Allowable assay error and regulations

EP27, the CLSI standard about error grids is about to begin its fifth year without being published as an approved level standard. The latest roadblock is that the CLSI Board of Directors had a slew of comments – they shouldn’t even be commenting but more of this later. So here is one of the comments:

“Regulatory requirements should not influence grids; the grids are based on risk of harm.”

This statement sounds good but is not helpful. Here’s why. The risk of patient harm depends on the amount of assay error. It is often suggested that there is an error limit below which there is no harm and above which, harm is certain (red line is above figure). This is also not true. The risk of harm is zero for zero assay error and rises as assay error rises (green line in above figure, which is taken from EP27P). And the harm itself can be different. For example, a glucose error can cause a small error in the amount of insulin given which is less harmful than a large glucose error which suggests hyperglycemia for a patient who is hypoglycemic.

If you’re still with me, error grids have to use the red lines from the figure. That is, they have dichotomized, the continuous risk of harm into an on off level for each zone in the error grid. To simplify things, consider only the innermost zone in an error grid, the one that separates no harm from minor harm. One can ask, where should this limit be placed? Remembering the green line, the limit should be placed as close to zero error as possible. [OK, it is possible that the green line is “S” shaped rather than as shown but that doesn’t change things.] The problem with placing the limit too close to zero is that it is too expensive. Thus, to have each assay run in triplicate reduces error but increases cost. The reality is that the socioeconomic practice of medicine dictates where the limit is placed and the socioeconomic practice is often realized by regulation, which can differ in different countries.

So in the real world, regulatory requirements influence error grids.

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