I am investigating MICs for colistin (15000 Units/mg) and polymyxin B (8000 IU/mg). Up until now I have just been weighing the antibiotics and reporting the MICs as mg/L.
I am confused by this equation: weight (mg) = (volume (ml) x drug concentration(mg/ml))/potency (mg/mg).
Since I have not been using this equation, how does the potency affect my MIC results and what is difference between units/mg and IU/mg?
When I use the equation, I get really strange results. Would someone help me with the calculation?
Could I just report my MICs without taking the potency into a account and just state the units/mg or IU/mg?
What really counts is the units and not the weight. You can buy one prep of a drug at 100U/mg and another at 200U/mg and obviously you would need to add twice as much of the first in order to have the same amount of active ingredient. Unit and IU are essentially the same, IU is international unit and is just better defined as to what it means. The definition of unit is otherwise somewhat arbitrary and can (but may not) change from vendor to vendor.
The concept of the IU is a murky one, and that may be part of your problem.
See: https://en.wikipedia.org/wiki/International_unit
(Note that the IU has NOTHING to do with the International System of Units used in physics and chemistry.)
Also a general problem with the concept of potency is that some antibiotics produce an MIC by being bactericidal and some do it by being bacteriostatic.
See:
https://en.wikipedia.org/wiki/Bactericidal
https://en.wikipedia.org/wiki/Bacteriostatic
Finally, the concept of an IMC is itself problematic. It just means that no discernible growth has a occurred after an arbitrary time--e.g. 24 hours. Thus use of an MIC concentration of an antimicrobial does NOT mean all the microbes have been killed. It is entirely possible that the microbes will show growth a that antimicrobial concentration at e.g. 36 hours.
From where do we get the potency if antibiotic to find the MIC by using this formula weight (mg) = (volume (ml) x drug concentration(mg/ml))/potency (mg/mg).
Four years late but the potency of an ATB has nothing to do with the units. It refers of how much of the actual weight is active compound. To answer that question (and to weight accurately) you need to know how pure the powder is, how much water comes attached to the molecule and how much is not actual compound but something else, such as a salt. All of this information is available in the CoA of the compound. It sound like you may have been under weighing the powder for your stock solution.
https://mypharmatools.com/othertools/iu
Follow above this link and you will get your answer easily
https://mypharmatools.com/othertools/iu
Follow above this link and you will get your answer easily.
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