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I got a list of Influenza-specific antibody titers from a hemagglutination inhibition assay.

They look like this:

  • 40
  • 640
  • 160
  • <10

One site says: “The swine influenza HI scale is geometric: 20, 40, 80, 160, 320, 640 (successive values increase by a factor of 2). The geometric scale is logarithmic. It is best to express an average influenza HI titer as a geometric mean. A geometric mean is calculated by averaging the logarithms of the test values and then converting the mean to a real number. This prevents a few obviously high positive values from making the mean unrealistically large.”

What does that mean for my sample? How do I deal with <10 in this calculation? This is surely not rocket science, but honestly I don’t get it right now.

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Typically "<10" is converted to a number that is below the limit of detection; often "9". After that simply following the instructions you have yields a GMT of 77.9. If you are using spreadsheet apps such as Excel or Google Sheets, they almost certainly offer "GEOMEAN" functions which will give the number directly.

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