Blood testing provides health care providers with a snapshot of the body’s internal state. Glucose, often called blood sugar, is a primary energy source for the body’s cells, making its measurement an important part of diagnostic blood work. Different blood tests analyze glucose levels in distinct ways, providing either an immediate reading or a long-term average. The specific test a doctor orders depends on the patient’s symptoms or risk factors for evaluating glucose metabolism.
What the Complete Blood Count (CBC) Actually Measures
The Complete Blood Count (CBC) is one of the most commonly ordered blood tests, but it does not provide any information about blood glucose levels. The CBC’s primary function is to offer a detailed look at the cellular components circulating within the blood plasma. It is used to screen for or monitor a wide range of conditions that affect the blood cells.
The panel quantifies and examines three major types of cells: red blood cells (RBCs), white blood cells (WBCs), and platelets. Red blood cells are measured by their total count and the amount of hemoglobin they contain, the protein responsible for oxygen transport. The test also calculates the hematocrit, which is the percentage of total blood volume made up of red cells.
The white blood cell count assesses the components of the immune system, often indicating the presence of an infection or inflammatory process. Platelets, which are small cell fragments, are tracked to evaluate the blood’s ability to form clots and prevent excessive bleeding. Because the CBC focuses on the quantity and characteristics of these formed elements, glucose is not a metric included in this panel.
Tests That Measure Current Blood Glucose Levels
Tests that measure current blood glucose offer an immediate, real-time look at the sugar concentration circulating in the blood when the sample is drawn. The most common is the Fasting Plasma Glucose (FPG) test, which requires the patient to fast for at least eight hours prior to the blood draw. Fasting ensures the result reflects the baseline level of glucose the body maintains without recent dietary influence. The FPG test is a simple, effective, and relatively inexpensive method often used as a first step in screening for prediabetes or diabetes.
Another immediate measurement is the Random Plasma Glucose (RPG) test, which can be performed at any time without a fasting requirement. This test is generally used when a patient presents with symptoms suggesting very high blood sugar, such as excessive thirst or frequent urination, and a rapid diagnosis is necessary. An elevated RPG result may indicate hyperglycemia, prompting further confirmatory testing.
A more involved assessment of glucose processing is the Oral Glucose Tolerance Test (OGTT), frequently used to diagnose gestational diabetes in pregnant individuals. The procedure begins with a fasting blood draw, followed by the patient drinking a standardized liquid containing a specific amount of glucose (typically 75 grams for non-pregnant adults). Subsequent blood samples are collected at set intervals, usually two hours later, to track how efficiently the body moves glucose from the bloodstream into the cells.
Measuring Long-Term Glucose Control (HbA1c)
Unlike immediate glucose tests, the Hemoglobin A1c (HbA1c) test provides a historical average of blood sugar control over an extended period. This test measures the amount of glycated hemoglobin—hemoglobin that has chemically bonded with glucose from the bloodstream. The “glycation” process is irreversible and occurs naturally over the lifetime of a red blood cell.
Since red blood cells typically live for about two to three months, the HbA1c result captures the average blood glucose level for the preceding 90 days. A higher percentage indicates that higher amounts of glucose were present in the blood, leading to more sugar-coated hemoglobin. This test is valuable because it is not affected by short-term fluctuations caused by stress, illness, or recent meals.
The HbA1c test is a standard tool used both to diagnose prediabetes and diabetes and to monitor how well a person with diabetes is managing their condition. It offers a reliable measure of long-term control, helping healthcare providers adjust medication or lifestyle recommendations to reduce the risk of complications. Because no fasting is required, this test is convenient for patients and can be performed at any time.
Understanding Blood Sugar Test Results
Interpreting the results from glucose tests involves comparing the numerical values against established reference ranges. For the Fasting Plasma Glucose test, a result below 100 milligrams per deciliter (mg/dL) is considered normal. Results between 100 mg/dL and 125 mg/dL suggest prediabetes, meaning blood sugar levels are elevated but not high enough for a diabetes diagnosis. A diagnosis of diabetes is made if the FPG result is 126 mg/dL or higher on two separate occasions.
The Hemoglobin A1c test results are reported as a percentage, reflecting the proportion of glycated hemoglobin in the blood. A normal A1c result is below 5.7%, while a range between 5.7% and 6.4% indicates prediabetes. A result of 6.5% or higher is used to diagnose diabetes. These results provide the context necessary for a healthcare provider to determine whether a person needs to monitor their glucose, make lifestyle changes, or start a treatment plan.