BLOOD GLUCOSE TEST STRIPS - Problems and fixes!! PART III

Methods of electrochemical analysis

All EBGTS (Electrochemical blood glucose test strips) produce a time varying current  which must be analyzed to deduce a glucose concentration. 
However, the method of analysis varies substantially, depending upon the strip design. 
In the broadest sense, the analysis can be either 
1. amperometric, in which the instantaneous current at a particular time is proportional to glucose, or 
2. coulometric, in which the integrated charge underneath the entire curve is proportional to glucose.


Amperometric strips predominate among electrochemical test strips. Typically the current is measured 5-15 seconds after the strip is filled with blood, and that current level is directly proportional to the glucose concentration. There are, however, many variations on this theme. For example:

1. Current can be measured and averaged over a short (a few seconds) interval.
2. Peak current can be measured.
3. A mathematical analysis of the declining portion of the current-time curve (termed a Cottrell analysis) can be performed.

In general, strips designed for amperometric analysis share some features:

1. The surface area and surface roughness of the working electrode must be highly reproducible to insure that current-time curves are reproducible from strip to strip.
2. The electrode surface-area/sample-volume ratio is low, so that glucose is not substantially depleted in the strip during the analysis.

 Enzymes used in the strips

Table I. Commonly used enzymes for EBGTS
Enzyme	Reacts with O2?	Reacts with maltose?
Glucose Oxidase	Yes	No
PQQ-Glucose Dehydrogenase	No	Yes
NAD-Glucose Dehydrogenase	No	No
FAD-Glucose Dehydrogenase	No	No

Four enzymes are commonly used in these strips






Glucose oxidase was first employed in EBGTS, and is still used in a few commercially available test strips, although its use is declining, since it reacts rapidly with oxygen. EBGTS made with glucose oxidase produce currents that are dependent on the amount of dissolved oxygen in the blood sample, which is heavily dependent on the hematocrit (% red blood cells) in the sample.
Glucose oxidase has been largely supplanted by the dehydrogenases, which do not react appreciably with oxygen. (In the table above: PQQ, NAD, and FAD refer to the electron transferring cofactors, which differ between each of the distinct enzymes.) However PQQ-GDH does react with many non-glucose sugars, particularly maltose, which can be elevated in patients undergoing peritoneal dialysis. For this reason, use of PQQ-GDH is decreasing, and the FDA has recently restricted its use in new commercial products.
Both NAD-GDH and FAD-GDH combine oxygen rejection with high specificity for glucose. Their use in EBGTS is increasing. Enzymes typically are deposited on at least the working electrode, and sometimes on one entire side of the sample chamber, and are codeposited with buffers and stabilizing agents, such as bovine serum albumin.

Copied by permission from the "Electrochemistry Dictionary" (http://electrochem.cwru.edu/ed/dict.htm) on "04/09/12." The original material is subject to periodical changes and updates.