Coulometry vs. Amperometry in Biosensors

Simplifying [Diabetic Test Strips](/sell-diabetic-supplies/): Coulometry vs Amperometry

Electrochemical test strips turn glucose into a number in one of two ways: Amperometry or Coulometry. Both start the same way. An enzyme on the strip reacts with glucose and frees electrons, and a mediator carries them to the meter [1][2]. The difference is what the meter counts.

• Amperometry measures the electrical current, the rate at which those electrons arrive. FDA's decision summary for the OneTouch Verio system describes a current proportional to the glucose in the sample [2].
• Coulometry measures the total charge, every electron released over the reaction time. For FreeStyle Lite strips, that charge is directly proportional to the glucose in the sample [1].

Counting the total charge doesn't by itself make a reading more accurate. The advantage Abbott's researchers claim for it is narrower, and the next section explains it.

How They Work

An amperometric strip reads the current a few seconds after the blood fills it. In textbook electrochemistry that current follows the Cottrell equation, which ties it to how fast molecules diffuse toward the electrode, and diffusion shifts with temperature and with hematocrit (the share of blood made up of red blood cells). A FreeStyle strip works differently. It holds a small, tightly controlled volume of blood between two facing printed electrodes about 50 micrometers apart and oxidizes the glucose in that volume [3]. Because the meter counts all of the charge rather than a rate, Abbott researchers describe coulometry as less sensitive to temperature and hematocrit [4]. The developers' 2000 paper on the original FreeStyle reported readings unaffected by temperature, altitude and hematocrit from 0% to 60% [5].

Here's what surprised us: in FDA records the hematocrit gap between the two methods is small. FreeStyle Lite, a coulometric strip, supports a hematocrit range of 15% to 65% [1]. The amperometric OneTouch Verio claims 20% to 60% [2]. Roche's amperometric Accu-Chek Guide is labeled for 10% to 65% [6]. If your hematocrit runs outside the range on your strip insert, which meter suits you is a decision for your care team.

Revolutionizing the Industry

TheraSense, now part of Abbott Diabetes Care, built its FreeStyle system around a tiny coulometer, and FreeStyle reached the market in 2000 [3]. Its strip needed 300 nanoliters of blood (0.3 µL), small enough to draw from the forearm, which the developers reported was significantly less painful than a fingertip [5]. In 2009, the FreeStyle system was used for more than one billion blood tests [3].

LifeScan and Roche went with amperometry: FDA decision summaries describe both the OneTouch Verio and the Accu-Chek Guide as amperometric systems [2][6]. Small samples didn't stay exclusive to coulometry. The Verio strip takes 0.4 µL and reports in 5 seconds, the same measurement time FDA lists for FreeStyle Lite [2][1].

Key Takeaways

• Coulometry: counts total charge from a small, fixed blood volume. Abbott's researchers describe it as less sensitive to temperature and hematocrit [4], and it let FreeStyle work from a 0.3 µL sample in 2000 [5][3].
• Amperometry: measures current, and it's the method behind LifeScan's OneTouch Verio and Roche's Accu-Chek Guide [2][6]. The amperometric strips we checked match FreeStyle Lite on test time and come close on sample size and labeled hematocrit range [1][2][6].