Labs to test for Insulin Resistance

You had your annual bloodwork done. Your fasting glucose came back normal, your A1c was fine, and your doctor said everything looks great. So why are you so tired after lunch, hungrier than you used to be, and gaining weight around your middle when nothing about your routine has changed?

I hear this from clients every week, and here is what I tell them: Normal blood sugar does not mean you don't have insulin resistance. It can simply mean your body is still working overtime to keep that number where it belongs. The standard labs can't see the overtime. A couple of inexpensive, rarely ordered labs can.

In this post I'm going to walk you through the labs to test for insulin resistance, including fasting insulin and HOMA-IR, what the numbers mean, and the habits that move them in the right direction.

What is insulin resistance?

Insulin is the hormone that moves sugar out of your blood and into your cells, where it gets used for energy or stored for later. Every time you eat, your pancreas releases some.

With insulin resistance, your muscle, liver and fat cells stop responding to insulin as well as they should. Your pancreas makes up for it by producing more. For a long time, that works. Your blood sugar stays in the normal range, but it now takes two or three times the insulin to keep it there.

This can go on for ten years or more before blood sugar finally starts to climb. That is the window where most people get missed, and it's also the window where food, movement and sleep make the biggest difference. High insulin on its own makes it easier to store fat, especially around the abdomen, and harder to tap into it. It also drives hunger and cravings, which is why this stage feels so frustrating.

Insulin resistance becomes more common in perimenopause and menopause, as estrogen falls and muscle mass gets harder to hold onto. It also shows up with PCOS, a history of gestational diabetes, fatty liver, and a family history of type 2 diabetes.

The labs you probably already have:

These two are on most annual panels. They are useful, but they are late markers. They tell you when blood sugar has already started to rise, not how hard your body is working to hold it steady.

Fasting glucose

This is the amount of sugar in your blood after at least 8 hours without food. Under 100 mg/dL is considered normal, 100 to 125 is prediabetes, and 126 or higher on two separate tests is diabetes.

I pay attention to the trend as much as the number. A fasting glucose that has crept from 84 to 91 to 97 over a few years is still "normal" every single time, but it is telling you something. Keep in mind that one reading can run high after a poor night of sleep, an illness or a stressful morning, so look at the pattern.

Hemoglobin A1c

A1c reflects your average blood sugar over the past two to three months. Under 5.7% is normal, 5.7 to 6.4% is prediabetes, and 6.5% or higher is diabetes.

A1c is convenient because you don't have to fast, but it's an average. It can't show you big swings after meals, and it can read falsely high or low with anemia, low iron, recent blood loss and certain hemoglobin variants. If your A1c doesn't match how you feel, that's a reason to look further.

Fasting insulin: the lab that often isn't tested

If I could add one lab to the standard annual panel, this would be it. Fasting insulin measures how much insulin is in your blood after an overnight fast. It answers the question glucose can't: how much effort is it taking to keep my blood sugar normal?

It isn't routinely ordered because it isn't part of the diagnostic criteria for prediabetes or diabetes, and because labs use different methods to measure it, so there is no single agreed cutoff. That doesn't make it useless, but it does make it a number to interpret in context and to track over time at the same lab.

How to interpret fasting insulin

Most lab reports list a reference range of roughly 2 to 20 or 25 µIU/mL. That range describes what is common, and insulin resistance is very common, so a result inside it isn't necessarily a good one.

Here is how I look at it with clients: Under about 8 µIU/mL generally suggests good insulin sensitivity. Between 8 and 12 is a gray zone worth watching, especially alongside other signs. Consistently above 12 to 15 suggests your pancreas is working harder than it should, even if your glucose is perfect.

HOMA-IR: putting glucose and insulin together

HOMA-IR stands for Homeostatic Model Assessment of Insulin Resistance. It isn't a separate blood test. It's a calculation that uses your fasting glucose and fasting insulin from the same blood draw to estimate how insulin resistant you are.

How to calculate HOMA-IR

Multiply your fasting glucose (in mg/dL) by your fasting insulin (in µIU/mL), then divide by 405. There are also simple online calculators.

So if your fasting glucose is 92 and your fasting insulin is 14, that's 92 × 14 = 1,288, divided by 405, for a HOMA-IR of 3.2. Notice that a glucose of 92 would never raise a flag on its own.

How to interpret HOMA-IR

Under 1.0 points to very good insulin sensitivity. Between 1.0 and 1.9 is generally considered a healthy range. From 2.0 to 2.9 suggests early insulin resistance, and 3.0 or above suggests significant insulin resistance.

Like fasting insulin, these cutoffs vary somewhat between studies and populations, so treat them as a guide. HOMA-IR also isn't reliable if you take insulin or certain diabetes medications, and both numbers have to come from a true fasting sample. What I find most helpful is watching your own number change. Going from 3.2 to 2.1 over four months tells you what you're doing is working, often before the scale or your A1c shows anything.

I’ve written about ways to manage your blood sugar and insulin resistance here.

If you'd like help making sense of your labs and building a plan that fits your life, reach out to schedule a visit.

This post is for general education and isn't a substitute for medical advice. Talk with your healthcare provider about your own labs, medications and health history.