Objective: Caries risk assessment is an important component of clinical practice. The goal of this project was to evaluate the diagnostic performance of two commercially available kits (the Saliva-Check mutans and the Plaque-Check pH kit; GC-America) in distinguishing caries-active from caries-free individuals. The effect of following instructions not to eat, drink, or perform oral hygiene, prior to testing, was also investigated on the effectiveness of these kits. Method and Materials: The subjects included 26 caries-affected children and 20 caries-free adults. Plaque and saliva samples were collected for analysis by the two kits following the manufacturer's instructions. For 63 additional subjects, instructions not to eat, drink, or practice oral hygiene were omitted prior to sample collection. The sensitivity and specificity in distinguishing caries-free from caries-active subjects was calculated for both kits. Results: The sensitivity (88%) and specificity (90%) of the Saliva-Check mutans kits were satisfactory in the subjects refraining from eating etc. In contrast, both the sensitivity and the specificity of this kit were reduced when subjects were tested without use instructions. When tested in subjects that followed instructions, the pH kit's sensitivity and specificity were 72% and 55% respectively. This modest diagnostic performance was reduced when pH values were measured in subjects not following use instructions. Conclusion: These kits, particularly the Saliva-Check mutans kits, can contribute to patient education by documenting microbial caries risk factors. Though difficult to implement in practice, subjects should refrain from eating or other activities that disturb the oral environment prior to sample collection. Keywords: caries risk assessment, patient education, plaque pH chairside test, Streptococcus mutans test