I have read provided SOP as well as USP chPter <55>for Biological indicator. But in this, testing Method has been defined but not mentioned Calculation formula.

In USP 42 chapter <55>, Pg no. : 6386 has mentioned one sentence, as like this :" calculate the average no. Of spores per test sample from the results, using the appropriate dilution factor. "

According to this, our sop have already defined one formula:

Spore population = avg. Colonies (cfu) x Reciprocal of Dilution /No. Of Strips

But in that, if

For 10^-4Dilution obtained avg. colonies 373 cfu/ml, then calculated spore population will be 1243333.33 spores/ strip.

For 10^-5 dilution obtained avg. Colonies 106 cfu/ ml, then calculated Spore population will be 3533333.33 spores /strip.

For 10^-6 dilution obtained avg. Colonies 48 cfu /ml, then calculated spore population will be 16000000 spores /strip.

Then how is possible , Higher dilution gives calculated maximum spores count per one strip in comparison to 10^-4 Dilution.

Because if 10^-4 dilution gives more no. Of colonies, then spores count must be more in tjis dilution.

But we obtained false reault.

If anyone have idea regrading to this, Kindly correct our calculation formula to count spore population.