4.51: Glucosemeter Nonlinearity

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Glucose meters are purchased and instrument linearity studies are performed on each instrument using the manufacturer's calibrators (see p 306, Chapter 17). Five calibrators range from about 40 to 460 mg/dL. The initial calibration meets the manufacturer’s specifications. After 6 months of use the glucometers are recalibrated and linearity re-validated. The POCT technologist brings to the attention of the laboratory supervisor that the linearity is now only good from 40 to 350 mg/dL. Above 350 mg/dL the values look like they are leveling off.

QUESTIONS

Is this a severe problem that requires intervention? Why?
The Laboratory Director makes the decision that values above 400 mg/dL are not reliable for proper patient care because of curve nonlinearity and this is an out-of-control situation. The problem is too widespread to use the few replacement instruments on hand. What are the options to improve the quality of results?

Questions to Consider

What steps should be taken to determine the severity of the linearity?
What factors must be considered when considering the response to an out-of-control situation?

Answer

This is an important issue for two reasons. First, patient values often extend to 600 mg/dL or greater and results may be clinically important when diagnosing certain hyperglycemic states, such as hyperosmolar coma. Second, nurses or doctors will adjust insulin dosage using the fingerstick glucose values. Thus meters that will produce falsely low readings when values are above 400 mg/dL, may cause erroneously low doses of insulin for therapy. See p 593-594, chapter 32.
Immediately, instruct all users of the meters that all glucose values above 350 mg/dL must be repeated on an automated chemistry instrument in the central laboratory before any treatment is initiated or dosage of medication adjusted. Medium-term; ask the manufacturer for total replacement of all devices. Long-term, consider replacement of the meters.

Answers to Questions to Consider

The determination must be made if the reduced linearity is an out-of-control situation that requires either restrictions placed on the use of the glucose meters or cessation of use of the instruments (see p 395, Chapter 21). If this is not an out-of-control situation, the laboratory must determine the immediate steps needed to correct the decreased linearity. If this is an out-of-control situation (see pp 394-396, Chapter 21), the laboratory director needs to determine the extent of response, that is restrictions on use versus total cessation of use. The laboratory should obtain glucose standards (or calibrators with higher values from a different manufacturer) and use these to determine the linear instrument response.
One need to determine the extent of the problem, in this case determining how far off the meters are from correct values and how many meters are affected with this poor linear response. What would be the clinical impact if the values >350 mg/dL are too falsely low by the amount determined? If it could adveresly impact on patient care, this will increase the likelihood of an-out-of-control decision.