Diagnosis of diabetes mellitus involves conducting specific glucose tests to evaluate blood sugar levels. But blood sugar level is not considered in isolation when diagnosing diabetes. This is because there are temporal or transient problems which can alter blood sugar levels. Blood sugar level will return to normal once the problems are sorted out. For example, trauma, medications such as steroids, diuretics and certain illnesses can alter your blood sugar levels. Vigorous exercise can also lower your blood sugar levels. Other factors such as physical examination, presence or absence of symptoms, and medical history must also be considered.
Confirming Diagnosis of Diabetes
Diagnosis of diabetes can be carried out using 4 major tests namely fasting blood sugar, glucose tolerance test, Hemoglobin A1C, and random blood sugar. To confirm diagnosis using one of these tests, the same test needs to be repeated using a different blood sample. The test should be repeated, preferably on a different day. However, if two different tests performed on the same blood sample give values above the diagnostic level, then the diagnosis is confirmed.
One of the two tests may have a value above the diagnostic level and the other below. In this situation, the one with value above the diagnostic level is repeated. Diagnosis is confirmed if the repeated test still gives value above the diagnostic level. But there might be no need to repeat the test to confirm diagnosis if there is unequivocal hyperglycaemia with acute metabolic decompensation or obvious symptoms.
Some individuals are asymptomatic although their blood glucose is above the diagnostic cut-off value. If the test for such people is repeated but fails to confirm diagnosis, then periodic testing should be done until the diagnosis becomes clear. Individuals with prediabetes or diabetes risk factor should get tested after every 1 or 2 years. But those with a history of gestational diabetes are recommended to have the test after every 3years.
Measured glucose values are reported in mg/dL or mmol/L. To convert mg/dL to mmol/L simply multiply by 0.0555.
Fasting blood sugar (FBS) test
Fasting blood sugar (FBS) also called fasting blood glucose (FBG) or fasting plasma glucose (FPG) measures your blood sugar levels after an overnight fast. You must not eat or drink anything except water for at least 8 hours before the test.
A fasting blood sugar level equal to or less than 99 mg/dL (5.49 mmol/L) is normal. Values between 100 to 125 mg/dL (5.6 to 6.9 mmol/L) indicate prediabetes (or impaired faster glucose). Fasting blood sugar greater than or equal to 126 mg/dL (7 mmol/L) indicates you have diabetes. The test is usually repeated on a different day to confirm diagnosis.
Glucose tolerance test (GTT)
Glucose tolerance test (GTT) also referred to as oral glucose tolerance test (OGTT) is a test used to evaluate how your body moves glucose from the blood into the tissues or cells. It requires that the patient fast and does not smoke, eat, or drink anything except water. Fasting should be done for at least 8 to 14 hours but not more than 16 hours before the test. But, days before the test, the patient needs to eat and drink normally or as usual. Some providers may provide special diet recommendations before the test. So, talk to your healthcare provider if not sure.
There are two types of GTT; 2-hour GTT which uses 75 g anhydrous glucose and measures blood glucose over two hours and 3-hour GTT which uses 100 g anhydrous glucose and measures blood glucose over three hours. The 3-hour GTT is used for diagnosis of gestational diabetes mellitus. But the 2-hour GTT can be used to diagnose any form of diabetes including type 1 diabetes, type 2 diabetes, or gestational diabetes. Before the test begins, an initial blood sugar is measured. Afterward, the patient is given a 200 to 300 mL drink containing high levels of sugar (75 grams glucose or 100 grams for pregnant women). The patient blood sugar will then be tested at 1 hour, 2 hours, or 3 hours after drinking the glucose drink.
Why is the glucose tolerance test performed?
Glucose tolerance test measures how your body handles glucose after a meal and before fasting blood glucose levels become abnormal. It is usually performed when fasting blood glucose level are high but not high enough to be considered as diabetes. This allows for further screening and confirmation of diabetes. It also offers the advantage that both fasting blood sugar and 1-hour, 2-hour, or 3-hour plasma glucose can be measured at the same time. Therefore, glucose tolerance test is usually recommended for diagnosis of diabetes.
In addition to diabetes, other conditions such as acromegaly, carbohydrate metabolism disorder and reactive hypoglycemia can be diagnosed using the glucose tolerance test.
Glucose tolerance test in pregnant women
Pregnant women who showed risk of developing diabetes before pregnancy or who have diabetic symptoms during pregnancy needs to take the GTT within 24 to 28 weeks of pregnancy.
The principle behind OGTT
Drinking a glucose drink will cause your blood glucose (or sugar) levels to rise. However, this should fall back quickly to normal if the person does not have diabetes. This is because the insulin produced in response to drinking the glucose drink will bring the glucose concentration in the blood back to normal. However, if the person has any form of diabetes, then glucose level will return very slowly to normal or may remain high. Slow return to normal means either insulin is not produced in response to the glucose or that it is produced but then, the cells of the body do not respond to it.
Interpreting results from glucose tolerance test
Values obtained at 2 hours (also called 2-h plasma glucose) are used to diagnose diabetes for all individuals except pregnant women after 75 g glucose load. A fasting blood glucose (FBG) level < 100 mg/dL and a glucose tolerance (GT) value < 140 mg/dL at 2 hours is considered normal. People with FBG between 100 and 125 mg/dL and a GT value of < 140 mg/dL at 2 hours are said to have impaired fasting glucose. Impaired fasting glucose is a risk factor that can result in diabetes in the future. So, frequent checkups are recommended.
FBG levels < 100 mg/dL and a GT value between 140 and 199 mg/dL at 2 hours indicate impaired glucose tolerance. This is also a high-risk factor indicating you could have diabetes in the future. Some refer to this as prediabetes. Talk to your doctor about possible early intervention and go for a routine checkup. If FBG is ≥ 126 mg/dl or 2 hours glucose is ≥ 200 mg/dL, then you have diabetes. But a second test on a different day needs to be performed to confirm this.
Interpreting results from Glucose Tolerance Test for pregnant women
For pregnant women, diagnosis of gestational diabetes is confirmed after a 75 g glucose load if any of the following are met; FPG is ≥ 92 mg/dL (5.1 mmol/L), 1 h GTT is ≥ 180 mg/dL or 2 h GTT is ≥ 153 mg/dL (8.5 mmol/L). But for a 3-hour GTT after 100 g glucose load, diagnosis is confirmed if:
- Fasting serum glucose ≥ 95 mg/dl (5.5 mmol/l)
- 1-hour serum glucose ≥ 180 mg/dl (10.0 mmol/l)
- 2-hour serum glucose ≥ 155 mg/dl (8.6 mmol/l)
- 3-hour serum glucose ≥ 140 mg/dl (7.8 mmol/l)
If only one of the values above are higher than normal, then the test will need to be repeated on a different day. But if 2 or more of the values are higher than normal then you have GDM.
Women diagnosed with GDM need a lifelong screening every 3 years. But those who are prediabetic and diagnosed with GDM need an immediate medical intervention to prevent the development of diabetes.
Drawbacks of the glucose tolerance test
Results from GTT usually fluctuate or vary. So, it is recommended that if a mildly or highly elevated blood glucose is detected, then the test should be repeated, preferably on a different day to confirm diagnosis. Reliability of the result also depends on your general health condition. You should be in good health (no other illness) and not taking any medication which can alter blood sugar levels. Vigorous exercise also lowers your blood sugar. Therefore, it is important that you talk to your doctor about your health and exercise regime before embarking on this test.
Other factors which influence the quality and outcome of GTT include how the blood sample was handled, quality of glucose measurements, physiological factors such as gastric emptying, or time of the day when the test is done. Venous plasma glucose levels depend on the timing of the day when it was done. Glucose tolerance decreases in the afternoon and evening as detected by glucose tolerance tests. This is due to reduced insulin sensitivity and beta cell response to glucose. Decrease insulin sensitivity and beta cell response will result to false positive. GTT should best be done in the morning. During the test you also need to stay very calm to prevent fluctuation of sugar levels.
Side effects and risks of glucose tolerance test
GTT causes side effects although they are rare and hardly severe. However, indicate to your doctor if you have any medical condition, have undergone a medical procedure, or have a history of symptoms related to blood tests. Side effects from glucose tolerance test include:
- Shortness of breath
- Moderate to no pain experience during injection
- Throbbing or slight bruise which eventually fades away.
Complications caused by glucose tolerance test are rare but occasionally the following may occur:
- Excessive bleeding may occur when blood is drawn.
- Building up of blood under the skin also called hematoma.
- Collecting blood means causing a rupture through the skin.
Hemoglobin A1C (HbA1C)
Hemoglobin is a protein in the red blood cells which carries oxygen from the lungs to the rest of the body’s organs and tissues. It also transports carbon dioxide from the organs and tissues back to your lungs where it is expelled out of the body. Glucose and other sugars can spontaneously bind to hemoglobin when present in the blood stream. The more sugar in the blood stream, the greater the percentage of sugar that will be bound to hemoglobin. When hemoglobin becomes bound to a sugar, it is referred to as hemoglobin A1C (HbA1C), glycated hemoglobin, glycosylated hemoglobin or simply A1C. Hemoglobin A1C test therefore measures the average amount of blood sugar attached to hemoglobin over a period of 3 months.
Unlike the FBS test and the OGTT, HA1C has the advantage that you don’t need to fast or drink any sugary drink. Also, there is less concern about day-to-day fluctuations of glucose levels whether in health or sickness. Hence, greater preanalytical stability. Despite these advantages, the test is expensive, not readily available in most regions of the world and displays poor sensitivity.
Why is a Hemoglobin A1C Test done over a period of 3 months?
A Hemoglobin A1C test or A1C is carried out over a period of 3 months. The test is limited to 3 months because the red blood cells which carry hemoglobin have a lifespan of 110 to 120 days. This is about 4 months. Again, the lifespan of individual red blood cells can vary. Therefore, the A1C test measures the level of glucose or sugar to which a red blood cell has been exposed during its lifespan.
Interpreting Hemoglobin A1C test results
The A1C test is carried out using a method certified by NGSP. Results for A1C are reported in percentage. A normal A1C level is less than 5.7%. An A1C level between 5.7% to 6.4% indicates prediabetes and an A1C level greater than 6.5% is indicative of either type 1 or type 2 diabetes. The test should be repeated on a different day using the A1C test or another diabetes tests to confirm the diagnosis.
Limitations of A1C
Several factors can interfere with A1C results causing you to have higher than normal A1C values even though you are not diabetic. Factors affecting A1C results include:
- Disorders affecting red blood cells or hemoglobin such as anemia, thalassemia, or myeloid leukemia.
- Acute blood loss.
- Recent blood transfusion.
- Deficiency in iron, vitamin B12 or folate.
- Diseases or conditions affecting the kidney or liver.
- Early or late pregnancy.
- Medications such as aspirin, antiretrovirals, dapsone, ribavirin, vitamin C and vitamin E.
- Ethnicity or race.
These limiting factors need to be brought to the attention of your doctor. Because of these limitations, it is recommended to do a repeat of the test with a different blood sample. Otherwise, a different diabetes test such as the FBG or OGTT is performed alongside the HA1C test.
When is A1C test done?
One of the major uses of A1C is to manage diabetes. For most adult, the target is usually to maintain the A1C levels below 7 % (53 mmol/mol). This will depend on the presence of other medical conditions or factors. But your doctor will be able to guide you. In addition, because of the day-to-day fluctuation in blood sugar levels, the A1C is used alongside other blood sugar tests.
The A1C is not recommended for diagnosing diabetes in children and adolescents. But, it may be used for the diagnosis of type 2 diabetes in children provided there’s no interference with the assay used. However, A1C test is not recommended at all in children with cystic fibrosis or children who demonstrate symptoms indicative of acute onset of type 1 diabetes.
Random blood sugar (RBS) test
Random blood sugar (RBS) test also called random plasma glucose (RPG) or casual blood glucose test measures your blood glucose level at any time of the day regardless of whether you have eaten or not. Glucose levels in healthy people do not vary widely throughout the day. Usually, several random measurements using RBS test can be conducted during the day. If blood glucose varies widely, then it may mean a problem.
Why is the test done?
The random blood sugar test is done to check for prediabetes and to confirm diabetes mellitus. Also, it is done during or after treatment of diabetes to monitor the disease by keeping track of blood glucose levels.
Interpreting RBS test results
Because random blood sugar test is done at any time of the day, the results will vary depending on whether it was done before or after a meal. Before eating, RBS values between 80 mg/dL to 130 mg/dL is normal, between 140 mg/dL to 180 mg/dl may indicate prediabetes and values ≥ 200 mg/dL (11.1 mmol/L) is diagnostic of diabetes. However, normal blood glucose can be as high as 180 mg/dL 1 to 2 hours after a meal.
Further Reading on Diagnosis of diabetes
- Standards of Medical Care in Diabetes—2018 Abridged for Primary Care Providers
- Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes—2018
- Classification and diagnosis of diabetes
- Screening and Diagnosis of Gestational Diabetes Mellitus, Where Do We Stand
- Definition, diagnosis and classification of diabetes mellitus and its complications. Part 1: diagnosis and classification of diabetes mellitus provisional report of a WHO consultation