Thioglycollate Culture Media and Tissue Samples: MedSun Small Sample Survey Summary
MedSun: Newsletter #54, November 2010
Survey Topic: Thioglycollate Culture Media and Tissue Samples
Year Conducted: 2009
Effective treatment of an infection hinges on reliable initial readings of Gram Stains that are confirmed by the results of the cultures. Since the culture can take days or even weeks to grow out the offending organism the initial Gram Stain reading is pivotal to directing the initiation of treatment. The tools and resources utilized in the Gram Stain and culture process can have great impact on the results which then may impact a patient’s treatment. The culture media, the cell and tissue transporter, grinders, etc., must not interfere or alter the tissue or cells for the Gram Stain and the culture. Non-viable organisms appear to be viable in the Gram Stain but do not grow out in the culture, hence creating a discrepancy between the initial reading and final culture results.
FDA received adverse events about thioglycollate media possibly containing high non-viable organism counts that may affect the reliability of results. A survey of nine laboratory professionals was conducted to better understand experiences with the use of thioglycollate media with gram stain procedures. The information that follows is a summary of the interviews.
All respondents grind their tissue specimens to perform Gram staining or other microbiological staining. The decision on whether to grind and mince the specimen depended on the size of the sample and how the tissue was going to be prepared. Most hospitals used sterile saline as a liquid culture medium when grinding and mincing tissue. All performed direct Gram stains on ground tissue samples. And in most cases the direct Gram stain results were reported directly back to the requesting physician.
Generally, the respondents have also experienced discrepant culture and direct smear results. Most believed those results were due to lab errors, previous antibiotic treatment, or nonviable organisms from various types of cultures, including thioglycollate (thio), or transport media. Most of the respondents re-examined the smears and/or repeated cultures when there was a discrepancy. Some also went back to the original sterile specimen and re-inoculated the plate, re-read the gram stains, or worked with infection control staff.
Many of the respondents also had some kind of experience with nonviable organisms seen on Gram stains. Possible reasons for those high nonviable counts included, problems with the stain or slides, and nonviable organisms in transport media, and previous antibiotic treatment. When erroneous results did occur, several respondents switched from thio based on literature recommendations and potential for contamination. To prevent misinterpretation of smear results, the respondents used quality control checks, read literature for best practices, and investigated nonviable or other unexpected organisms. Even though the respondents had experienced discrepant results at some point most were unaware of a patient ever having been treated based on a false positive Gram stain due to the presence of nonviable organisms.
Small scale surveys are one of many tools the Agency uses to evaluate the public health impact of the potential problems associated with the use of medical devices. Because these surveys utilize a small sample size that is not statistically derived, the results provide only qualitative and not quantitative results. Additionally, FDA continues to receive adverse event reports from its Medical Device Reporting program, and will continue to make use of the literature and other published information. FDA scientific, medical, nursing and engineering staff are made aware of the survey results as needed. If FDA believes a possible public health problem may be emerging from the results of a survey, it will combine those results with data gained from the other sources and may convene an Ad Hoc group of clinical and manufacturing representatives to discuss further actions.
FDA will work with the manufacturers and health care professional organizations to make important information known about medical devices to the clinical community.
Small Sample Survey Questions
1. Can you tell us about the method you use for performing gram stains on collected specimens?
a. Do you grind and mince tissue/biopsy in your laboratory?
2. What type of microbiological culture media do you use to grind and mince the tissue?
3. Do you perform staining on the ground tissue sample?
• If so, what type of staining method do you perform on the tissue specimens?
4. Do you report direct stain results to the physician or clinician who requested the diagnostic procedure?
5. Have you experienced high nonviable counts with your direct smear results?
• If so, were there any changes you observed with the media?
• Have you been able to determine possible reasons for the high nonviable counts?
6. Have you experienced any other types of erroneous results with nonviable counts in other media associated with direct Gram stains or other direct microbiological results?
• If so, what kind of results did you find and how frequently have you seen these erroneous results?
7. What actions have you taken when you experienced erroneous results?
• Have you made the manufacturer aware?
• If so, what was the manufacturer’s response?
8. Have you experienced a situation where a patient was treated based on a false-positive Gram stain result?
9. How do you communicate information about the possibility of false-positive Gram stain results to staff?