Interexaminer variability in common ratings in reading Streptococcus mutans dip-slides with or without a microscope.

The observer variability in reading 145 S. mutans dip-slides was assessed by a test panel consisting of a professional bacteriologist, practising dentists and unskilled laymen. The cultivated slides were read and classified into four categories by comparing them with standard photographs of previously cultivated slides with known numbers of S. mutans colonies. For reference, all the slides were also counted under a stereomicroscope. The results showed both under- and overestimation in the visual reading, but the scoring trend was logically increasing in relation to the microscopic reference scores in each case. The significance of the agreement on the ratings was high and it further increased when the results were recombined by classifying them into three categories. The individual agreement percentages varied then from 54 in the laymen to 79 in the bacteriologist. The use of a microscope did not significantly increase the number of common readings and cannot be considered necessary in practice. Reading the slides, however, definitely requires understanding and experience of the method.