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 blood sugar 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 patient has not eaten/drunk for at least 8 hours prior.  In healthy fasting patients, there should be normal glucose levels with low insulin.
If a high blood glucose test 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 non-diabetic patient, insulin should be released to lower blood sugar levels down to normal.  In a diabetic patient, 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 in obtained in a cup) suggests that tissues are not using glucose for energy, possibly because insulin isn’t available.
  • Hemoglobin A1C = this blood test examines the presence of high blood sugar in the body for more than 3 months.  Since red blood cells live for less than 4 months, those red blood cells accumulate the excess sugar that it is exposed to during those months.  Specifically, the hemoglobin molecule will pick up sugars in a process called glycosylation.  The hemoglobin will be more glycosylated only if there is excessive sugar exposed to the hemoglobin in the 3-4 months of the RBC life.

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).
  • Full fasting lipid 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, 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
Figure 7.6 BC Guidelines on diagnosis of diabetes mellitus using blood tests.
Diagnostic algorithm for Type 2 diabetes
Figure 7.7 Flowchart of blood sugar and A1C test results that determine where someone is in the spectrum between normal and diabetic levels of blood sugar

Section Summary

Diabetes is diagnosed through a blood test measuring glucose levels.  Throughout the day, blood glucose levels fluctuate depending on meals and activity: blood glucose levels should be between 4-11 mmol/L.  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 that 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 when, presumably, insulin levels will be at their highest with a decrease of blood sugar as the tissues respond to insulin.  As hyperglycemic blood 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 is needed to differentiate between the types of diabetes mellitus.

Review Questions

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



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