Specification ratios

Posted on September 9, 2010 by Dan

A quick post today on a topic that I see often:  incorrect specifications for potency assays.

It is fairly common to see specifications for a potency assays that look like this:
50% – 150%
80% – 120%
Etc…

What is the problem with these?  While they look to be symmetrical around 100% they are not.  Percentages are ratios, and therefore, the correct specifications should actually be:
50% – 200% (1/2 to 2/1)
80% – 125% (4/5 to 5/4)

Of course, the top set of specifications may actually be deliberately asymmetrical (for safety reasons on the high end for example), but in most cases I’ve seen they are simply listed this way by mistake.

Thanks for reading,

Dan

About Dan

My name is Dan Joelsson and I'm the main contributor to this blog. I have spent the last ten years or so in analytical development at a major pharmaceutical company, mainly working on potency assays of all kinds. While I'm a biologist by background I also have experience in process optimization using Six Sigma and Lean principles. If you want to contact me, please leave me a comment on the "about" page and I will get back to you as soon as possible.
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2 Responses to Specification ratios

  1. RG says:
    November 5, 2010 at 9:54 am

    Hi Dan,
    I just wanted to know from your experience if potency assays are conducted under GMP or GLP ? Could you also kindly provide me with references for this.

    Much appreciated
    Thanks
    RG

  2. A. Dos Santos says:
    April 19, 2011 at 11:01 am

    Hi Dan,

    I did not know where to post this comment. So here it is.
    I just wanted to thank you for creating this blog. I’m new in the field of Bioanalyticals. In the small company I am working in I am in charge of developing and qualifying potency assays (which are for the moment immunoassays). I am learning everyday and your blog, with your advices and comments, is of great help.

    So thank you and please keep writing.
    Anaëlle

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