Glucose and HbA1c tell you how much sugar is in the blood. They do not tell you how hard the body is working to keep it there. Insulin, the hormone made by the pancreas that moves sugar out of the blood and into cells, is part of that hidden effort. When cells respond less well to insulin, the pancreas makes more, and blood sugar can stay normal for a while.
That state is called insulin resistance. The NHS describes it simply: the body does not respond to the hormone insulin properly. It also links it to metabolic syndrome. An insulin resistance test looks at glucose and insulin together to estimate it.
What the insulin resistance test measures
The Insulin Resistance (Fasting) test in the BPR Clinic catalogue is processed by the LML and reports three results:
- Fasting glucose: blood sugar after a period without food.
- Fasting insulin: how much insulin is circulating at the same moment.
- HOMA-IR: a calculation that combines the two into a single index of insulin resistance.
According to the LML, the result can indicate how well the body responds to insulin and may contribute to the assessment of prediabetes, type 2 diabetes and metabolic syndrome.
The limits of HOMA-IR
HOMA-IR is simple and widely used, but it has an important limitation. A review comparing studies found a high degree of variability in the threshold used to define insulin resistance, and noted that the distribution of HOMA-IR changes with age, sex and ethnic background (Tahapary et al., 2022). There is no single cut-off that applies to everyone.
That is why HOMA-IR is best used as a trend, or read together with weight, waist measurement, blood pressure and the lipid profile, rather than as a pass or fail.
Insulin on its own
The Insulin test measures insulin alone. According to the LML, insulin levels give information about how well the body produces and uses insulin. The LML also lists other reasons a doctor may request it, such as investigating episodes of low blood sugar.
What C-peptide adds
When the pancreas makes insulin, it also releases a fragment called C-peptide. The LML describes it as a by-product of insulin production, used to evaluate how much insulin the body itself is making.
A review in Diabetes, Obesity and Metabolism describes C-peptide as a valuable marker of the function of the insulin-producing cells, mainly in autoimmune diabetes and especially when diabetes starts in adulthood (Maddaloni et al., 2022). The same review notes that evidence for its routine use in type 2 diabetes is limited, partly because insulin resistance complicates interpretation, and that results are not always comparable between laboratories.
The C Peptide test in the catalogue is therefore more of a specialist test, usually requested by a doctor who wants to understand the type of diabetes or how much insulin the pancreas still makes.
When an insulin resistance test tends to make sense
As a general orientation, the test is most often considered in adults with excess weight, and particularly when there is:
- weight concentrated around the waist;
- a family history of type 2 diabetes;
- raised triglycerides or low HDL in a previous cholesterol test;
- borderline glucose or HbA1c results;
- a wish to follow the effect of changes in diet, activity or weight.
In the UK, NICE guidance on people at high risk of type 2 diabetes focuses on identifying them and offering an intensive lifestyle-change programme. Knowing where you stand on insulin resistance can be part of that conversation.
How often
For people who use it, the test is often repeated once a year. Talk to a healthcare professional about whether it adds to the tests you already have and at what interval.
