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Diabetes Mellitus

Diagnosis of Diabetes

Jennifer Kong

Learning Objectives

By the end of this section, you will be able to:

  • Briefly describe some of the laboratory tests of diagnosing diabetes mellitus.

All types of diabetes mellitus are diagnosed most commonly using measurement of blood glucose levels.

In addition to taking a patient’s medical history (which highlights possible risk factors), the patient does a blood test with or without fasting (normal values are different).

  • Random blood glucose test = blood is taken at any time of the day, regardless of whether the patient has eaten/drunk recently
  • Fasting blood glucose test = blood is taken when a patient has not eaten/drunk for at least 8 hours. In healthy fasting patients, there should be normal glucose levels with low insulin.

If high blood glucose is detected, a subsequent test is performed in case the patient failed to follow instructions.

  • Oral glucose tolerance test = patient is given a known amount of sugar to drink and then waits for 2h before blood sample is taken for glucose levels. In a patient without diabetes, insulin should be released to lower blood sugar levels down to normal. In a patient with diabetes, blood glucose levels will be high.
  • Ketone test in blood or urine = the presence of ketones in the blood (draw blood) or urine (urine sample obtained in a cup) suggests that tissues are excessively using fat breakdown and not glucose for energy, possibly because insulin isn’t available.
  • Hemoglobin A1C (glycosylated hemoglobin) = this blood test examines the values of blood glucose in the last 3 months. Red blood cells take up glucose, and since red blood cells live for less than 4 months, if blood glucose was high red blood cells accumulate the excess sugar that was exposed to them during those months. Specifically, the hemoglobin molecule will bind to glucose in a process called glycosylation. The hemoglobin will be more glycosylated only if exposed to excessive sugar in the 3-4 months of the red blood cell life.
Hemoglobin a1c.
As more glucose (white) accumulates in the blood, more red blood cells have glucose attached to them forming the glycosylated hemoglobin or HbA1c. This is measured by the hemoglobin a1c test and provides an estimated information about the last 3-4 months of blood glucose levels. © Bruce Blaus is licensed under a CC BY (Attribution) license

Diagnosis of Diabetes

Meeting ANY of the following criteria confirms diagnosis when classic symptoms of hyperglycemia are present

Absence of classic symptoms of hyperglycemia requires a repeat confirmatory test
FPG ≥7.0 mmol/L
  • Test needs to be done after fasting (i.e., no caloric intake) for 8 hours (if fasting lipid test is ordered concurrently extend to 10 hours).
  • A full fasting panel rarely needed for most people with Type 1 Diabetes.
  • Since a number of people with Type 2 Diabetes have elevated triglycerides to the point where this affects management, a full fasting lipid panel may be needed.
A1C ≥6.5% (in adults)
  • Not used for diagnosis in suspected Type 1 Diabetes, pregnant women, children, or adolescents.
  • Results may be inaccurate in patients with hemoglobinopathies, very low eGFR (estimated glomerular filtration rate), hematologic malignancies, anemia. See Diabetes Canada for more information.
2hPG in a 75 g OGTT ≥11.1 mmol/L
Random PG ≥11.1 mmol/L in a patient with classic symptoms of hyperglycemia
  • Random = any time of the day, regardless of the interval since the last meal
BC Guidelines on diagnosis of diabetes mellitus using blood tests.
Diagnostic algorithm for Type 2 diabetes
Flowchart of blood sugar and A1C test results that determine where someone is in the spectrum between normal and diabetic levels of blood sugar. Retrieved from Diabetes Canada.

Section Review

Diabetes is diagnosed through a blood test measuring glucose levels. Throughout the day, blood glucose levels fluctuate between 4-11 mmol/L depending on meals and activity. Hence, patients are instructed to not take in any nutrients (i.e. fast) for >8h to ensure – presumably – low levels of insulin with a normal range of blood glucose. If a patient’s fasting plasma glucose is above normal range (i.e. hyperglycemia), this indicates a lack of insulin response: whether due to lack of circulating insulin (T1DM) or resistance to insulin at the the tissue level (i.e. T2DM or gestational). If hyperglycemia is present for more than 3 months, the excess sugar will modify the red blood cells and can be detected by measuring glycosylated hemoglobin – known as “Hemoglobin A1C”. A further test for suspected diabetes is the oral glucose tolerance test (OGTT) where the patient ingests a known amount of sugar orally and their blood is measured 2h later. At this time, presumably, insulin levels will be at their highest with a decrease of blood glucose as the tissues respond to insulin. As blood high in glucose is being filtered by the kidney to produce urine, urine can be examined for the presence of glucose and/or ketones. Ketones will be present in both blood and urine if an abnormal amount of fat metabolism is occurring to compensate for the lack of glucose entering tissue for metabolism. When more than one test measures an inappropriately high amount of blood sugar/presence of ketones, diabetes mellitus is suspected and further investigations are needed to differentiate between the types of diabetes mellitus.

Optionally, check out Diabetes Canada‘s website where they outline screening for diabetes in adults.

Review Questions

1. Which of the following are tests taken to determine diabetes mellitus diagnoses? Select all that apply.

  • Patient’s medical history
  • Random blood glucose test
  • Oral glucose tolerance test
  • Hemoglobin A1C
  • Lipid urine test
  • Lipid blood test
  • Fasting cholesterol blood test
  • Iron blood test

2. Place the tests and definitions in the correct section – Preliminary Tests or Secondary Tests.  

Examining the glycosylation of sugar in red blood cells Random Blood Glucose Test
Testing for the presence of ketones in blood or urine Oral Glucose Tolerance Test
Blood taken at any time (regardless of most recent food/drink consumption) Ketone Blood & Urine Test
Give known amount of sugar; wait 2 hours before taking blood sample Fasting Blood Glucose Test
Blood taken without food/drink consumption 10 hours prior Hemoglobin A1C
Preliminary Tests Secondary Tests 
Test Definition Test Definition
 

 
 

 
 

 

 

3. If a patient is suspected to have diabetes mellitus, which of the following preliminary tests should they take to determine a diagnosis? Select all the apply.

  • Random blood glucose test
  • Fasting blood glucose test
  • Oral glucose tolerance test
  • Ketone urine test
  • Ketone blood test
  • Hemoglobin A1C

 

Answer Key

  1. Random blood glucose test, Oral glucose tolerance test, Ketone urine test, Hemoglobin A1C
  2. Review table below.
  3. Random blood glucose test, Fasting blood glucose test
Preliminary Tests Secondary Tests 
Test Definition Test Definition
Fasting Blood Glucose Test

 

 Blood taken without food/drink consumption 10 hours prior
Hemoglobin A1C

 

 Examining the glycosylation of sugar in red blood cells
Random Blood Glucose Test Blood taken at any time (regardless of most recent food/drink consumption) Ketone Blood & Urine Test

 

 Testing for the presence of ketones in blood or urine
 

 

 Oral Glucose Tolerance Test Give known amount of sugar; wait 2 hours before taking blood sample

Media Attributions

Diabetes Canada Clinical Practice Guidelines Expert Committee. Diabetes Canada 2018 Clinical Practice Guidelines for the Prevention and Management of Diabetes in Canada. Can J Diabetes. 2018;42(Suppl 1):S1-S325. [Internet]. [cited 2020 Oct 16]. Available from: http://guidelines.diabetes.ca/cpg

 

BC Guidelines for Diabetes Care  https://www2.gov.bc.ca/gov/content/health/practitioner-professional-resources/bc-guidelines/diabetes#3

 

Media Attributions

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

Pathology Copyright © 2022 by Jennifer Kong, Zoe Soon, and Helen Dyck is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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