Optimizing glucose control is critical for patients with diabetes to minimize risks of micro- and macrovascular complications associated with hyperglycaemia. To this end, individuals with diabetes who depend on insulin replacement therapy, particularly those on basal-bolus regimens, need to be aware of their blood glucose values to guide treatment decisions.
In recent years, continuous or semi-continuous glucose monitoring devices have become available and are increasingly adopted in clinical care. Nevertheless, most patients still rely on self-monitoring of blood glucose (SMBG), which typically involves measuring the glucose concentration in a drop of blood taken from the fingertip, using a glucose meter or glucose strips. This introduces a range of potential errors that could impact on the reliability of the reading. It should be remembered that reliability of glucose readings can be divided into two components: accuracy and precision. Accuracy refers to how closely a reading matches the laboratory reference value, while precision denotes the degree of reproducibility in multiple readings.
It has been estimated that 90% of deviant glucose readings are “operator dependent.” The blood sample may be too small for some meters to record or properly report the error.2 The patient may have anaemia, which could affect the reading in glucose meters that assume normal haematocrit in their calculations. Deviations of up to 20% may occur when blood is taken from the forearm and thigh, rather than the fingertip.3 Taking measurements at extreme temperatures may likewise skew the results. Although rare, acetaminophen, ascorbic acid and mannitol can impact readings from meters that use the glucose oxidase method. Glucose strips, if used, may lose accuracy when exposed to light or moisture.
Such circumstantial errors aside, hand-held instruments do not equate to clinician-grade laboratory instruments. This is why SMBG cannot be used to diagnose diabetes, particularly type 2. Some glucose meters do not adequately sense hypoglycaemia, particularly for asymptomatic events in the ICU setting.4,5 Of note, glucose meters that err on the side of underreporting glucose values are associated with a higher HbA1c than those that deviate in the other direction.6
There are literally dozens of glucose meters available. For market introduction, at least in Europe and the US, glucose meters need to comply with the new ISO15197:2013 standard for SMBG systems.1 This standard states that accuracy should be within 15% at glucose values > 4.2 mmol/L (75 mg/dL) and within ≤ 0.8 mmol/L for lower glucose values, for 99% of all results. This is considerably more stringent than the 2003 standard, which allowed an accuracy error of no more than 20% in at least 95% of measurements. That said, even glucose meters that meet the current standard may differ considerably in accuracy and precision, with clinically significant consequences around the boundary of hypoglycaemia.
For example, a value of 3.2 mmol/L can vary between 2.4 and 4.0 mmol/L without falling outside the ISO standard’s accuracy limits. Such deviations may lead patients to miscalculate the amount of insulin they actually require. A bias of just 10% can lead patients to misdose their insulin by 2 units in half of cases.7 If they overestimate their insulin requirements, they may tip into the hypoglycaemic range.
Proper patient education can help minimize, if not fully eliminate, such risks. Helpful SMBG strategies include:
- Always using the same glucose meter
- Washing hands with water and soap (not alcohol) prior to taking a reading
- If hand washing is not feasible, wipe off the first drop of blood and use the second drop of blood
- In extreme weather conditions, wearing glucose meters close to the body and conducting measurements indoors, if possible
- Using modern glucose meters that make quick measurements that can be electronically converted to bolus insulin calculations
Finally, although continuous glucose monitoring or flash glucose monitoring have many benefits over SMBG, it should be acknowledged that they may be less reliable in the extremes of the glucose spectrum. This means that patients are advised to always check a low or (extremely) high glucose value reported by such a device with a SMBG.
- ISO 15197:2013. Accessed Nov. 12, 2017 at https://www.iso.org/standard/54976.html
- Pfützner A et al. Evaluation of the effects of insufficient blood volume samples on the performance of blood glucose self-test meters. Diab Sci Technol 2013;7:1522.
- Bina DM et al. Clinical impact of prandial state, exercise, and site preparation on the equivalence of alternative-site blood glucose testing. Diabetes Care 2003;26:981-5.
- Sonmez A et al. The accuracy of home glucose meters. Diab Tech Ther 2010;12:619.
- Voulgari C et al. Accuracy and precision of glucose monitoring are relevant to treatment decision-making and clinical outcome in hospitalized patients with diabetes. Diab Tech Ther 2011;13:723.
- Boettcher C et al. Accuracy of blood glucose meters for self-monitoring affects glucose control and hypoglycemia rate in children and adolescents with type 1 diabetes. Diab Tech Ther 2015;17:275.
- Boyd JC, Bruns DE. Quality specifications for glucose meters: assessment by simulation modeling of errors in insulin dose. Clin Chem 2001;47:209-14