diabetes test diagnosis
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Expert-reviewed by Ashwini S.Kanade, Registered Dietician and Certified Diabetes Educator with 17 years of experience

Diabetes is one of the most common ailments among people worldwide. Nearly 179 million people across the globe may be living with undiagnosed diabetes. In fact, every 7 seconds a person dies because of diabetes. [1]

This is a serious situation, but one that can be easily improved by increasing awareness about diabetes and its screening tests. No matter the type of diabetes, be it type 1, type 2, gestational (pregnancy-related) or prediabetes, they can all be determined by different types of blood tests. Read on to learn more about these diabetes screening blood tests.

1. Fasting plasma glucose test:

As the name suggests, this test is conducted when you are in a fasting state, meaning that you have not eaten for a minimum of 8 hours. A fasting plasma glucose (FPG) test is thus usually conducted in the morning before breakfast.

The blood is either taken by pricking your fingertip and placing it on a strip attached to the glucometer, or a technician takes the blood from a vein in your arm. An FPG test tells you how much sugar is in your blood at the time of checking.

According to Mayo Clinic, these values suggest what stage of diabetes you are at:[2]

FPG statusAmount of sugar in mg/dLAmount of sugar in mmol/L
Diabeteshigher than 1257.0 or higher


However, researchers suggest that if your FPG is below 100 but in the range of 91-99 mg/dL, you are at risk of developing type 2 diabetes. [3]

 2. Postprandial blood sugar test:

The postprandial blood sugar test (PPBS) is conducted after your FPG test and after you have eaten a meal. The test, thus, is generally done in the morning two hours after breakfast.

Here too, you can do the test yourself by pricking your fingertip with a lancet (that comes with the glucometer) and placing it on the strip attached to a glucometer, or if you got to a lab, the technician will take the blood from a vein in your arm.

Values for PPBS:

PPBS status

Amount of sugar in mg/dLAmount of sugar in mmol/L


Less than 140





Diabetes200 or higher

Higher than 11.1


Inform your doctor if you have any illness or if you are taking any medication prior to the PPBS test since these can affect your test results.

How often should you be testing your blood sugar levels? Find out.

3. Oral glucose tolerance test (OGTT)

Glucose tolerance test, or oral glucose tolerance test (OGTT), measures abnormalities in the way your body removes sugar from your blood. In other words, it checks your insulin resistance. This is a standard test to determine type 2 diabetes.

The technician will draw blood from a vein in your arm more than once; first after an overnight fasting and then after you drink a sugary solution they give you.

So, this test involves two steps:

  • First, you need to do an FPG test (after fasting overnight).
  • After the FPG test, you will be given a sugary solution to drink. Your blood sugar levels will then be tested after 2 hours (or at intervals of 30 or 60 min).

A reading of 140-199 mg/dL after 2 hours indicates prediabetes, and a reading of more than 200 mg/dL indicates diabetes.

4. Random plasma glucose test:

The random plasma glucose (RPG) test also measures your blood sugar levels but it does not have the time or diet restrictions. It is random because it is taken at any time of the day, and regardless of when and what you ate. Blood is taken by pricking your fingertip and placing it on a strip attached to the glucometer. A reading of 200 mg/dL or more suggests diabetes.

According to the American Academy of Family Physicians, you will need to repeat the OGTT and HbA1c (discussed below) test on consecutive days (2 days in a row) to get a definitive confirmation of diabetes. But a single RPG reading of 200 mg/dL or more is usually sufficient to diagnose diabetes. [4]

RPG is also a useful test to determine the risk of diabetes in early pregnancy called gestational diabetes. Although gestational diabetes develops after 20 weeks of gestation, an RPG in the first trimester can reveal whether diabetes will develop in the later stages of pregnancy, [5]

If you are a woman with diabetes, you need to know this.

It is thus advisable to get a screening done for gestational diabetes in your first trimester, especially if you meet any of these criteria:

  • Aged 35 or older
  • Are overweight or obese, especially in the abdominal area
  • Have a BMI of less than 25 kg/m2  before pregnancy, but excessive weight gain during pregnancy
  • Have high blood pressure
  • Have a family history of type 2 diabetes
  • Have polycystic ovary syndrome (PCOS)
  • Have a history of miscarriages
  • Have had gestational diabetes during earlier pregnancies [6]

All the tests mentioned thus far tell you your sugar levels at the time the test is conducted. To further corroborate the findings of these tests, your physician may recommend you to do additional tests that show your blood sugar levels going back a few weeks to a few months. We’ll discuss these tests in detail below.

As a person with diabetes, you must own a glucometer, here’s why.

5. HbA1c test:

HbA1c is also called glycated haemoglobin or simply A1c. By far, the HbA1c test is the most important test to diagnose diabetes. It shows the average amount of sugar present in your blood over the last 2-3 months. This is significant because the higher the HbA1c, the greater the chances of developing the dangerous complications of diabetes.

If the test results show A1c to be equal to or greater than 6.5%, it is a diagnosis of diabetes. Prediabetes is diagnosed if the results are in the range of 5.7%-6.4%. Less than 5.7% is considered as no diabetes.

Advantages of the HbA1c test:

It has no time or diet restrictions or requirements like FPG or OGTT. It can also determine diabetes complications, especially heart and vessel diseases. HbA1c on the higher side predicts an unfavourable lipid profile, that is, high total cholesterol, high ‘bad’ LDL cholesterol, and low ‘good’ HDL cholesterol. [7]

Drawbacks of the HbA1c test:

The HbA1c test measures blood sugar in a different manner. If you had high blood sugar in the couple of weeks right before your test, your results will be impacted even if your actual glucose levels 2 to 3 months before the test were normal. Moreover, the results are affected by age and ethnicity. So, it is necessary for children to get the OGTT with 2-hour plasma glucose done in addition to HbA1c. [8]

Who should not do the HbA1c test:

  1. People with cirrhosis do not benefit from an HbA1c test. Cirrhosis is a liver disease in which healthy liver cells are destroyed and replaced with scar tissues, which over time stop the liver from working and cause insulin resistance leading to high blood sugar. Most people with cirrhosis have an iron deficiency, which shortens the lifespan of red blood cells and results in the falsely lower HbA1c test. [9]
  2. HbA1c test is not reliable for those recovering from acute blood loss, those with chronic kidney failure or those receiving blood transfusions.
  3. Most people have a form of haemoglobin called HbA. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), many people in India have less common forms of haemoglobin, such as HbS, HbC, or HbE. The test is not reliable for them as well.

Researchers suggest that it is necessary to get all three tests (FPG, OGTT and HbA1c) done to get a correct diagnosis. Otherwise, there are chances your diabetes will not be detected in time. [10]

6. Fructosamine test (Glycated serum protein or glycated albumin test):

The fructosamine (FA) test determines the amount of blood sugar over the preceding 2-3 weeks. FA is formed when blood sugar interacts with the protein present in the blood. The higher the FA levels, the higher the amount of blood sugar.

Since 80% of all fructosamine is the protein called albumin, the glycated albumin test also falls under the same category.

This test has an advantage over the HbA1c test in that it can detect changes in blood sugar levels more quickly and, thus, treatment can begin earlier.

The normal range of fructosamine levels is 200–285 mmol/L when the albumin concentration is 5 g/dL.

The FA test is done for checking blood sugar when:

  • the HbA1c test is unreliable
  • a narrow time frame is required, for example, sugar control at the time of conception in women with diabetes who recently became pregnant (gestational diabetes)
  • it becomes necessary to check the effect of the change in diet or medication within 2 months
  • a surgery is about to be performed. A study found that FA levels of more than or equal to 292 mmol/L may lead to a higher risk of surgical site infection. [11]

Who should not do the Fructosamine test:

The test is not reliable if you have:

  • liver disease such as cirrhosis
  • nephrotic syndrome (kidney disease in which high amount of protein is excreted in the urine)
  • thyroid disease
  • paraproteinaemia (presence of excessive amounts of a tumour causing proteins in the blood)

If you have kidney disease, it is best you get only the glycated albumin test done, as this test measures the ratio of glycated (sugar-interacted) albumin to total albumin and not just the glycated albumin in the blood. [12]

Caution: Do not take ascorbic acid supplements for 24 hours prior to sample collection.  

All diagnostic tests have their advantages and disadvantages and with this article, we aim to educate you on the important need-to-know points. However, your doctor ultimately will decide which test is best suited for you depending on your personal factors.


  1. S.I. Sherwan, H.A. Khan, A. Ekhzaimy, A. Masood, M.K. Sakharkar. Significance of HbA1c Test in Diagnosis and Prognosis of Diabetic Patients. Biomarker Insights. 2016. 11:95-104. doi:10.4137/BMI.S38440.
  2. Mayoclinic.org. Diabetes: Diagnosis & Treatment. https://www.mayoclinic.org/diseases-conditions/diabetes/diagnosis-treatment/drc-20371451
  3. P. Brambilla, E. La Valle, R. Falbo, G. Limonta, S. Signorini, F. Cappellini, P. Mocarelli. Normal Fasting Plasma Glucose and Risk of Type 2 Diabetes. Diabetes Care. 2011. 34(6), 1372–1374. http://doi.org/10.2337/dc10-2263.
  4. K. Pippitt, M. Li, H.E. Gurgle. Diabetes Mellitus: Screening and Diagnosis. Am Fam Physician.  2016 Jan. 15;93(2):103-109.
  5. C.L. Meek, H.R. Murphy, D. Simmons. Random plasma glucose in early pregnancy is a better predictor of gestational diabetes diagnosis than maternal obesity. Diabetologia. 2016;59:445-452. doi:10.1007/s00125-015-3811-5.
  6. R.S.Pons, F.C. Rockett, B. de Almeida Rubin, M.L.R. Oppermann, V.L. Bosa. Risk factors for gestational diabetes mellitus in a sample of pregnant women diagnosed with the disease. Diabetology & Metabolic Syndrome. 2015. 7(Suppl 1): A80. doi:10.1186/1758-5996-7-S1-A80.
  7. A. Klisic, N. Kavaric, M. Jovanovic, E. Zvrko, V. Skerovic, A. Scepanovic, A. Ninic. Association between unfavorable lipid profile and glycemic control in patients with type 2 diabetes mellitus. Journal of Research in Medical Sciences : The Official Journal of Isfahan University of Medical Sciences. 2017. 22:122. doi:10.4103/jrms.JRMS_284_17.
  8. H.K. Nam, W.K. Cho, J.H. Kim, Y.J. Rhie, S. Chung, K.H. Lee, B.K. Suh. HbA1c Cutoff for Prediabetes and Diabetes Based on Oral Glucose Tolerance Test in Obese Children and Adolescents. Journal of Korean Medical Science. 2018. 33(12), e93. http://doi.org/10.3346/jkms.2018.33.e93
  9. T. Sehrawat, A. Jindal, P. Kohli, A. Thour, J. Kaur, A. Sachdev, Y. Gupta. Utility and Limitations of Glycated Hemoglobin (HbA1c) in Patients with Liver Cirrhosis as Compared with Oral Glucose Tolerance Test for Diagnosis of Diabetes. Diabetes Therapy. 2018. 9(1), 243–251. http://doi.org/10.1007/s13300-017-0362-4.
  10. D-L Kim, S-D Kim, S.K. Kim, S. Park, K-H Song. Is an Oral Glucose Tolerance Test Still Valid for Diagnosing Diabetes Mellitus? Diabetes & Metabolism Journal. 2016. 40(2):118-128. doi:10.4093/dmj.2016.40.2.118.
  11. N. Shohat, M. Tarabic+B3hi, E.H. Tischler, S. Jabbour, J. Parvizi. Serum Fructosamine: A Simple and Inexpensive Test for Assessing Preoperative Glycemic Control. J Bone Joint Surg Am. 2017. 15;99(22):1900-1907. doi: 10.2106/JBJS.17.00075.
  12. S.H. Senapathi, R. Bhavsar, R. Kaur, P. Kim, I. Sachmechi. A Case Report of Fructosamine’s Unreliability as a Glycemic Control Assessment Tool in Nephrotic Syndrome. Muacevic A, Adler JR, eds. Cureus. 2017. 9(9):e1694. doi:10.7759/cureus.1694.

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