Monday, 24 August 2020

Assessing Gram-stain error rates within the pharmaceutical microbiology laboratory

The implications of errors with the Gram-stain can influence the selection of the test method (and test kit) for the next stage of identification, whether this is a manual biochemical identification method (such as API) or a semi-automated method (such as Vitek or Omnilog). Getting an identification wrong could lead to an incorrect root cause analysis (which impacts on all types of pharmaceutical processing, including sterile products) and potential errors relating to batch release (especially with non-sterile pharmaceuticals where understanding the pathogenic nature of the organism is a key requirement).

In relation to this fundamental aspect of pharmaceutical microbiology, Tim Sandle has written an article.

The abstract reads:

Gram-staining remains the fundamental method for determinative bacteriology, dividing bacteria into Gram-positive and Gram-negative organisms. This test provides information as to the origin of any contamination and is a pre-requisite for many microbial identification methods. Despite the longevity of the test, the test is highly reliant upon analyst technique and therefore errors occur. While there are a few studies looking at errors in the clinical context, research has not been extended to the pharmaceutical microbiology laboratory context. In this study, we present a review of over 6,000 Gram-stains and establish an error rate of around 3%, with the most common reason for error being an over-decolourisation step resulting in organisms that should be Gram-positive appearing as Gram-negative. The analysis enables others to benchmark their facilities against.

The reference is:

Sandle, T. (2020) Assessing Gram-stain error rates within the pharmaceutical microbiology laboratory, European Journal of Parenteral and Pharmaceutical Sciences, 25 (1):

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (

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