I have calculated form this formula
Percentage of potency= 100- [T/C*100]
T= Zone of inhibition of Test sample (Plant extract)
C= Zone of inhibition of Control (Standard)
but in some cases, i have got negative percentage potency of extracts. Is it OK to have a negative percentage potency?
Please provide proper references.
what is your control test? an antibiotic?
if your control sample is more potent than your test sample, it is very normal to have a negative result.
i can suggest a better method to testify your sample and it is the microdilution broth method.
good luck
Methods for Dilution Antimicrobial
Susceptibility Tests for Bacteria That Grow
Aerobically; Approved Standard—Ninth
Edition
Thanks for your reply.
I used Erythromycin and Bacitracin as control drug.
in some cases, my plant extracts are more potent than control drugs and by this formula, i have got negative percentage potency of those extracts.
I will give you an example:
Zone of inhibition by plant extract= 33.00 mm
Zone of inhibition by erythromycin= 18.23 mm
percentage of potency= 100- [T/C*100]
= 100- [33/18.23*100]
= -81.02
I just wanna ask is it OK to have a negative percentage of potency of that extract which show more inhibition than control drugs?
If you have any reference regarding this formula or any other way of calculating the percentage of potency, then please tell me.
Thank you.
I think you got mised up. First things first. Your result with high activity for the plant extract is fine. But you should never get a negative value. The formula you used is for spectroscopic method and not suitable for paper measurement. In case you need to calculate the percentage of inhibition your formula should look something like:
I%=[Zone of inhibition by test material/Actual diameter of the culture plate/area]x100
But I don't know anyone using that! In addition, drug and experimental samples cannot and should not be calculated together. If comparison is required, use statistical analysis (like Student's t-test). In practice, we stop there at the measurements. In fact even a small zone can indicate good activity.
You may benefit from previous threads:
https://www.researchgate.net/post/What_is_the_proper_way_to_measure_the_zone_of_inhibition_of_an_antibiotic_disc
https://www.researchgate.net/post/Calculation_of_percent_inhibition_of_bacteria
https://www.researchgate.net/post/What_is_the_proper_way_to_measure_the_zone_of_inhibition_of_an_antibiotic_disc
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