If you don't have diabetes, probably neither. The 25-trial meta-analysis of CGM as a behaviour-change tool found the benefit driven almost entirely by participants who had diabetes, and commercial insulin assays disagree so badly that a HOMA-IR cut-off from one laboratory does not transfer to another. If you want one number about your glucose metabolism, an HbA1c through a clinician is cheaper and standardised.
| Compared on | CGM | Fasting insulin / HOMA-IR |
|---|---|---|
| Evidence grade | Measurement tool | Measurement tool |
| What it is | The FreeStyle Libre is a factory-calibrated, intermittently scanned ("flash") continuous glucose monitor made by Abbott. A small sensor worn on the back of the upper arm measures glucose in the interstitial fluid just below the skin and reports current glucose, a trend arrow, and glucose history… | The homeostatic model assessment of insulin resistance (HOMA-IR) is a simple index derived from a single fasting blood sample using fasting glucose and fasting insulin (classically HOMA-IR = fasting insulin (uU/mL) x fasting glucose (mmol/L) / 22.5). |
| Best human evidence | “Across 25 RCTs (n=2,996), CGM as a behavior-change tool produced modest glycemic improvements (HbA1c −0.28%, time in range +7.4%), driven mostly by participants with diabetes, with no…” The international journal of behavioral nutrition and physical activity 2024 · PMID 39716288 | “insulin resistance is independently associated with greater risk of cardiovascular or all-cause mortality” Biosci Rep 2017 · PMID 28811358 |
| Dose / protocol studied | Treat it as a two-week experiment, not a permanent feed. Test specific questions — how your usual breakfast compares with a higher-protein version, what a 15-minute post-meal walk does to the curve — then take the sensor off. Do not chase a flat line: post-meal rises are normal physiology, and there is no validated target range for people without diabetes. | Fasting 10-12 hours, and only ever compared against your own previous result from the same laboratory. Twice a year at most. Treat the trend as the signal and ignore the absolute HOMA-IR figure printed on the report. |
| Price | $139.00 Typical listed price | $79.99 Cost per test |
| Who it suits | Someone with diabetes, under clinical guidance — or a curious person running a deliberate two-week experiment with specific questions and no expectation of a health outcome. | Someone tracking their own trend through the same laboratory over time, who will read the direction rather than the absolute number. |
| Who should skip it | Anyone without diabetes hoping to optimise a flat line. Healthy people already spend roughly 15% of the time in the prediabetic range on CGM, and expert clinicians handed non-diabetic reports could not agree on who needed follow-up. | Anyone planning to compare their result against a threshold they read online — those cut-offs are assay-specific and may not apply to your report at all. |
| Key caveat | Two questions matter for a CGM worn without diabetes, and both answers are uncomfortable. On accuracy: in healthy adolescents the Libre's mean absolute relative difference against fingerstick plasma glucose was 13.1%, with only 68% of paired readings meeting the ISO 15197:2013 criterion, and in children without diabetes it showed magnitude bias at fasting levels and correlated poorly with true fasting glucose (r-squared 0.06) — it is built to track the… | HOMA-IR is a reasonable idea with a real problem underneath it. The formula itself has been in use since 1985 and correlates acceptably with clamp measures at a population level, but it is built on a fasting insulin value, and insulin has no reference measurement procedure - when 12 commercial assays from 9 manufacturers were compared against isotope-dilution mass spectrometry, differences ran from roughly -298 to +303 pmol/L and only one assay agreed… |
CGM lists at about 1.7× the price of Fasting insulin / HOMA-IR. Prices are the typical listed prices in our catalog, not live Amazon prices, and a true cost-per-studied-dose is not shown because our catalog does not record servings per container — we would have to guess, so we don't.
One of these is a continuous stream with no validated target range for healthy people; the other is a single number computed from an assay with no reference measurement procedure.
CGM, with your clinical team. In randomized trials and meta-analyses, continuous and flash monitoring modestly lowers HbA1c and, more strikingly, cuts time spent in hypoglycemia — by 38% in type 1 and roughly half in insulin-treated type 2 diabetes. Time in range is validated against long-term microvascular complications.
CGM as a two-week experiment. Test defined questions — your usual breakfast versus a higher-protein version, what a 15-minute post-meal walk does to the curve — then take the sensor off. Do not chase a flat line: post-meal rises are normal physiology.
Fasting insulin, same lab every time. HOMA-IR correlates acceptably with clamp measures at a population level and the formula has been in use since 1985. Its weakness is between-assay variation, which cancels out if you never change laboratory and only read your own trend.
Neither — ask for an HbA1c. Our own review of this kit says it plainly: if that is the question, an HbA1c through a clinician is cheaper, standardised and interpretable.
On the CGM side: in healthy adolescents the sensor's mean absolute relative difference against fingerstick plasma glucose was 13.1%, with only 68% of paired readings meeting the ISO 15197:2013 criterion, and in children without diabetes it correlated poorly with true fasting glucose. It is built to track the large swings of diabetes, not to resolve the narrow band a normoglycemic person lives in.
On the insulin side: when 12 commercial assays from 9 manufacturers were compared against isotope-dilution mass spectrometry, differences ran from roughly −298 to +303 pmol/L, and only one assay agreed fully with the reference — despite all of them claiming traceability to the same WHO standard. A HOMA-IR of 2.4 from one lab and 2.4 from another are not the same finding.
Cited: The international journal of behavioral nutrition and physical activity 2024 · PMID 39716288 · Diabetes Care 2004 · PMID 15161807
Each quote below is taken verbatim from the cited paper. Null and negative results are included on purpose — they are the reason a grade means anything.
“Across 25 RCTs (n=2,996), CGM as a behavior-change tool produced modest glycemic improvements (HbA1c −0.28%, time in range +7.4%), driven mostly by participants with diabetes, with no significant effect on weight or BMI.”
“CGM improves glycemic control by expanding TIR and decreasing TBR, TAR, and glucose variability in both type 1 and type 2 diabetes.”
“higher time in range was associated with reduced cardiovascular and all-cause mortality”
“insulin resistance is independently associated with greater risk of cardiovascular or all-cause mortality”
“The estimate of insulin resistance obtained by homeostasis model assessment correlated with estimates obtained by use of the euglycaemic clamp (Rs = 0.88, p less than 0.0001), the fasting insulin concentration (Rs = 0.81, p less than 0.0001), and the hyperglycaemic clamp, (Rs = 0.69, p less than 0.01).”
“In conclusion, the HOMA model has become a widely used clinical and epidemiological tool and, when used appropriately, it can yield valuable data. However, as with all models, the primary input data need to be robust, and the data need to be interpreted carefully.”
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The evidence is weak. A meta-analysis of 25 randomized trials of CGM as a behaviour-change tool found modest glycemic improvements driven mostly by participants with diabetes, no significant effect on weight or BMI, and only three trials in people without diabetes.
There is no universal cut-off. Thresholds vary by insulin assay, age and ethnicity, and published cut-offs are effectively assay-specific — which is why the number is only interpretable as your own trend from one laboratory.
For the common question — is my glucose metabolism drifting — HbA1c is cheaper, standardised and interpretable, and it comes through a clinician who can act on it. That is our recommendation for most people.
Post-meal rises are normal physiology. Healthy people already spend roughly 15% of the time in the prediabetic range on CGM, and when 18 expert clinicians were handed non-diabetic CGM reports they could not agree on who needed follow-up.
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