RIDLEY&;S LOGARITHMIC SCALE FOR BACTERIOLOGICAL INDEX: Bacteriological Index(BI): Density of bacilli in smears, including both living and dead bacilli . Ridley&;s logarithmic scale: Based on the number of bacilli seen in an average High power microscopic field, using an oil-immersion objective (1/12 in or 2mm) When several smears are taken: Mean index is calculated Score Description 6+ Many clumps of bacilli in an average field (Over 1000) 5+ 100-1000 bacilli in an average field 4+ 10-100 bacilli in an average field 3+ 1-10 bacilli in in an average field 2+ 1-10 bacilli in 10 fields 1+ 1-10 bacilli in 100 fields In the given question, Two samples have 10 bacilli in 100hpf (each 1+ score) and Tw samples with >1000 bacilli in average field (each 6+ score) BI= 1+1+6+6/4 = 3.5 Ref: Park 25th edition Pgno: 337