Enzyme Engineering and Stability

Enzyme Engineering and Stability in [Diabetic Test Strips](/sell-diabetic-supplies/)

The enzyme on a test strip is a protein, and proteins lose their shape and their activity with heat and moisture. Strip makers fight that on several fronts. Protein engineers have built enzyme variants that survive heat better, strips carry more enzyme than a reading needs, and the vial or foil packet keeps humidity out. Before a strip reaches a pharmacy shelf, FDA guidance asks its maker to show how long it stays accurate, both sealed and after the vial is opened [1].

Why a strip enzyme needs protecting

Here's the contrast that put the problem in perspective for us. A 2017 review of glucose oxidase, one of the main strip enzymes, lists its half-life at about 30 minutes at 37 °C (body temperature), and says a freeze-dried preparation stays stable for at least 6 months only when kept at −20 °C [2]. A strip has to do far better than that at room temperature. When the FDA cleared OneTouch Verio strips, LifeScan claimed a 12-month shelf life unopened and 3 months after first opening, stored between 5 and 30 °C (41 to 86 °F) [3].

That durability has limits. A 2009 review of meter errors describes strips as fragile, with a finite life of about 2 years under ideal storage. High temperature, high humidity, or leaving the vial open so moist air reaches the strips can all shorten it [4].

Engineering a sturdier enzyme

Two strips can carry enzymes with the same name and still behave differently. A 2011 review of glucose-sensing enzymes makes that point directly: an enzyme's properties depend on the organism used to produce it, on engineered mutations, and on chemical modifications, not only on its family name [5]. For PQQ-dependent glucose dehydrogenase (PQQ-GDH, an enzyme that uses the helper molecule pyrroloquinoline quinone), the same review says protein engineering has improved both how selectively it reacts with glucose and how stable it is [5].

The individual studies show how small those changes can be. In 2000, a Japanese research group swapped a single amino acid in soluble PQQ-GDH and got a variant whose half-life at 55 °C was more than 8 times that of the natural enzyme, with similar activity toward glucose [6]. A 2004 paper from the same lab went after a different weakness. It changed amino acids at the enzyme's active site to make it pickier about glucose, and the best variant kept most of its activity and its stability [7]. That balance is the hard part: a mutation that fixes one property can cost another.

Engineered enzymes do reach strips. Roche's Accu-Chek Aviva Plus strips, cleared in 2011, use what the FDA review calls a modified (mutant) GDH-PQQ, described as "a variant of glucose dehydrogenase" [8]. The review doesn't say which mutations it carries, and we found nothing tying it to the research variants above. For glucose oxidase, the 2017 review reports that rational redesign and directed evolution (making many random variants and screening for the best) improved its stability and catalytic function, mostly with industrial uses in mind [2]. Why strips moved between enzyme families is its own story, covered in our article on GOx versus GDH enzyme chemistries.

The recipe around the enzyme

Strip makers rarely publish their full reagent recipe. The FDA review of OneTouch Verio strips lists an FAD-dependent glucose dehydrogenase from Aspergillus, the mediator potassium ferricyanide, and then simply "other ingredients (buffer, etc.)" [3]. Research on glucose oxidase gives a sense of what such ingredients can do: polyhydric alcohols such as glycerol, sorbitol and polyethylene glycol have shown stabilizing effects, and binding the enzyme to a surface (immobilization) has improved its stability in several studies [2]. Whether any given strip uses those additives isn't in the public record we read.

Another safeguard is quantity. Ginsberg's review explains that strips usually carry excess enzyme and mediator, so a moderate loss doesn't change the reading. Bare spots are different: if part of the sample well loses its enzyme coating, the strip reads low [4]. The same review notes that the mediator breaks down too, and calls mediator reduction a common problem in outdated or poorly stored strips [4].

How shelf life gets proven

The FDA's 2020 guidance for over-the-counter glucose meters asks for two stability studies. A closed-vial study supports the expiration date. An open-vial study mimics real use, with the vial opened and closed throughout its claimed open life. Both should span the labeled temperature and humidity ranges at several time points, and a mix of real-time and accelerated testing is acceptable [1].

Here is what the FDA reviews we read record for current strip lines. These are the claims at clearance; the vial label is the current word.

• OneTouch Verio: 12 months unopened, 3 months after opening, 5 to 30 °C [3]
• OneTouch Ultra Plus: 12 months closed, 6 months open, 41 to 86 °F at 65% relative humidity, with real-time studies still running at clearance [9]
• Accu-Chek Guide: up to 18 months closed and open, 4 to 30 °C and 10 to 90% humidity; labeling says not to freeze the strips [10]
• Accu-Chek Aviva Plus: 18 months closed and open, 2 to 30 °C [8]
• Contour Next: 24 months shelf life and open-vial life, 5 to 30 °C and 10 to 80% humidity [11]
• FreeStyle Precision Neo: 18 months; each strip is sealed in its own foil packet, so there is no open-vial claim [12]

One comparison surprised us. When Abbott moved FreeStyle Lite strips from GDH-PQQ to GDH-FAD in 2010, the FDA review listed the new strip's shelf life as 12 months, against 18 months for the GDH-PQQ version it replaced [13]. The review doesn't explain the gap.

When storage goes wrong

Strips don't all fail the same way. Ginsberg's review shows strips kept at 40 °C (104 °F): after 9 months, samples near 110 mg/dL were reported reading as high as 300 mg/dL, and depending on the brand, a failing strip can read either too high or too low [4].

Two storage studies fill in the details. In a 2005 test of one maker's point-of-care strips, capped vials held up longer than uncapped ones overall. Open vials left in humidity or direct light failed control checks within 3 to 14 days, and the authors concluded the results support storing strips capped in the original vial at room temperature [14]. In a 2023 study from Thailand, the five health centers monitored averaged above 30 °C and 84% relative humidity. Strips stressed under five simulated conditions for 3 or 30 days gave results that differed significantly from unstressed strips on all four meters tested [15].

All of that engineering reaches the user as a few lines on the label: the expiration date, the open-vial limit, and the storage range. Our article on environmental stability and packaging engineering covers the vial and desiccant side. If strips were stored outside their labeled range, the insert and the manufacturer can say whether they are still within claim, and what a doubtful reading means for your care is a question for your care team.