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Antibiotic Testing USP

What is Antibiotic Potency Testing?

Antibiotic Potency Testing is a biological assay used to determine the potency (concentration) of antibiotic active pharmaceutical ingredients (API’s) and the products which contain them.

FDA requires that the potency of antibiotic drug products conform to the potency appearing on their labeling in order to be distributed within the United States. USP General Chapter <81> entitled “Antibiotic Potency Assays” provides validated methods for determining the potency of multiple antibiotic active ingredients based upon their actions against specific microorganisms.

For the highest quality testing, antibiotic potency assessments should be performed by qualified personnel with expertise in microbiology and aseptic techniques as well as analytical biological methods. CPT Labs performs all antibiotic potency testing according to validated procedures outlined in USP <81> Antibiotics—Microbial Assays. USP <81> is the primary and most commonly used reference document for biological antibiotic potency evaluations.

How is Antibiotic Potency Testing Conducted?

USP General Chapter <81> entitled “Antibiotic Potency Assays” provides  method types for determining antibiotic potency.  CPT performs the Cylinder-Plate Assay for the antibiotics indicated in USP <81>.

Cylinder-Plate Assay:

This antibiotic potency test extracts the antibiotic active ingredient(s) from the product being tested using solvents and phosphate buffers as outlined in USP <81>. After extraction, the sample preparations are diluted to the median dose of the active ingredient that is found on a Standard curve which is created for the analysis.

The sample and corresponding USP Reference Standard preparations are pipetted into penicylinders that are placed into a petri dish already containing a base layer of antibiotic growth medium and a seed layer of antibiotic growth medium which also contains a USP-specified microorganism that is sensitive to the antibiotic being tested.

The plates containing the cylinders are then incubated as specified within the method. The Cylinder-Plate Assay is based upon the ability of the antibiotic being assayed and its corresponding Reference Standard to diffuse through the agar and form a ‘zone of clearing’ (‘zone of inhibition’) thereby killing the microorganism which was present within the seed media.

The ‘zone’ sizes are measured (in millimeters) using either an automated zone reader or manually using either calibrated calipers or a manual antibiotic zone reader. The size of each ‘zone’ is proportional to the quantity of antibiotic present. Results are calculated using a formula comparing the size of the ‘zone’ obtained from known quantities of antibiotic contained in the Reference Standard to the size of the ‘zone’ obtained from the sample. The Cylinder-Plate Assay is performed by conducting three independent assays on three different days.

USP <81> lists 18 antibiotics that require the use of the  cylinder plate assay for the antibiotic raw materials and the Finished Products.  Some of the most common performed at CPT include Neomycin sulfate, Bacitracin zinc, Polymyxin B Sulfate, Nystatin, Amphotericin B, Gentamicin sulfate, and Erythromycin.

Other antibiotics which require the cylinder plate assay are  Amoxicillin,  Bleomycin, Cloxacillin, Colistemethate, Colistin, Dihydrostreptomycin, Nafcillin, Novobiocin, Paromomycin, Penicillin G and Vancomycin.

Method Verification

Although both Antibiotic Potency Testing methods appearing in USP <81> are validated, they still require verification when employed for the first time by a laboratory for a specific sample type (formulation). Method Verification includes a determination of method precision, accuracy, linearity and specificity.

This ensures that accurate results are obtained by a laboratory for each sample type under actual conditions of use. Method Verification is performed only once for each sample type (formulation) unless the USP method or sample formulation undergoes change.

CPT℠, offers expert capabilities to not only perform antibiotic potency testing but to also provide expert consultation services every step of the way. We employ the use of a Trinity V3 Automated Antibiotic Zone reader and we routinely test a wide variety of antibiotic products. Click here to view a list of antibiotic products commonly tested by CPT℠.

Please contact CPT℠ today regarding your antibiotic testing needs for active ingredients and products containing one or more antibiotic ingredients.



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Our Departments

Based on the individual needs of our clients, we offer a wide range of support across various departments. Since initial product testing can be fairly complex in terms of a high-quality, safe and trusted product, our clients typically explore the benefits of numerous departments and areas of expertise, including:

  • Clinical — Efficacy trials can be performed for safety, consumer in-use, and to support claims such as ‘dermatologist-tested’ and ‘ophthalmologist-tested.’
  • In-Vitro Toxicology — Our In-Vitro Toxicology department assists clients when they are looking to perform non-animal (animal-free) toxicology testing.
  • Microbiology — Microbial contamination and preservative efficacy testing are key to cosmetic and OTC drug products.
  • Photobiology — From SPF and Water-Resistant claims to Photoallergy and Phototoxicity studies, CPT℠ is the world leader in sunscreen testing.
  • Analytical — From raw materials, product batch release, and stability shelf-life testing to “free-of” and low-level detection claims, our analytical department ensures that all requirements are met based on industry standards.
  • Consulting – Our consulting staff can perform GMP and GLP audits of your facilities or vendors, help create SOPs, develop testing programs, etc.

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Please Note: CPT℠ only performs testing for companies. We do not perform product testing for the general public.