Determination of 50% endpoint titer using a simple formula

Table 1 Calculation of virus titre in mice using the Reed and Muench method

Log10 virus dilution	Mice		Cumulative total			Percent mortality
	Died	Survived	Died	Survived	Total
-1	10	0	57	0	57	57/57 × 100 = 100
-2	10	0	47	0	47	47/47 × 100 = 100
-3	10	0	37	0	37	37/37 × 100 = 100
-4	10	0	27	0	27	27/27 × 100 = 100
-5	10	0	17	0	17	17/17 × 100 = 100
-6	6	4	7	4	11	7/11 × 100 = 63
-7	1	9	1	13	14	1/14 × 100 = 7

Difference of logarithms = (63-50)/(63-7) = 0.23; log₁₀ 50% end point dilution = -6 - (0.23 × 1) = -6.23; 50% end point dilution = 10^-6.23; the titre of the virus = 10^6.23 LD₅₀/mL.

Table 2 Calculation of virus titre in mice using the Spearman-Karber method

Log10 virus dilution	Mice
	Died	Inoculated
-1	10	10
-2	10	10
-3	10	10
-4	10	10
-5	10	10
-6	6	10
-7	1	10

x₀ = 5; d = 1; log₁₀ of 50% endpoint dilution = - [5 - ½ + 1 (17/10)] = -6.2; 50% end point dilution = 10^-6.2; the titre of the virus = 10^6.2 LD₅₀/mL.

Table 3 Calculation of virus titre in mice using the new method

Log10 virus dilution	Mice		Death score
	Died	Inoculated
-1	10	10	10/10 = 1
-2	10	10	10/10 = 1
-3	10	10	10/10 = 1
-4	10	10	10/10 = 1
-5	10	10	10/10 = 1
-6	6	10	6/10 = 0.6
-7	1	10	1/10 = 0.1
Total	57		5.7

By using formula 1: log₁₀ 50% end point dilution = - (57/10 + 0.5) × 1 = -6.2; 50% end point dilution = 10^-6.2; the titre of the virus = 10^6.2 LD₅₀/mL. By using formula 2: log₁₀ 50% end point dilution = - (5.7 + 0.5) × 1 = -6.2; 50% end point dilution = 10^-6.2.