Quality and suitability of antimicrobial discs: theoretical and practical sources of error and variability

ABSTRACT

Introduction: The use of antimicrobial discs remains one of the main methods for assessing antibiotic activity. Most discs are made to one of three main standards (FDA, WHO, DIN); these all describe an assay method for assessing the quality of discs using a linear method. Theory predicts a curved relationship, and this is backed up in many cases in practice. In such cases, the assays are potentially invalid. Other sources of uncertainty arising from the manufacturing processes employed are also discussed.

Areas covered: This includes error arising from applying FDA, WHO, and DIN standards, the manufacturing techniques employed, and variation in the materials used in production. The need for a specification that relates directly to the use of the discs is also discussed.

Expert opinion: Manufactured discs, some of which may be out of specification due to curvature and other sources of error, have been used to establish quality zone sizes. Quality zone sizes have then been used to measure the quality of discs. This circular quality system where there is no quantitative check is potentially unsafe. In the many decades of their use, there has been no comprehensive check on the quality of manufactured discs using quantitative, validated assays.

KEYWORDS:

• Antimicrobial susceptibility test
• assay
• AST
• Kirby–Bauer test
• quality zone ranges
• validation

Article highlights

• The three main standards for testing AST discs (DIN, FDA, WHO) are potentially invalid due to curvature.

• The standards only specify an average content. This is inadequate for their intended use, which would require a measure of bias, an error of the mean, and variation in the batch.

• Secondary measures such as quality zone ranges are hence also potentially invalid. Quality zone ranges are not quantitative and therefore cannot replace a validated assay.

• Discs selected from a limited number of vials may not be representative of the batch since it depends on the packing method of the manufacturer. Therefore, such comparisons between manufacturers may not be valid.

• The clinical requirement for discs by the end user is not known. Similarly, the quality of individual discs is not generally known. Therefore, it is not known if current manufactured discs are fit for purpose.

• Despite the many years of their use, there has been no comprehensive check on the quality of manufactured discs using quantitative, validated assays.

Declaration of interest

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewers Disclosure

Peer reviewers on this manuscript have no relevant financial relationships or otherwise to disclose.

Additional information

Funding

This paper was not funded.