Dear Stanley: What is Mean Assay Value?

Dear Stanley,

I use EZ-Accu Shot™ for growth promotion testing (GPT), and sometimes I don’t get counts within 50-200% of the count printed on the label. Is there something wrong with my media or the control? 



Fresno, CA

Dear Beverly,

Great question! The value listed on Microbiologics enumerated product labels is the mean assay value. This value is the average count our quality control team obtains prior to releasing the lot. We employ specific methods and materials to achieve the mean assay value. Your results may differ from the value listed on the product’s label due to many factors including:

  • Materials
    • Brands and formulations of culture media
    • Age of the media being tested
    • Incubator environments
    • Length of incubation
  • Methods
    • Spread plates
    • Pour plates
    • Membrane filtration
  • Techniques
    • Pipetting
    • Inoculation

We recommend testing in parallel with a previously approved lot of media during GPT to counteract these differences. The average of the counts on the previously approved media become your standard to calculate the 50-200% range. This method uses your standard, which eliminates variability from the mean assay value printed on the label, so you can focus on what’s important: testing the media.

Microbiologics guarantees that if processed as directed your results will be within the specific CFU concentration range designated for each enumerated product line:

Product CFU
Epower™ E2: 1.0-9.9E+02 per pellet

E3: 1.0-9.9E+03 per pellet

E4: 1.0-9.9E+04 per pellet

E6: 1.0-9.9E+06 per pellet

E7: 1.0-9.9E+07 per pellet

E8: 1.0-9.9E+08 per pellet

EZ-Accu Shot™ 10 – 100 CFU per 0.1 ml
EZ-CFU™ 10 – 100 CFU per 0.1 ml
EZ-CFU™ One Step 10 – 100 CFU per 0.1 ml
EZ-FPC™ Qualitative: 1.0-9.9E+02 CFU per pellet

Quantitative: 1.0-9.9E+03 CFU per pellet

EZ-PEC™ 1.0E+05 to 1.0E+06 CFU per ml of product tested

(2.0-9.9E+07 CFU per pellet)

EZ-SPORE™ 1.0-9.9E+04 CFU per pellet

Visit the Frequently Asked Questions page on our website for answers to common questions about our products. If you’re unable to find the information you need, our team of experienced scientists is here to help. You can reach the Microbiologics Technical Support team at or 1.320.229.7045.



Stanley Staphylococcus is a Master Micro-Technologist at Microbiologics, where he is responsible for helping customers understand why microorganisms behave the way they do. You could say he’s somewhat of a psychologist. Microbiologics has been lucky to have Stanley, a graduate of Gram-Positive Cocci University, as a member of their renowned Technical Support Team for over 20 years. Stanley says his favorite type of people are microbiologists and he enjoys traveling far and wide to meet them. Amazingly, Stanley has been on every continent – even Antarctica!

How to submit inquiries: There are two ways to get Stanley’s help. You can email your questions to or you can simply submit an inquiry from our Dear Stanley page. For urgent issues, please contact our Technical Support Team at or 1.320.229.7045.


Read Next – Top 10 Tips for Growth Promotion Testing