CLSI C51 – measurement uncertainty – or the classic comic version of GUM

GUM (Guide to the Expression of Uncertainty in Measurement) for laboratories (and manufacturers) is what CLSI C51 is all about.  (GUM was originally used to provide information about reference materials). I have previously commented that I didn’t think that GUM was a good idea for laboratories (1). I was also initially on the C51 subcommittee but since I couldn’t convince anyone about my point of view, I bailed.

To recall some of the problems with GUM …

  1. bias is not allowed – it must be corrected. But you could ignore big, rare biases (outliers) as well as real small biases.
  2. To obtain the standard deviations or bias corrections applied by manufacturers was impractical if not impossible for laboratories as in … Let’s set up a fixture and measure the variability of 10 pumps we just bought for this experiment.
  3. The math required to put together an estimate will make most people’s head spin.

In the C51 version of GUM, there is only 1 example – that of measuring a bunch of controls. This is not GUM! and will not provide an uncertainty estimate for patient samples since controls do not estimate the non specificity assay errors in patient samples.


  1. Krouwer JS A Critique of the GUM Method of Estimating and Reporting Uncertainty in Diagnostic Assays Clin Chem 2003;49:1818-1821.

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