Bacterial Index for Leprosy

The bacterial index was proposed by Ridley. He developed a logarithmic scale, from 0 to 6+. The scale is based on the average number of bacilli per microscopic field using an oil-immersion objective. In infections with a high bacterial load, it usually takes 5-8 years from the beginning of therapy before the bacterial index is negative. [A rule of thumb is 1+ per year].

 

Bacterial index
0	0 bacilli in 100 oil-immersion fields
1+	1 to 10 bacilli per 100 fields
2+	1 to 10 bacilli per 10 fields
3+	1 to 10 bacilli per field
4+	10 to 100 bacilli per field
5+	100 to 1000 bacilli per field
6+	> 1000 bacilli per field