Testing For Diabetes- What Are The Most Reliable Markers?

August 26, 2022

Diabetes mellitus, commonly just called “diabetes”, is a metabolic disorder where there is excess sugar in the blood because the body’s ability to process sugar in the blood gets impaired. This has to do with a malfunction in insulin functioning because insulin is the hormone primarily responsible for blood sugar processing.

While around 5-10 per cent of cases are caused by an autoimmune condition where the insulin production gets hampered(Type-1 diabetes), the rest are primarily caused due to insulin resistance(Type-2 diabetes).

In this article, we are going to discuss the different tests used for diabetes diagnosis or to track diabetes risk.

Conventional Diagnostic Tests

The standard tests used for diabetes diagnosis in conventional medicine are:

Fasting blood glucose:

Your blood glucose is tested 10-12 hours after a fast for the fasting glucose reading. It tells us how a patient’s blood glucose levels behave in a fasted state.

Post-prandial blood glucose or OGTT(oral glucose tolerance test):

These tests are used to check how a patient’s blood glucose responds to food intake. A post-prandial reading is when a patient eats a non-standardised meal(the patient is usually recommended to stick to their usual meal) and their blood glucose is measured after a fixed time period that is generally an hour.

In an OGTT, the patient is made to ingest a standardised meal consisting of a glucose solution consisting of 75g glucose and blood glucose is monitored one, two and three hours after. OGTT is not commonly used.


HbA1C is a form of haemoglobin that is bonded to glucose. This test measures the amount of HbA1C in the blood, which is supposed to be indicative of the average blood glucose levels in the last 3 months.

The conventional ranges for diabetes, pre-diabetes and normal values are as follows:

Result A1C Test Fasting Blood Sugar Test Glucose Tolerance Test
Diabetes 6.5% or above 126 mg/dL or above 200 mg/dL or above
Prediabetes 5.7 – 6.4% 100 – 125 mg/dL 140 – 199 mg/dL
Normal Below 5.7% 99 mg/dL or below 140 mg/dL or below

Source: American Diabetes Association

What Is The Purpose of Disease Diagnosis?

Before we get into what the best markers to predict diabetes risk may be, we need to understand why we call what we call diabetes. When a specific blood marker exceeds a specific cut-off, we assign an official disease diagnosis. This is because when the diagnostic criteria for a disease are fulfilled, it means the patient is at risk for experiencing certain known symptoms or is already experiencing them and it is happening due to a certain known condition which is supposed to be the disease. Hence, the best diagnostic test is one that is best able to predict and correlate with symptoms. That is the purpose of medicine.

Keeping in mind the above, we are going to assess the merit of the different tests used for diabetes:

Flaws in Current Standard Tests

Fasting blood glucose:

This is the most commonly used test for diabetes diagnosis. The issue with this is it tells us how our blood glucose behaves in a fasted state but it doesn’t tell us anything about how it responds to food intake, which is a key feature of diabetes. Therefore it should not be used as a sole diagnostic criterion for diabetes.

OGTT(Oral glucose tolerance test):

The drawback with this test is that it’s not practical. It involves ingesting 75g of refined glucose which is an unrealistic amount of refined sugar. Even people eating very unhealthy diets do not eat such a high amount. It’s better to test the body’s sugar management by putting it through a sugar load that the body actually experiences on a day-to-day basis.

Besides accuracy issues, this test can cause very harsh and potentially dangerous side effects for diabetic/pre-diabetic individuals since their blood sugar management is already impaired.

Post-prandial blood glucose:

This test remedies the issue we mentioned with the OGTT. The patient is made to eat a regular meal instead of a glucose solution. One issue here is that hospitals and clinics most often provide a standard meal that may not be what the patient normally eats, thus producing sub-optimal results. Very often, juice boxes with a high amount of added refined sugar are provided as part of the meal. Secondly, a single post-prandial reading is not sufficient. You need to track your post-prandial reading across a range of different meals at different meal times to get a fair idea of how your blood sugar responds to food.


As we mentioned, this test is supposed to indicate our average blood glucose levels in the past 3 months. It does so by measuring the amount of our haemoglobin that is bonded with glucose(which is HbA1C). The flaw with this test is that the result is dependent on not just blood glucose levels but a second factor as well- haemoglobin levels. So a patient with low haemoglobin despite having high blood glucose would still get a normal HbA1C score, wrongly implying that the patient is healthy.

The HbA1C test assumes a normal haemoglobin level, which muddies its accuracy.

The very easily available and highly efficient test that conventional medicine ignores

As we mentioned, the vast majority of Type 2 Diabetes cases(and other metabolic diseases as well) are caused by insulin resistance. Even if not the root cause, it is a major upstream factor in disease progression-in almost all chronic diseases today.

You can test for insulin resistance very easily by testing serum insulin. Despite this, this very simple test is ignored by conventional medicine and not included in health checkups. 

Serum insulin levels are able to predict diabetes much sooner than blood glucose levels. Because insulin levels start malfunctioning much sooner than blood glucose levels do.

Healthy ranges for serum insulin

Fasting serum insulin should be within 5 uIU/mL and post-prandial within 30 uIU/mL.

Other Useful Tests:

HOMA-IR(Homeostatic Model Assessment of Insulin Resistance):

This is a scientifically derived formula to diagnose insulin resistance-  calculated by multiplying fasting plasma insulin (FPI) by fasting plasma glucose (FPG), then dividing by the constant 22.5, i.e. HOMA-IR = (FPI×FPG)/22.5


This measures the level of triglycerides in the blood. Triglycerides are large lipid particles circulating in the blood and doing the vast majority of fat transportation. It is a very reliable marker for metabolic disease.


C-Peptide is a substance produced by the pancreatic cells along with insulin. Measuring C-Peptide levels helps us track the insulin production in the body and thus serves as a useful adjunct in diabetes diagnosis.


Similar to how the HbA1C tests for glycated(sugar-bonded) haemoglobin, fructosamines are glycated proteins in the blood, mostly albumin. This too gives us an average estimate of our blood sugar levels over the past.

CGM(Continuous glucose monitor):

This is a device attached to the body and can be kept on for multiple days. It tracks blood glucose levels in real-time and is thus extremely helpful for complete detailed tracking of blood glucose and how it behaves at different levels of fasting, different times of the day under different situations and in response to various foods. It is one of the best tools we have for diabetes diagnostics. It also tells patients exactly what foods and behaviors are causing problems. If only we had continuous insulin trackers too, we would have the near-perfect tool for diabetes risk tracking, (but those don’t exist yet).

The only issue is that CGMs are still very expensive and most people can’t afford them.

Sub-optimal reference ranges in conventional medicine

The reference ranges for diabetes diagnosis in conventional medicine(mentioned at the head of this article), as is the case with a lot of diseases, are quite inadequate. Only when the blood sugar metabolising ability of the body is completely impaired is a diabetes diagnosis given. There is no treatment in the “pre-diabetes” stage(not that conventional forms of treatment with drugs and insulin supplementation are any help in reversing the condition in the first place). We need to address and start reversing the condition at an earlier stage.

Reference ranges are based on an unhealthy population with whom diets high in refined carbs, refined seed oils and processed foods are rampant. This is what makes unhealthy levels “normal” and thus skews the reference ranges.

Best way to track diabetes risk

You shouldn't rely on a single marker such as fasting blood glucose which is often the case. It’s important to run a full panel including fasting and post-prandial values for glucose and insulin, HbA1C and C-Peptide. The more data we have the better of course. 

A CGM to track blood glucose behaviour is best. But if unavailable, getting a glucometer and testing blood glucose at different points in the day is a cost-effective solution to help achieve the same effect.  

India has been referred to as the diabetes capital of the world, with 72 million cases of Type-2 diabetes reported in the country in 2017. So it is paramount that we test and track our metabolic health status closely.


Ranodeep Seth
Content Writer

An IT graduate, Ranodeep has worked in IT operations and as a tech blog writer in the past. He is very passionate about health, owing partly to his own chronic health issues which plunged him much deeper into the field. He works for iThrive both as a writer and in the software design team. He is a strong advocate for “informed consent” in healthcare and aims to promulgate the same through his work.

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