CGM Accuracy Comparison (New Study)

This seems a telling aspect, but worth it to read the rest a bit deeper…

Stratification by sensor type revealed that mean GMI and HbA1c were similar in rtCGM users ( n = 341 CareLink Pro/Personal, n = 64 Dexcom G5; 7.6 ± 0.7% vs. 7.6 ± 1.1%), whereas iscCGM users ( n = 1,568 FreeStyle Libre) had higher mean GMI than HbA1c (7.9 ± 0.9% vs. 7.6 ± 1.2%). Similar patterns were observed after stratification by glucose variability or weight while absolute differences between GMI and laboratory HbA1c were almost symmetrically distributed around 0 in rtCGM users, GMI was higher than the laboratory HbA1c value in almost three-fourths of iscCGM users. This finding was still observed after additional stratification by age-group (data not shown). Stratification by HbA1c level revealed higher GMI than HbA1c in most individuals with HbA1c <7.5%, with GMI-HbA1c differences being higher in iscCGM users. In contrast, three-fourths of all rtCGM users with HbA1c ≥7.5% had lower GMI than HbA1c, whereas differences in iscCGM users with HbA1c ≥7.5% were almost symmetrically distributed around 0.

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Good study to post, James. Thanks for posting. Lots of clicks already. Lots of interest.

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Yes, I found this conclusion particularly interesting-

“ Stratification by HbA1c level revealed higher GMI than HbA1cin most individuals with HbA1c <7.5%, with GMI-HbA1cdifferences being higher in iscCGM users. In contrast, three-fourths of all rtCGM users with HbA1c ≥7.5% had lower GMI than HbA1c, whereas differences in iscCGM users with HbA1c≥7.5% were almost symmetrically distributed around 0 (Fig. 1).”

People with a lower A1c tended to have a higher GMI vs A1c whereas those with A1c > 7.5% had a mean difference between GMI and A1c of 0 (Freestyle) and lower GMI than A1c (Dexcom). The boxplot they show does seem to have a wider range for this measurement (more variance in difference between GMI and A1c) for those with an A1c above 7.5%. It’s an interesting graph! I think I’m reading that correctly…

IscCGM is freestyle. RtCGM is Dexcom

image

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Clarifying a bit, since it’s dense…

Term

  • GMI: glucose management indicator estimates A1c from meter readings

Real-Time (rtCGM)

  • CareLink Pro/Personal, Dexcom G5
  • Symmetric variance of GMI around A1c

Intermittent Scanning (iscCGM)

  • FreeStyle Libre
  • Higher GMI compared to actual on 75% of users

Generally

  • About 50% had differences between GMI and A1c of >0.5%
  • Similar patterns were observed regardless of weight or age
  • With average A1C < 7.5, GMI was higher than actual, more often in isCGM
  • With average A1C > 7.5, A1c was higher than GMI, more often in rtCGM
  • With average A1C > 7.5, isCGM was generally accurate

Synopsis

  • If you have good control, you can trust your Dexcom or CareLink, but your Freestyle might be overestimating average A1c.
  • An alternative assessment might turn up that the pattern of scanning by FreeStyle users is different, and they more often scan when they are high, but not when they are low.
  • If you have poor control, the real-time CGM is possibly underestimating your A1c.
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