Mediator Chemistries (Ferrocene vs. Osmium vs. Ruthenium)

Mediator Chemistries (Ferrocene vs. Osmium vs. Ruthenium)

A mediator is the molecule on an electrochemical test strip that carries electrons from the glucose enzyme to the electrode, where the meter counts them [1]. Side by side, the families differ on four practical questions: what voltage the meter has to apply, what else in blood can leak into the reading, how much oxygen gets in the way, and how carefully the strips must be packaged. Ruthenium and osmium complexes work at lower voltages than potassium ferricyanide, the most common mediator in commercial strips, and a lower voltage generally means fewer interferences [2] [3]. Ferrocene looks good on paper [1], but we didn't find a current FDA strip record that names it.

How mediators replaced oxygen in the first sensors is covered in our article on electron mediator technologies. Here we stick to how each family behaves once it's printed on a strip.

Operating potential sets the interference risk

The operating potential is the voltage the meter applies to the strip to pull electrons back off the mediator. Anything in the blood sample that also gives up electrons at that voltage adds current that didn't come from glucose, so the reading comes out falsely high [1]. A mediator with a lower redox potential (the voltage at which it trades electrons) lets the strip run at a lower operating potential. That single number is why chemists keep looking past ferricyanide [2].

In the sources we read, every value is measured against a silver/silver chloride (Ag/AgCl) reference electrode.

Abbott's authors say the low 40 mV potential "minimizes the oxidation of electroactive species at the working electrode and thereby minimizes susceptibility to interferents" [3]. The FreeStyle strip developers describe a different reason for their osmium choice. The strip's two electrodes face each other only 50 micrometers apart, and the negative potential keeps the mediator from shuttling electrons back and forth between them, which would corrupt the count [4].

One strip, two mediators, two interference lists

The cleanest real-world comparison we found sits in FDA's files on True Metrix strips. In August 2014 FDA cleared a change from hexaammineruthenium(III) plus a second mediator, Meldola's Blue, to potassium ferricyanide [5]. Before the switch, the strip listed one interferent, xylose. After it, uric acid and ascorbic acid (vitamin C) joined the list, which the decision summary calls "the result of a mediator change" [5].

The new labeling put numbers on it: ascorbic acid may interfere above 2 mg/dL and uric acid above 5 mg/dL [5]. That is what the chemistry predicts when a strip moves to a mediator with a higher potential [2]. Our article on electrochemical interferences covers other common culprits.

A lower potential doesn't settle everything, though. In the 2017 study, ruthenium paired with a bacterial FAD-dependent glucose dehydrogenase (FAD-GDH) picked up more bias from maltose than ferricyanide did: +39% versus +18% at 20 mM maltose [2]. Maltose isn't normally in blood, but some intravenous drugs contain it [2]. The same study found that ruthenium couldn't take electrons from a fungal FAD-GDH at all [2]. A mediator performs only as well as its match with the enzyme.

Oxygen is mostly an enzyme question

A good mediator shouldn't react with oxygen [2], and ferrocenes pass that test [1]. In a real strip, though, oxygen sensitivity depends more on the enzyme. With glucose oxidase (GOx), oxygen from red blood cells can compete with the mediator for the electrons glucose gives up [1]. GDH doesn't react with oxygen, one reason it suits mediated strips [2].

So the mediator name alone says little about oxygen. The 2017 study notes that hexaammineruthenium(III) is used in some commercial GOx strips [2]. The ruthenium-era True Metrix strip, by contrast, used a fungal FAD-GDH [6], and FreeStyle's osmium strips used GDH as well [4]. The enzyme listed in a strip's labeling is the better clue.

Stability, packaging and shelf life

Every mediator has to survive on the strip in both its oxidized and reduced forms [2]. Ferrocenes are stable in both [1], and hexaammineruthenium(III) chloride is "sufficiently stable" in both [2]. Storage is where the families split. Strips made with potassium ferricyanide need special packaging that keeps them dry and dark. Ruthenium strips have less stringent packaging requirements, which the 2017 authors put down to the ruthenium compound's higher storage stability [2].

We found one product where FDA records let us line up shelf-life claims for each mediator. The original True Metrix strip, cleared in April 2013, claimed a 24-month shelf life and a 4-month open-vial life at 40 to 86°F and 10 to 85% relative humidity [6]. A modification later that year added event tags and widened the operating temperature without changing the underlying technology [7], and FDA's 2014 summary names that version's mediators as hexaammineruthenium(III) and Meldola's Blue [5]. The ferricyanide strip cleared in 2014 claimed 21 months, with the same open-vial life and storage range, based on accelerated studies while real-time studies continued [5].

FDA's summaries don't say the mediator caused that three-month gap, and the 2014 figure rested on accelerated testing, but it points the same way as the lab study [2]. Our article on environmental stability and packaging goes further into vials and desiccants. As of October 2026, FDA recommends that all True Metrix users move to another way of testing when possible, because of a meter error-code problem rather than the strip chemistry [8]. Whether and when to switch is a decision to make with your care team.

Osmium's published stability data is about something else. In Abbott's Wired Enzyme sensors, worn on the body, median sensor sensitivity stayed constant from day 2 through day 14, which the authors read as stable sensor chemistry [3]. That's stability during wear, not on a shelf, and we didn't find shelf-life figures tied to osmium strips.

Where each family shows up

Ferricyanide is the most common mediator in commercial strips, according to the 2017 study [2], and it has been in True Metrix strips since the 2014 change [5]. Ruthenium was "gaining attention" for commercial strips in 2017 [2], and it was in True Metrix strips from their 2013 clearance until that switch [5] [6] [7].

Osmium went into FreeStyle strips, which reached the market in 2000, by the developers' own account [4]. Abbott also used an osmium mediator in the Wired Enzyme chemistry of its earlier FreeStyle Navigator CGM and of the FreeStyle Libre sensor as Abbott described it in 2017 [3]. We couldn't confirm from public records that the FreeStyle strips sold as of October 2026 still use the same complex.

Ferrocene has the longest pedigree and the thinnest current paper trail. MediSense's ExacTech, the first electrochemical meter for home testing, used a ferrocene derivative in 1987, and a 2010 review said various self-monitoring strips used ferrocene or ferricyanide [1]. Which strips, if any, use ferrocene today is still an open question in the sources we could reach.