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# Preparation of antibiotic disc

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### #1 dingdong

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Posted 29 June 2016 - 11:54 PM

Hi, I'm new in this field. My customer gave me some antibiotic in powder and liquid form and would like me to prepare antibiotic disc for antibiotic sensitivity test. I'm confused on the calculation and units though.

For example,

Amoxicillin, each gram contain 500mg. Dosage used: 100g/100 gallons drinking water.

So I just take 0.264g and mix with 1L water for my antibiotic solution and then pipette 50ul onto my paper disc.

So the disc concentration would be 132ug/ml.

Am I correct? Then how about the commercial disc that having disc potency 2mcg? Meaning the disc I prepared is actually less concentrated than the commercial disc?

Please hep me on this. Thank you.

### #2 bob1

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Posted 30 June 2016 - 05:40 AM

What final amount do you want to be on the disk? If it is 2 ug then you need to go about the calculation differently.

You know that you have 500 mg/g so if you dissolve this in 1 L of water you will have 500 mg/L or 0.5 mg (500 ug) per ml.

You then need to use the calculation:

V=N/C

V= 2 ug/500 ug/ml

V=0.004 ml or 4 ul

Note that this is an example!!! do your calculations yourself, we are happy to check them.

### #3 dingdong

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Posted 01 July 2016 - 01:29 AM

What final amount do you want to be on the disk? If it is 2 ug then you need to go about the calculation differently.

You know that you have 500 mg/g so if you dissolve this in 1 L of water you will have 500 mg/L or 0.5 mg (500 ug) per ml.

You then need to use the calculation:

V=N/C

V= 2 ug/500 ug/ml

V=0.004 ml or 4 ul

Note that this is an example!!! do your calculations yourself, we are happy to check them.

Hi,  Bob. Thanks for your reply. I need to prepare the same concentration as the dose that customer used.

My customer is using 100g in 100gallon,

100g ->100 gallon/378.541 L

1L=0.264g

So I just dissolve 0.264g powder in 1L water. Correct?

By doing this, the final concentration of the disc would be

[]=(0.264g x 500mg/g) / 1000ml

= 132mg/ml

Is my calculation correct? I used concentration = Weight x Potency/Volume

Edited by dingdong, 01 July 2016 - 01:29 AM.

### #4 mdfenko

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Posted 01 July 2016 - 04:11 AM

in the dosage, is that supposed to be 100gm of the powder or 100gm of the antibiotic? the answer will determine if your calculation is correct or if the dosage is half.

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### #5 dingdong

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Posted 01 July 2016 - 06:03 PM

in the dosage, is that supposed to be 100gm of the powder or 100gm of the antibiotic? the answer will determine if your calculation is correct or if the dosage is half.

Hi, is 100gm of the powder.

### #6 mdfenko

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Posted 05 July 2016 - 03:42 AM

[]=(0.264g x 500mg/g) / 1000ml

= 132mg/ml

Is my calculation correct? I used concentration = Weight x Potency/Volume

you have 132mg/1000ml (or 132ug/ml)

talent does what it can
genius does what it must
i used to do what i got paid to do