How Hyperglycemia Is Diagnosed

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Hyperglycemia, otherwise known as high blood sugar, can be diagnosed with a blood test such as a fasting plasma glucose (FPG) test, an A1C test, or a fructosamine test. Additionally, hyperglycemia can be identified using a glucose monitor or by drinking a beverage and monitoring the body's glucose response—a test referred to as an oral glucose tolerance test (OGTT).

To confirm a diabetes diagnosis, a doctor should conduct two different tests. Your doctor will explain the results and what they mean.

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Self-Checks/At-Home Testing 

If you have diabetes, frequent blood glucose monitoring can help you manage your blood sugars and prevent or detect hyperglycemia.

Testing your blood sugar in the morning before you've eaten, two hours after a meal, and before bed can help you understand what causes your blood sugar to rise and how much.

Your medical team will provide you with individualized blood sugar targets based on a variety of factors such as age, length of diagnosis, activity level, weight, and your overall health history.

Generally speaking, hyperglycemia for most non-pregnant adults is defined as:

  • Fasting: Blood glucose reading higher than 130 mg/dL
  • Two hours after a meal: Higher than 180 mg/dL
  • Random blood sugar test: Higher than 200 mg/dL

If you have a random blood sugar result that is above normal, there is no reason to panic, especially if you know the cause. Perhaps you ate too many carbs at dinner or underestimated your insulin needs.

It makes sense to call your healthcare provider if you notice a pattern of elevated blood sugars. For example, if your blood sugar is higher than 130 milligrams per deciliter (mg/dL) after an eight-hour fast multiple days in a row, you may need to adjust your meal plan, medicines, or activity, and your medical team can help you to do that.

If you do not have diabetes but have risk factors such as prediabetes, obesity, or a family history of diabetes, or you are experiencing symptoms including increased thirst, increased hunger, and increased urination, schedule an appointment to get screened so that you can determine whether or not your blood sugar is elevated.

Note that blood sugar checks can yield inaccurate results if you do not have clean, washed hands or if test strips are expired or have been exposed to extreme temperatures.

Labs and Tests

Your doctor may have you take one or more tests to check for hyperglycemia. This may be done to screen for diabetes or to monitor prediabetes or diabetes.

These tests may be part of a routine exam or ordered by your doctor if you are experiencing any symptoms of hyperglycemia.

Fasting Blood Glucose Test

The fasting plasma glucose test (FPG), which is also known as the fasting blood glucose test (FBG) or fasting blood sugar test, measures blood sugar levels and is used to diagnose diabetes and impaired glucose tolerance.

It can also help people with diabetes to detect hyperglycemia.

If the test is being done to screen or diagnose you, a small vial of blood is taken from your arm. If the test is being done to monitor you, a drop of blood will be obtained from a finger prick. A lab will then analyze the sample.

The American Diabetes Association (ADA) recommends a fasting glucose test to screen for diabetes in people age 45 years and older. If the results are normal, it is repeated at least every three years. The test is also recommended for people who have had symptoms of diabetes or multiple risk factors for the disease.

If you have diabetes and check your blood sugars regularly, you can test your own fasting blood sugar using a glucometer. Prior to testing, you must avoid eating or drinking for at least eight hours. Because of the need to fast, the test is usually done in the morning.

For people without diabetes, hyperglycemia is indicated when the reading is:

  • 100 mg/dL to 126 mg/dL: This suggests impaired glucose tolerance or prediabetes, indicating an increased risk of developing diabetes.
  • Above 126 mg/dL: This is the threshold at which diabetes is diagnosed. Typically this must be confirmed twice or cross-checked with another diagnostic test.

For those with diabetes, hyperglycemia is indicated when a reading over 130 mg/dL occurs a few consecutive days. This can indicate a pattern of high morning blood sugar.

A1C Test 

The A1C test (also known as HbA1C, hemoglobin A1C, glycated hemoglobin or glycosylated hemoglobin) indicates a person’s average blood glucose level over the past two to three months.

It is a good general measure of diabetes care and can help to determine hyperglycemia in people with diabetes. It can also be used to diagnose diabetes.

You can get an A1C reading through a regular blood draw. Additionally, many doctors' offices have A1C testing machines that enable them to get a result using a small drop of blood obtained by pricking your finger with a lancet. No fasting is required during this test.

For a person without diabetes, a typical A1C level is about 5%. A borderline A1C that indicates hyperglycemia or prediabetes falls within the range of 5.7% to 6.4%.

For those with diabetes, the ADA recommends an A1C target of less than 7% without significant hypoglycemia (low blood sugar). The American Association of Clinical Endocrinologists recommends a level of 6.5% or below. However, the ADA also emphasizes that A1C goals should be individualized.

If you have diabetes, it's important to understand what your A1C target is and what value indicates hyperglycemia.

Most of the time, when blood sugar control is good, an A1C test is done twice a year. However, if you have hyperglycemia, the level may be checked more frequently, especially if medication changes have been made.

Fructosamine Test

The fructosamine test is another blood test, similar to the hemoglobin A1C test, measures blood glucose levels over the course of two to three weeks.

It measures glycated protein in the blood and is typically used to measure blood sugar in people who have sickle cell anemia or other hemoglobin variants.

Unlike the A1C test, the fructosamine test is not used as a screening test for people who do not have diabetes or who have well-controlled diabetes.

The fructosamine test can be used in addition to blood glucose testing when you've had a recent change in your medicines or insulin. It can help monitor the effectiveness of the new treatments after just a few weeks; with an A1C test, you would need to wait months.

Lastly, the fructosamine test is used in gestational diabetes because changes can happen very quickly during pregnancy. The shorter time span of the test allows a doctor to track your blood glucose levels more closely than an A1C test.

Hyperglycemia is indicated when fructosamine levels are above the following ranges:

  • For people without diabetes: 175 to 280 mmol/L
  • For people with controlled diabetes: 210 to 421 mmol/L
  • For people with uncontrolled diabetes: 268 to 870 mmol/L

Oral Glucose Tolerance Test

The oral glucose tolerance test (OGTT), also referred to as the glucose tolerance test, measures the body’s ability to clear glucose out of the bloodstream.

The test can be used to diagnose diabetes, gestational diabetes, or prediabetes. The OGTT test is typically not indicated in diagnosing hyperglycemia in people who already have diabetes.

All pregnant women must undergo a glucose challenge between 24 to 28 weeks gestation. It can either be a 75-gram OGTT or a 50-gram OGTT followed by a 100-gram OGTT (if the first test indicates hyperglycemia).

The OGTT is also used four to 12 weeks postpartum in women who've had a history of gestational diabetes in order to confirm persistent diabetes. Additionally, a doctor may recommend an OGTT if they suspect diabetes even though a fasting blood glucose level is normal.

Compared to the FBG test, the OGTT test is more time-consuming. According to the ADA, the OGTT test is the preferred test used in diagnosing type 2 diabetes in adolescents and children.

The test begins after an eight to 12-hour fast. Next, blood is drawn to establish a fasting glucose level.

Following the blood draw, you'll be asked to drink a sugary (glucose-rich) beverage that typically contains 75 grams of carbohydrate. Blood will be drawn at various intervals to measure glucose levels, usually one hour and two hours after the beverage is consumed.

The test reveals how your body metabolized sugar and if it is clearing out of the blood efficiently.

The normal rate of glucose clearing depends on the amount of glucose ingested. After fasting, the normal blood glucose rate is 60 to 100 mg/dL.

Normal blood glucose values after consuming the drink (for those who are not pregnant) are:

  • After 1 hour: Less than 200 mg/dL
  • After 2 hours: Less than 140 mg/dL. Between 140 to 199 mg/dL indicates impaired glucose tolerance (prediabetes). If test results are in this range, a patient is at an increased risk of developing diabetes. Greater than 200 mg/dL indicates diabetes.

For those who are pregnant, normal blood glucose values in this situation are:

  • Fasting: Less than 92 mg/dL
  • After 1 hour: Less than 180 mg/dL
  • After 2 hours: 153 mg/dL 

If 100 grams of glucose are given instead, normal glucose values (for those who are pregnant) are:

  • Fasting: Less than 95 mg/dL
  • After 1 hour: 180 mg/dL 
  • After 2 hours: 155 mg/dL
  • After 3 hours: 140 mg/dL

The diagnosis of gestational diabetes is made with either of the following:

  • Any levels exceeded for a 75-gram OGTT
  • Levels are 130 mg/dL or greater after an hour for a non-fasting 50-gram OGTT plus at least two levels exceeded for a 100-gram OGTT

Differential Diagnosis

In the event that one of the tests you've had confirms hyperglycemia, you'll likely need another test to determine whether or not you have diabetes, prediabetes, insulin resistance, or some sort of glucose intolerance.

The good news is that detecting hyperglycemia early can increase your chance of preventing diabetes. Most of the time, the treatment is a change in lifestyle, such as adhering to a modified carbohydrate diet, increasing exercise, and weight loss.

If you are pregnant and fail your first glucose tolerance, you'll definitely need to take another one. Sometimes women don't pass the first one but pass the second. 

If blood sugars are very high at diagnosis, you may need to start oral medications or insulin. If you have diabetes and your blood sugars are high then you will likely need a change to your treatment plan.

In the event that you have had a regular check-up and your fasting blood sugar comes out to be high, the result can be skewed if you did not fast. Candy, gum, even cough syrup can cause your blood sugar to be elevated, so make sure you tell your healthcare providers if you haven't truly fasted. 

And if your blood glucose monitoring indicates that your blood sugar is high at certain times of the day, contact your healthcare provider to see if you need a treatment plan adjustment. 

Frequently Asked Questions

  • How do I know what my blood sugar should be?

    Your doctor will set targets for you as an individual. Taking into consideration your age, other medical conditions, medications you take, whether or not you’re pregnant, and other factors, your doctor will let you know what your blood sugar levels should be before meals and after meals. You can then monitor your blood sugar at home.

  • Can hyperglycemia be misdiagnosed?

    High blood sugar can be accurately diagnosed with blood tests. However, the underlying cause of hyperglycemia might be misdiagnosed. The greatest concern is that those with hyperglycemia may be misdiagnosed with type 2 diabetes instead of type 1 diabetes, resulting in delayed insulin treatment and a greater risk of diabetic ketoacidosis.

  • Do I need to fast before a blood sugar test?

    For some tests, yes. You will need to fast for at least eight hours before a fasting blood glucose test, which checks your blood sugar levels. For an oral glucose tolerance test, you’ll need to drink a sugary substance before a blood draw to see how well you metabolize sugar. Random blood glucose and A1C tests don't require fasting.

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