Over the past three decades, self-monitoring of blood glucose (SMBG) with finger-stick blood samples, test strips, and portable meters has revolutionized diabetes management. These glucose meters have evolved to become more user-friendly and efficient, providing rapid results with minimal blood samples. Consequently, they are now employed extensively in hospitals, intensive care units, dialysis units, and infusion centers to offer point-of-care results. This advancement has significantly streamlined diabetes management, but it is crucial to remain aware of the limitations of finger-stick glucose, including precision and selectivity.
Precision
While no universally accepted standard exists, the International Organization for Standardization (ISO) guidelines are widely recognized. ISO guideline 15197 recommends that for glucose levels below 75 mg/dl, a meter should read within 15 mg/dl of the reference sample, and for levels equal to or above 75 mg/dl, the reading should be within 20%. Meters should meet these targets for at least 95% of the samples tested.
Various scenarios highlight the consequences of this level of imprecision. For example, an actual glucose level of 55 mg/dl could produce an SMBG reading between 40 and 70 mg/dl. This could be especially dangerous for patients with hypoglycemia unawareness. Conversely, a genuine value of 350 mg/dl might display readings between 280 and 360 mg/dl. In critical care situations, the error tolerance limit for bedside glucose testing should be 5 mg/dl.
Educating patients and providers about the potential inaccuracy of glucose meters is essential. Factors that can impact accuracy include an insufficient amount of blood on the strip, expired or improperly stored strips, and incorrect code entry.
Selectivity
The gold standard for enzymatic glucose concentration measurement in clinical laboratories is based on hexokinase. Test-strip systems employ enzymes such as glucose oxidase, glucose dehydrogenase nicotinamide adenine dinucleotide (GDH-NAD), GDH flavin adenine dinucleotide (GDH-FAD), and GDH pyrroloquinolinequinone (GDH-PQQ).
Glucose oxidase-based sensors are more substrate-specific than GDH-based sensors; however, oxygen can negatively influence the results from glucose oxidase-based sensors. While GDH-FAD and GDH-NAD strips do not exhibit cross-reactivity with sugars other than glucose, GDH-PQQ is nonspecific. GDH-PQQ-based sensors can misinterpret maltose, galactose, and xylose as glucose, which holds clinical significance in certain situations.
The FDA advises against using GDH-PQQ glucose test strips in healthcare facilities and cautions against their use in patients receiving interfering products. A potential solution is the utilization of mutant forms of GDH-PQQ with amino acid substitution, which maintains enzymatic activity for glucose but reduces reactivity for other sugars.
In conclusion, while glucose meters have revolutionized diabetes management, it is essential to understand and address their limitations in terms of precision and selectivity. This will ensure that patients and healthcare providers make well-informed decisions and use the technology safely and effectively.