Glucose Meter Error Statistics

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The CLSI and ISO glucose meter guidelines imply the use of Normal statistics in their error limit guidelines. For example, the CLSI guideline states that meter performance is acceptable when 95% of the results are within 12.5% of reference at or above 100 mg/dL and 98% must be within 20%.

Assume that a glucose meter has a CV of 5% (or an SD of 5) and the reference is 100 mg/dL. Normal statistics say that 95% of the results will meet the guideline. If all of the errors follow the Normal distribution, where will they be? The following table shows this.

n std dev Glucose No more than 1 in … Percent in

2

90

20

94.99995

3

85

370

99.73002

4

80

15,787

99.99367

5

75

174,428

99.99994

6

70

507,000,000

 

7

65

3,910,000,000,000

 

Thus, 19 out of 20 results will be no lower than 90 (10%), but 1 of out 20 is still a lot of glucose errors. As shown by the table, most of the errors beyond 2 standard deviations are associated with the lower standard deviations (e.g., 3 and 4). And to get a hypoglycemic result (<70) requires 7 standard deviations and would be expected no more than once in almost 4 trillion times.

But larger glucose errors are more frequent than this table. The reason is that these larger errors are coming from different glucose error processes; hence the table no longer applies.

The point is that the table should not be used to judge the frequency of larger glucose errors.

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