Environmental Interference Correction

Environmental Interference Correction in [Diabetic Test Strips](/sell-diabetic-supplies/)

A blood glucose reading is a chemical reaction timed by a meter, and the air around you takes part in it. Like every biochemical process, the reaction on a test strip is influenced by temperature, so readings from current strips depend on it [1]. Humidity and altitude matter as well, and the FDA asks makers of over-the-counter meters to test all three [2]. Most of the correction happens inside the meter. It measures the conditions, then either adjusts the result or refuses to test [1][3].

This article covers the moment you test. How heat and moisture age strips in the vial is a separate story, told in our piece on environmental stability and packaging engineering.

What the FDA asks meter makers to prove

The FDA's final guidance for over-the-counter blood glucose systems, dated September 2020, describes an "operating conditions" study [2]. It should mirror real use, cover the temperature and humidity range printed on the label, and push the system past that range. The guidance names four corners to test: hot and dry, cold and humid, hot and humid, cold and dry. Each result is compared with the same meter at a nominal condition such as 23 °C (73 °F) and 40% relative humidity, or RH, a measure of how close the air is to saturated with water [2].

Altitude gets its own study. The FDA points out that high altitude is a bundle of changes, not one: extremes of temperature and humidity, plus shifts in blood pressure and in hematocrit, the share of blood volume made up of red cells [2]. Makers should test at 10,000 feet or higher with an actual change in air pressure, real or simulated in a chamber [2]. Lowering oxygen alone doesn't count, and "sea level" can't be defined as anything above 500 feet [2].

Operating ranges differ by meter

Those studies become the operating range in each meter's labeling, and FDA clearance records show a real spread:

• Accu-Chek Guide (cleared 2016): 43 to 113 °F (6 to 45 °C) at 10 to 90% RH; testing at 10,150 feet showed no significant effect [4].
• Contour Next One (cleared 2016): 41 to 113 °F (5 to 45 °C) at 10 to 93% RH; simulated-altitude testing supported a claim up to 20,674 feet [5].
• FreeStyle Lite (cleared 2010): usable from 4 to 40 °C (39 to 104 °F) at 5 to 90% RH; results up to 10,000 feet stayed within the sponsor's ±10% limit against sea level [6].
• Accu-Chek Aviva Plus (cleared 2011): 14 to 40 °C (57 to 104 °F) at 10 to 80% RH; a glove-box chamber simulating 10,000 feet showed no significant effect [7].

Here's what surprised us: the Contour Next One's altitude claim is more than double the 10,000-foot minimum the FDA asks for [5][2]. Its user guide says the system has not been tested above that height [8].

How a meter corrects for temperature

Roche's FDA record for the Aviva Plus explains its method step by step. Once blood reaches the strip, the meter sends small alternating-current (AC) signals at four frequencies, and the responses carry information about the sample and the surrounding temperature. A steady direct-current (DC) signal then produces a current in proportion to the glucose, and the meter combines the AC and DC information into a result compensated for hematocrit and temperature [7]. The 2016 Accu-Chek Guide record describes the same idea in one line: the sample and environmental conditions are evaluated using AC and DC signals [4]. Our article on AC impedance and multi-pulse technology covers the signals in more depth.

Other meters lean on a thermometer. A 2016 review notes that many meters carry a built-in temperature sensor and use its reading to correct the result. The catch is placement: the sensor usually sits inside the meter housing, not at the strip where the reaction happens, and the two can be at quite different temperatures [1]. Most meters also report an error at extreme temperatures instead of guessing [3]. On the Contour Next One, that means code E27 for "Too Cold to Test" or E28 for "Too Hot to Test" [8]. The same guide lists E6 for a moisture-damaged strip, though it doesn't say how the meter detects one [8].

What independent studies found about temperature

A 2011 Norwegian study held nine systems at 5 °C or 30 °C for an hour, then moved them to room temperature [9]. Tested right away, six of the nine read more than 5% high or low. Testing after an hour near the low and high ends of their operating ranges caused fewer problems: two systems read low at 10 °C, and at 39 °C one read high and another low [9].

A 2015 study pushed harder, heating five meters to 50 °C or chilling them to −5 °C for an hour [10]. They took 1 to 8 minutes to start working again and 15 to 30 minutes to give stable readings. Stressing only the strips had little effect; stressing the meters added bias for the first 15 minutes and not after. The authors, most of them employees of AccuNostics Ltd, concluded that the built-in compensation performed well [10].

Not every study is reassuring. In a 2007 pilot study, eight systems spent 50 days in an environmental chamber that failed and drifted between 54 and 87 °F and 49 to 100% humidity [11]. Temperature, and to a lesser degree humidity, affected nearly every system, and the authors judged them unreliable even inside the makers' recommended limits [11]. That study used control solution rather than blood, and the 2016 review cautions that much of this literature tested older meters [11][1].

What studies found about altitude

The altitude evidence doesn't point in one direction. The usual explanation involves oxygen: on glucose oxidase strips, oxygen competes with the mediator, the chemical that carries electrons to the electrode, so low oxygen can push a reading high; strips that use glucose dehydrogenase are less affected [3][1]. In a high-altitude climbing study summarized in a 2009 review, glucose oxidase meters read 6% to 15% high while the glucose dehydrogenase meters stayed within 5% [3].

Other studies disagree on direction and cause. On Mount Rainier, a 2002 field study of seven meters found readings ran about 1% to 2% low for each 1,000 feet climbed, before adjusting for other factors, and temperature and humidity also affected accuracy [12]. A 2010 study of nine meters found that low oxygen pressure at simulated altitude didn't affect even the glucose oxidase meters, though most meters tended to read high at altitude [13]. At a simulated 8,000 feet, the cabin altitude of a commercial airliner, a 2012 study found no difference between the two enzyme types, and its authors judged that none of the errors would have led to a dangerous treatment mistake [14].

What the label can and can't tell you

The operating range on a meter's label is the set of conditions its maker proved in the studies the FDA reviewed [2]. Staying inside it isn't a guarantee, since the 2007 pilot found errors within the makers' recommended limits [11]. We also couldn't find an FDA record that explains how much weight a given meter gives its temperature reading, and the newest independent study we cite is from 2015 [10]. If a reading doesn't fit the situation after a big change in temperature or altitude, what to do about it is a decision for your care team.