives: To review the literature on validation and application of the quantitative light-induced fluorescence (QLF) method for quantitative assessment of early enamel lesions in vivo and in vitro. Methods: QLF uses light with wavelengths around 405 nm to excite yellow fluorescence at wavelengths above 520 nm. Its diagnostic capacity is based on the mechanism that the intensity of natural fluorescence of a tooth is decreased by scattering due to a caries lesion. The equipment, the data processing and the interaction between equipment and operator are described. Results: The method has been validated by many authors; the results are presented and compared. For artificial lesions, the validation line is curved. For larger mineral losses, the curve is linear with a slope of 10% fluorescence loss corresponding with a mineral loss of 0.15 kg(.)m(-2). For lesions caused by natural caries, it is tentatively concluded that fluorescence loss is linear to mineral loss with a slope similar to that of artificial lesions. Reliability and reproducibility have been tested in vivo and show interexaminer values of the interclass correlation coefficient, r, of 0.93 <0.99. Confounding factors are inadequate reconstruction of sound fluorescence values and drying of the lesion before or during measurement. In vivo application showed that statistically significant changes between different preventive regimes could be proven in only 6 months of study time. Conclusion: QLF offers a potential tool to reduce the time needed for clinical research. Its objectivity will prove useful in epidemiological surveys. QLF provides visual and quantitative feedback to patients.