Abstract
To the Editor:
Continuous glucose monitors (CGMs) offer improved diabetes management for over 9 million patients worldwide.1,2 Introduction of new over-the-counter (OTC) CGMs in 2024–2025 is anticipated to increase use among an additional 25 million patients with diabetes and among non-insulin-dependent diabetes and prediabetes patients to support behavior modification.2-4 Increased access will also likely increase CGM-related allergic contact dermatitis (ACD) and the need for clinical guidance. CGM adhesives are proprietary, thus limiting patient-physician transparency. However, independent identification of acrylates and colophony derivatives in specific models has allowed for improved device selection for sensitive patients. 5 Allergens in available OTC CGMs have not been reported to date.
Gas chromatography/mass spectrometry analysis was performed on whole-sensor, acetonic extracts from the Dexcom Stelo (San Diego, CA) and Abbott Lingo (Chicago, IL) devices. Recent Dexcom G7 sensors were also analyzed for comparison. Spectra were identified using the National Institute of Standards and Technology Mass Spectral Library (NIST08) and cross-referenced with available CGM data. Match scores >700 were deemed to be a reasonable match, particularly when identified as a previously reported CGM allergen. Table 1 depicts identified, known allergens as well as additional identified compounds with unknown sensitizing potential. Spectra from the Dexcom Stelo and G7 extracts were found to be identical.
Identified Compounds in the Abbott Lingo and Dexcom Stelo, and G7 Sensors
Abbott Lingo (ART 43407–301 Rev. A) and Dexcom Stelo (LBL-1005733 Rev001) and G7 (LBL-1003893 Rev_006) models were analyzed using the Agilent 7820A-5975 GC-MSD system and Agilent J &W 128–1012 column. Compounds were identified using the National Institute of Standards and Technology Mass Spectral Library and Compound Scoring systems and cross-matched with previously reported allergens. Spectral matches to library compounds are indicated using a Match Factor score wherein a score of 999 indicates a perfect match (>900, Excellent Match; 800–900, Good Match; 700–800, Fair Match; <600, Poor Match). Identified compounds, match factor scores, and associated CAS identification numbers are described. Relevant allergens refers to identified compounds known to cause ACD in previously reported CGMs or insulin pumps. Possibly relevant allergens indicate compounds or derivatives of known CGM allergens. Other identified compounds refers to additional compounds identified in these CGMs with unknown sensitizing potential.
We have listed all spectra that were identified, regardless of Match Factor for completeness and future reference of emerging allergens. All compounds identified as relevant or possibly relevant share both fair to excellent match scores as well as relevance in prior CGM-related ACD. Additional compounds with unknown sensitizing potential and match scores <700 likely carry limited relevance.
2,3,4,5-tetramethyl-6-phenylpyridine (TM6PP) was a close spectral match to the detected compound, however it was noted to have an additional peaks at m/z 281 and 337, suggesting a structural similarity to TM6PP but unlikely match.
3,3′,5,5′-Tetra-tert-butylstilbene 4,4’-quinone has been reported as a breakdown product from Butylated Hydroxytoluene (BHT), an antioxidant and allergen previously reported in CGMs. The NIST08 library includes BHT for identification, however this compound was not identified in any sample.
The Dexcom Stelo and Abbott Lingo models do not share common allergens. The primary allergen in the Stelo is 1-hydroxycyclohyexyl phenyl ketone, a photoinitiator previously reported to cause ACD related to UV-curing adhesives used in insulin infusion needles. 5 Stabilizing agent P-p-dioctyldiphenylamine and oxidation product 3,3′,5,5′-Tetra-tert-butylstilbene 4,4′-quinone was also identified. Known allergens in the Lingo include the adhesive component, 4,4-Methylene diphenyl diisocyanate, and 2,4 di-tert-butylphenol (2,4-DTB). Bisphenol A was also identified, which may represent plastic components used in the CGM housing or our analytical materials. As this compound was not found in other samples, it may also represent a more specific breakdown product, such as from an epoxy-based resin or the known allergens Bisphenol A dimethacrylate. AAcrylate ester, 2-(prop-2-enoyloxy) tridecane, was also identified in this sample, which may suggest the presence of an additional acrylate additive. Further identified compounds of possible future interest include the monomer, caprolactam, and the stabilizer p-tert-butylphenol, which is structurally similar to butylated hydroxytoluene and (2,4-DTB).
As allergens between OTC models do not overlap or cross-react, patients experiencing ACD with one of these models may benefit from a trial of the alternative brand. Patch testing with these allergens may also support CGM selection. Our findings demonstrate identical allergen profiles between Dexcom OTC and prescription models; however, these data are incongruent with prior G7 analyses. Clinically, we have observed tolerance of newer G7 models among formerly sensitized patients, suggesting that adhesive systems may have been updated. Future comparison of G7 revisions may help to confirm this assertion. It is unclear whether Abbott’s Freestyle Libre 3 shares allergens with its OTC models. We were unable to obtain FreeStyle Libre models for analysis; however, currently available models are planned for discontinuation and re-release in Fall 2025. Future work should include these models, which have not been reported to date.