11 Responses

  1. Bernard Farrell
    Bernard Farrell November 13, 2012 at 7:07 am | | Reply

    Now that’s out of the box thinking and two very useful metrics. For Dexcom, then can probably already calculate GVI and PGS with ease — I hope we see this come out soon.

    One question I do have. If one end of the line is elevated (say from 120 to 240), then L0 would we bigger than L0 if the line was flat. Do they allow for that? Even if they don’t it probably doesn’t make a huge difference.

  2. Terry
    Terry November 13, 2012 at 7:45 am | | Reply

    Amy – Thank you for writing on this important topic. As a long term (since 2009) CGM user, I’ve been using standard deviation (SD) and coefficient of variation (CV) to analyze the quality of my control. When my line is flatter I do feel better.

    While I know just enough about math to be dangerous, I’d be grateful if you did a follow-up column to this one. I’m most interested in figuring out the actual steps to implement Excel to crunch the numbers needed to calculate a GVI and PGS. Discussion of a step by step example would be instructive.

    Thank you for keeping on top of emerging diabetes topics like this one.

  3. Bernard Farrell
    Bernard Farrell November 13, 2012 at 10:37 am | | Reply

    @Terry, the distance between two lines is Sqrt( (x1-x2)**2 + (y1-y1)**2).

    In Excel the formula is =SQRT(POWER(A1-A2,2)+POWER(B1-B2,2)). If you assume the X value (time) is in minutes, and the Y value is readings, it’s not too hard to do.

    Send me your email (bernard (dot) farrell (at) and I’ll email you a sample spreadsheet.

  4. mcityrk
    mcityrk November 13, 2012 at 7:18 pm | | Reply

    Possible correction: PTIR looks like should be the fraction of time in the target range [i.e. it can never be greater than 1] not the percentage of time in the target range. This implies that the PGS goes to zero when all values are within the target range.

    One suspects with all the data crunching of CGM data over the last decade, this concept was already qualitatively understood by researchers. However, it would not be useful to introduce it to physicians and patients until the CGM manufacturers believed data was always of very high quality. Apparently we have now reached that point!!

    1. Bernard Farrell
      Bernard Farrell November 14, 2012 at 11:27 am | | Reply

      @mcityrk is on to something, PTIR can never be 1. How does this work for a non-diabetic where PTIR could be equal to 1. Amy, can you point us to the original paper?

      1. Daniele
        Daniele November 17, 2012 at 1:42 am | | Reply

        PTIR = 1 means 100% values in range = non diabetic

        PTIR can never be > 1, ok, and for that PGS is always >= 0

        Percentage is intended as 20% = 0,20

  5. Mike Ratrie
    Mike Ratrie November 14, 2012 at 8:18 pm | | Reply

    Just one question (yeah, right ….):

    When researchers do “… a study of diabetic and non-diabetic subjects to validate these values … “. Does that mean they have achieved “valu-dation”?

  6. diabetic survival kit
    diabetic survival kit November 17, 2012 at 4:08 am | | Reply

    Variability of blood sugar does correlate with diabetic complications as well as immediate problems from hypoglycemia. What may sound like theoretical math actually has a great impact on people’s lives. This was discussed a little bit in the meetting in Boston: sounds like they went into much greater detail at your meeting. Thanks for a great post.

  7. c.a. costova
    c.a. costova November 23, 2012 at 6:23 am | | Reply

    I don’t understand the math (I didn’t pass algebra) but as a cgs pump User for 2 years – medtronic-I understand the WIDE VARIABILITY as a extremely brittle diabetic. Can you put this into EASY understandable language for an excel spreadsheet? Thanks!!

    1. Bernard Farrell
      Bernard Farrell November 26, 2012 at 7:32 am | | Reply

      @c.a. costova.

      Send me an email and I’ll forward you a working spreadsheet that does this calculation for you. bernard(dot)farrell(at)

  8. Daniele
    Daniele November 26, 2012 at 7:45 am | | Reply

    Anyone knows how much is the constant L0 ? It’s needed to do the calcs….

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