The previous blog entry criticized the updated FDA POCT glucose meter performance standard, which now allows 2% of the results to be unspecified.

What follows is an explanation of why this is wrong. My logic applies to:

- Total error performance standards which state that 95% (or 99%) of results should be within stated limits
- Measurement uncertainty performance standards which state that 95% (or 99%) of results should be within stated limits
- The above FDA standard which states that 98% of results should be within stated limits

One argument that surfaces for allowing results to be unspecified is that one cannot prove that 100% of results are within limits. This is of course true. But here’s the problem of using that fact to allow unspecified results.

Using a glucose meter example, with truth = 30 mg/dL. Assume the glucose meter has a 5% CV and assume that the precision results are normally distributed. One can calculate the location of glucose meter errors using various SD multiples and also note their location in a Parkes error grid and the number of times 1 of these errors due to precision could occur.

Truth |
SD multiple |
Observed glucose |
Parkes grid |
Occurs 1 in |

30 |
2 |
33 |
A zone |
20 |

30 |
3 |
34.5 |
A zone |
370 |

30 |
8 |
42 |
A zone |
7E+14 |

30 |
22 |
63 |
C zone |
1E+106 |

(To get an error in the E zone, an extremely dangerous result, would require 90 multiples of the standard deviation, and Excel refuses to tell me how rare this is). I think it’s clear that not specifying a portion of the results is not justified by worrying about precision and / or the normal distribution.

Now errors in higher zones of the Parkes error grid *do* occur including E zone errors and clearly this has nothing to do with precision. These errors have other causes by other sources such as interferences.

A better way to think of these errors are “attribute” errors – they either occur or don’t occur. For more on this, see: Krouwer JS. Recommendation to treat continuous variable errors like attribute errors. Clinical Chemistry and Laboratory Medicine 2006;44(7):797–798.

Note that one cannot prove that attribute errors won’t occur. But no one allows results to be unspecified the way clinical chemistry standards committees do. For example you don’t hear “we want 98% of surgeries to be performed on the correct organ on the correct patient.”