Not everyone is aware that the results of blood glucose readings obtained by continuous glucose monitoring systems (CGM) may be affected by the use of certain medications.

In particular, overestimation of actual glucose levels can lead to inappropriate insulin administration and the development of hypoglycemia. One such substance is the widely used acetaminophen (paracetamol).

How can acetaminophen alter the glucose readings with CGM ?

Strictly speaking, this can happen not only when measuring glucose with CGM, but also in any other case when glucose is oxidized. This creates a current in the circuit proportional to the concentration of glucose in the medium surrounding the electrode.

The chemical structure of acetaminophen also allows it to undergo an oxidation reaction on the surface of the electrodes, leading to a false increase in glucose level in test.

Although there are some general similarities in the structure of these substances, this does not mean that the enzymes in biological systems that process glucose can also process acetaminophen. The only thing they have in common is that both substances can be oxidized at the electrode during test or sensor readings.

A dose-dependent effect of acetaminophen on falsely elevated glucose levels was observed in 1982 using the YSI analyzer (Yellow Springs Instrument). Acetaminophen had no effect on the results of a strong>Beckman analyzer which used a different photometric/ion-selective method.

• the method underlying the chemical reaction. In descending order of influence:   
  Oxidation (GOx) > Glucose dehydrogenase (GDH-PQQ)>   
  GDH-FAD / GDH-NAD
• acetaminophen concentration in blood. For example, very high concentrations in a suicide attempt can lead to measurement error during a point-of-care glucose check done by Nova StatStrip® glucose meter. Althouth it's specially designed to be resistant to common interfering substances, including acetaminophen.

The oxidation method is rarely used in modern home blood glucose meters. For example, the CONTOUR glucose meters from Bayer, are based on the (GDH)/flavin dinucleotide (FAD) method. This makes it resistant not only to the influence of oxygen in the reaction, but also to presence of maltase, which is important for patients on dialysis with the carbohydrate polymer icodextrin.

Let us now turn directly to glucose monitoring systems 

The above applies to them to a greater extent than to modern glucometers.

In general, acetaminophen is metabolized quite rapidly to form conjugates, so a high initial dose is necessary to distort the results of glucose values: when dosed over 1000 mg for 6 hours per os or intravenously. The maximum peak is reached in 1-1.5 hours from the time of drug administration.

In these cases, additional glucose monitoring with a glucometer is recommended when using the Medtronic Guardian and Dexcom G4 and G5 sensors.

Some modern CGM systems have additional protection against this phenomenon. Sensoren Dexcom 6, Dexcom 7 and FreeStyle Libre 3 are quite resistant to it.

In addition to acetaminophen, there are other drugs and chemicals that can falsely overestimate glucose levels when measured by the oxidase method. These include vitamin C, uric acid, levodopa, and the earlier mentioned Icodextrin (dialysis fluid).

Therefore, in the case of regular intake of these drugs by patients using glucose monitoring or dialyzing with Icodextrin, it is useful to read the device instructions and find out how they can alter sensor readings. This is usually provided by the manufacturer in the documentation or is available online.