The problem with the FDA standard explained

October 25, 2016

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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:

  1. Total error performance standards which state that 95% (or 99%) of results should be within stated limits
  2. Measurement uncertainty performance standards which state that 95% (or 99%) of results should be within stated limits
  3. 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.”

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The updated FDA POCT glucose meter performance standard has a big problem

October 21, 2016

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As readers may be aware, I have ranted against glucose meter standards for some time. Although the standards have many flaws, the most egregious one is the failure to specify 100% of the results. For POCT glucose meters, the CLSI standard C30-A2 (2003) adopted the ISO glucose meter standard 15197, which accounts for 95% of the results.

In 2013, CLSI updated its standard, now called POCT 12-A3 to include 98% of the results.

In 2014, FDA issued a draft POCT glucose meter guidance which covers 100% of the results.

But, now FDA has updated its POCT glucose meter guidance to cover only 98% of the results.

There’s no reason to allow 2% of the results to be unspecified – I don’t know why the FDA did this.


Interesting paper about Hb A1c

October 21, 2016

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An upcoming paper about Hb A1c showed that MCH (mean corpuscular hemoglobin, a CBC parameter) correlated negatively with Hb A1c. In the patients studied, the effect at the extreme range of observed MCH could be as much as 0.7%-0.8% on HbA1c.