What Is the Diabetes Blood Test Called?

The test most people mean when they ask about “the diabetes blood test” is the hemoglobin A1c test, usually shortened to A1C or HbA1c. It measures how much glucose has attached to your red blood cells over the previous two to three months, giving a snapshot of long-term blood sugar control rather than a single moment in time. But A1C is far from the only test doctors use. Depending on the situation, you might encounter a fasting plasma glucose test, an oral glucose tolerance test, or one of several lesser-known markers, and the differences between them matter more than most people realize.

The HbA1c Test

HbA1c stands for glycated hemoglobin. Hemoglobin is the protein in red blood cells that carries oxygen, and glucose in your blood naturally sticks to it. The higher your blood sugar has been over the past couple of months, the more glucose-coated hemoglobin you’ll have. The test captures this as a percentage. An A1C below about 5.7% is considered normal, 5.7% to 6.4% falls in the prediabetes range, and 6.5% or higher on two separate occasions points toward a diabetes diagnosis.

What makes A1C so widely used is convenience. You don’t need to fast beforehand, and a single blood draw gives your doctor a reliable picture of average blood sugar over roughly 8 to 12 weeks. It’s central to both diagnosing diabetes and tracking how well treatment is working over time.¹ Results are reported in two different unit systems around the world: a percentage (the format used in the United States and many other countries) and millimoles per mole (mmol/mol), which is the international reference system. Both are anchored to the same standardized measurement framework, so they’re interchangeable once you know the conversion.²

Fasting Plasma Glucose

The fasting plasma glucose test, or FPG, is the other workhorse of diabetes diagnosis. You fast for at least eight hours, then have blood drawn, and the lab measures how much glucose is circulating in your blood at that moment. A result below 100 mg/dL is normal, 100 to 125 mg/dL suggests prediabetes, and 126 mg/dL or higher (confirmed with a second test) meets the threshold for diabetes.

FPG is simple and cheap, but it captures only one point in time. Your blood sugar the morning of the test could be affected by what you ate the night before, how well you slept, stress, or whether you accidentally had coffee with sugar on the drive to the lab. That’s why doctors often pair it with an A1C or repeat it on a different day before making a diagnosis. In pregnancy, FPG is sometimes used as a first-pass screen for gestational diabetes, though its sensitivity as a standalone test is limited compared to the more comprehensive oral glucose tolerance test.³

The Oral Glucose Tolerance Test

The oral glucose tolerance test, or OGTT, is more involved. After fasting overnight, you drink a sugary solution containing a set amount of glucose (typically 75 grams), and your blood is drawn at intervals, usually at fasting and then one and two hours later. The test measures how quickly your body clears the sugar from the bloodstream, and it’s particularly good at catching people whose fasting numbers look fine but whose blood sugar spikes excessively after eating.

For gestational diabetes specifically, the OGTT remains the gold-standard screening method.³ Pregnant women are routinely screened between 24 and 28 weeks. The test is not pleasant. Drinking a concentrated glucose solution on an empty stomach makes some women nauseous, and waiting around for multiple blood draws takes time. Researchers have looked into whether a simpler fasting glucose measurement could substitute, but the sensitivity of FPG alone at standard cutoffs is low enough that it misses a large share of gestational diabetes cases.⁴ So for now, most guidelines stick with the OGTT for this purpose.

Outside of pregnancy, the OGTT is used less commonly for routine diabetes diagnosis, mainly because the A1C and fasting glucose tests are simpler and faster. But in research settings and in cases where other test results are ambiguous, the two-hour OGTT still provides useful information.

When HbA1c Results Are Unreliable

A1C is convenient, but it has blind spots. Because it measures glucose stuck to hemoglobin in red blood cells, anything that changes how long those cells live or how hemoglobin behaves can throw off the reading. If your red blood cells are destroyed faster than normal, as happens in certain types of anemia, sickle cell disease, or after significant blood loss, the A1C will read artificially low. A study on patients with type 2 diabetes found that shorter red blood cell lifespans caused noticeable underestimates of actual blood sugar levels.⁵ That’s a problem if your doctor is using that number to decide whether to adjust your medication.

Iron-deficiency anemia can push the number the opposite direction, making it read falsely high, because iron-depleted red blood cells tend to live longer, giving glucose more time to attach. Kidney disease, recent blood transfusions, and certain inherited hemoglobin variants can all create similar distortions. One case report documented an abnormally low A1C caused by a hemoglobin variant that didn’t produce any obvious symptoms, meaning the patient’s actual blood sugar was significantly worse than the test suggested.¹ This is relatively rare, but it highlights why a single test type isn’t always sufficient.

Alternative Blood Sugar Markers

When A1C can’t be trusted, doctors turn to other markers that don’t depend on red blood cell lifespan. The two most established alternatives are glycated albumin and fructosamine. Both work on a similar principle to A1C: sugar in the blood attaches to proteins, and measuring how much has accumulated tells you about recent blood sugar levels. The difference is that albumin and other serum proteins turn over faster than red blood cells, so these markers reflect a shorter window, roughly two to four weeks rather than two to three months. That shorter window also makes them useful for tracking rapid changes in blood sugar control, such as after starting a new medication.⁶

A third option, 1,5-anhydroglucitol (sometimes sold under the brand name GlycoMark), works differently. It’s a naturally occurring sugar-like molecule that’s normally present in your blood at stable levels. When blood glucose spikes above the kidney’s reabsorption threshold, 1,5-AG gets flushed out in urine instead of being reclaimed, so its blood level drops. Low 1,5-AG signals that you’ve been experiencing glucose spikes over the past one to two weeks.⁷ It’s especially good at catching post-meal glucose spikes that A1C and fructosamine can miss, since those tests average out the highs and lows. Research has also linked low 1,5-AG levels to markers of cardiovascular risk, including more severe coronary artery plaque.⁸

None of these alternatives have replaced A1C as the primary test for most patients. They’re supplements, used when something makes A1C unreliable or when doctors need a finer-grained picture of glucose fluctuations. Understanding that they exist is useful, though, because if your A1C numbers don’t seem to match how you feel or what your home glucose readings show, your doctor has other tools to investigate.

C-Peptide and Autoantibody Testing

The tests above all measure blood sugar itself or its trail on various proteins. But sometimes the question isn’t “how high is your blood sugar?” but rather “why is it high?” That’s where C-peptide and autoantibody tests come in, and they serve very different purposes.

C-peptide is a byproduct of insulin production. When your pancreas makes insulin, it first creates a precursor molecule called proinsulin, which then gets cut into insulin and C-peptide in equal amounts. Measuring C-peptide in blood tells your doctor how much insulin your pancreas is actually producing.⁹ This is valuable in several scenarios. If you’re already on injected insulin, a regular insulin test would mix up the insulin your body makes with the insulin you inject. C-peptide isn’t found in therapeutic insulin, so it isolates what your pancreas is contributing. Low or undetectable C-peptide in someone with diabetes strongly suggests type 1 disease, where the immune system has destroyed the insulin-producing cells. Higher C-peptide points toward type 2, where the pancreas still makes insulin but the body doesn’t respond to it well enough.

Autoantibody testing takes this a step further for type 1 diabetes specifically. In type 1, the immune system attacks the beta cells in the pancreas, and it produces specific antibodies during this process. Testing for these diabetes-related autoantibodies can identify people who are in the early stages of developing type 1 diabetes, sometimes years before they have any symptoms or blood sugar problems.¹⁰ This has become increasingly important as new treatments have emerged that can delay the onset of full-blown type 1 diabetes in high-risk individuals, making early detection more actionable than it used to be.

Home Glucose Meters and Their Limitations

Most people with diabetes also use finger-stick blood glucose meters at home, usually called blood glucose monitors or BGMs. These give an instant reading of your blood sugar from a tiny drop of blood, and they’re the backbone of day-to-day self-management for anyone on insulin or certain oral medications.

Home meters are useful but imperfect. Various factors can skew the readings, including extreme temperatures, high or low red blood cell counts, and certain medications.¹¹ The type of blood sample matters too. Most people test capillary blood from their fingertip, which gives slightly different results than venous blood drawn from your arm. Research has shown that the gap between capillary and venous readings varies depending on the meter’s technology, with some meters more sensitive to oxygen differences between the two blood types than others.¹²

International standards require that home glucose meters hit certain accuracy benchmarks, but not all devices meet them equally well. A study evaluating one non-invasive home glucose device found that only about 19% of readings met the international accuracy standard, and only about two-thirds fell within the acceptable safety zones on error analysis.¹³ That’s a sobering result and a reminder that not all devices marketed as glucose monitors deliver the same reliability. If you’re choosing a meter, looking for one that has performed well against recognized accuracy standards is worth the effort.

Continuous Glucose Monitoring

Continuous glucose monitors, or CGMs, represent a different approach entirely. Instead of pricking your finger several times a day, a CGM uses a small sensor inserted just under the skin (usually on the arm or abdomen) that measures glucose in the interstitial fluid every few minutes. You get a rolling stream of data on your phone or a dedicated receiver, including trends and alerts for high or low readings.

CGMs don’t measure blood glucose directly. They measure glucose in the fluid surrounding your cells, which lags behind blood glucose by a few minutes. Clinical validation studies have shown strong agreement between CGM readings and standard blood glucose tests, with one study finding a correlation coefficient above 0.98 and the vast majority of paired measurements falling in the no-risk zone of error-grid analysis.¹⁴ Still, CGMs are not a replacement for lab-based A1C or fasting glucose tests when it comes to formal diagnosis. They’re monitoring tools, not diagnostic ones.

One of the more useful concepts to come out of CGM technology is “time in range,” which tracks what percentage of the day your glucose stays between roughly 70 and 180 mg/dL. Studies have found that CGM use helps people spend more time in that target zone while reducing time in both dangerously low and dangerously high ranges.¹⁵ Time in range gives a much richer picture than A1C can offer alone, because two people with the same A1C could have very different glucose patterns: one might be stable all day, while the other swings wildly between highs and lows but averages out to the same number.

How HbA1c Got Standardized

If you’ve ever wondered why your A1C result from one lab should be comparable to a result from a different lab across the country (or across the world), the answer involves decades of behind-the-scenes work. In the early days of A1C testing, different laboratories used different methods, and results could vary substantially from one lab to another. Two organizations tackled this problem from different directions.

In the United States, the National Glycohemoglobin Standardization Program, or NGSP, was established to align commercial A1C assays with the method used in the landmark Diabetes Control and Complications Trial, the study that firmly established the link between A1C levels and diabetes complications. Internationally, the International Federation of Clinical Chemistry and Laboratory Medicine developed a reference measurement system based on purified HbA1c calibration material, giving labs a universal standard to calibrate against.¹⁶ An international consensus agreement recognized the IFCC system as the global anchor, with results reported in both percentage units and mmol/mol units.²

This standardization work is the reason you can move between doctors, clinics, or even countries and still compare your A1C results meaningfully. It wasn’t always the case, and it remains an ongoing effort to keep lab methods aligned as new testing technologies emerge.

Emerging Research on Early Detection

All of the tests described so far measure things that are already happening: sugar in the blood, sugar stuck to proteins, insulin production. Researchers are working on biomarkers that could flag diabetes risk even earlier, before blood sugar levels change at all. One active area involves tiny molecules called microRNAs, which are short strands of RNA that circulate in the blood and play roles in regulating gene activity. Certain microRNA patterns appear to be altered in people heading toward type 2 diabetes, raising the possibility that a blood test could eventually identify prediabetic changes in high-risk individuals before conventional tests pick up anything unusual.¹⁷

This work is still in early stages and hasn’t made it into clinical practice. But it’s worth knowing that the future of diabetes testing may involve markers that detect the disease process before blood sugar rises, rather than waiting for glucose itself to signal the problem. For now, A1C, fasting glucose, and the OGTT remain the tools your doctor will use. If you’ve been told to get “the diabetes blood test,” chances are excellent they mean the A1C, and understanding what it does and what it can’t do puts you in a better position to have a useful conversation about your results.

1
PubMed Central. Abnormally Low Hemoglobin A1c Due to a Heterozygous Alpha-Globin Variant (Hb I; HBA2 c.49A>G, p.Lys17Glu) in a Geriatric Patient
2
PubMed Central. The NGSP: Over 20 Years of Improving HbA1c Measurement
3
European Journal of Human Health. Comparison of Oral Glucose Tolerance Test and HOMA-IR in Screening for Gestational Diabetes Mellitus in Pregnant Women
4
PubMed Central. Characteristics of gestational diabetes subtypes classified by oral glucose tolerance test values
5
PubMed Central. The influence of shorter red blood cell lifespan on the rate of HbA1c target achieved in type 2 diabetes patients with a HbA1c detection value lower than 7%
6
PubMed Central. Glycemic assessment when hemoglobin A1c is unreliable: a clinical-laboratory framework for kidney disease, anemia, and hemoglobinopathies
7
PubMed. 1,5-anhydroglucitol (GlycoMark) as a marker of short-term glycemic control and glycemic excursions
8
PubMed Central. 1,5-Anhydroglucitol as a Marker of Acute Hyperglycemia in Cardiovascular Events
9
PubMed Central. A Practical Review of C-Peptide Testing in Diabetes
10
PubMed Central. Recommendations for Screening and Monitoring the Stages of Type 1 Diabetes in the Immune Therapy Era
11
PubMed Central. Interferences and Limitations in Blood Glucose Self-Testing: An Overview of the Current Knowledge
12
PubMed Central. Capillary and Venous Blood Glucose Accuracy in Blood Glucose Meters Versus Reference Standards: The Impact of Study Design on Accuracy Evaluations
13
PubMed Central. Accuracy of a Non-Invasive Home Glucose Monitor for Measurement of Blood Glucose
14
PubMed Central. Clinical validation of continuous glucose monitoring in glycemic management of critically ill sepsis patients: A prospective randomized clinical study
15
PubMed Central. Continuous Glucose Monitoring Versus Self-monitoring of Blood Glucose in Type 2 Diabetes Mellitus: A Systematic Review with Meta-analysis
16
PubMed Central. HbA1c standardisation: history, science and politics
17
PubMed Central. Novel Micro-Ribonucleic Acid Biomarkers for Early Detection of Type 2 Diabetes Mellitus and Associated Complications-A Literature Review