How is diabetes diagnosed? Explaining important factors beyond blood glucose levels
Hello!
I am Saori Murakawa, a medical writer.
Do you have the impression that diabetes simply means having "high blood sugar," and that you can be diagnosed with diabetes just by having a high blood glucose reading once during a blood test?
While blood glucose level is certainly an important test item, it is also important to make a comprehensive judgment based on other factors such as HbA1c and the presence or absence of typical diabetes symptoms.
In this article, I will explain in an easy-to-understand way the diagnostic criteria for diabetes established by the Japan Diabetes Society.
In the previous article on diabetes, I explained the causes of high blood sugar and the typical symptoms of diabetes.
If you haven't read it yet, please take a look.
Review: Diabetes is a disease where high blood sugar levels persist
Diabetes is a disease where insulin, a hormone that works to lower blood sugar levels, is either insufficient in secretion or becomes less effective, causing glucose in the blood, or a state of high blood sugar, to persist.
Diagnosis is made comprehensively based on blood glucose levels, HbA1c, and the presence of symptoms or complications
To diagnose diabetes, it is necessary to prove that a state of high blood sugar is persisting.
To do this, we perform blood tests to check blood glucose levels and HbA1c. We make a diagnosis by confirming whether the test results fall under the "
diabetes type" category, and in some cases, by checking for typical symptoms or complications of diabetes.
HbA1c is
a test item that reflects the average blood glucose level over the past 1-2 months. While blood glucose levels are easily affected by the time between a meal and the blood draw, HbA1c is
less affected by meals, making it useful for early detection of diabetes and for evaluating the state of control.
What is the diabetes type?
If any of the following four conditions are confirmed, it is classified as "diabetes type."
Fasting blood glucose level is 126 mg/dL or higher
👉 Blood glucose level when blood is drawn after not eating for 10 hours or moreBlood glucose level at 2 hours in a 75g oral glucose tolerance test (OGTT) is 200 mg/dL or higher
Casual blood glucose level of 200 mg/dL or higher
👉 Blood glucose level measured at the time of blood draw, regardless of food or drink intake before the testHbA1c of 6.5% or higher
What is a 75g Oral Glucose Tolerance Test (OGTT)?
A test where you drink a solution containing 75g of glucose after measuring your fasting blood glucose level, and then check your blood glucose levels 30 minutes, 1 hour, and 2 hours later.
Diagnostic process
When making a diagnosis,it is important that the blood glucose level is in the diabetic range because HbA1c can also be elevated by factors other than diabetes.
In some cases, a diagnosis is made after a single test, while in others, a re-test on a different day is required.
[Cases diagnosed with a single test]
・When it can be confirmed that both blood glucose and HbA1c are in the diabetic range
① Fasting blood glucose level of 126 mg/dL or higher
② OGTT 2-hour value of 200 mg/dL or higher
③ Casual blood glucose level of 200 mg/dL or higher
Any one of ① to ③ above
+
HbA1c of 6.5% or highermet simultaneously in a single blood draw teststate
・When blood glucose is in the diabetic range and there are typical symptoms or complications of diabetes
① Fasting blood glucose level of 126 mg/dL or higher
② OGTT 2-hour value of 200 mg/dL or higher
③ Casual blood glucose level of 200 mg/dL or higher
Any one of ① to ③ above
+
④ Typical symptoms of diabetes are present
👉 Dry mouth, excessive thirst, frequent urination, weight loss despite eating, etc.
or
⑤ Definite diabetic retinopathy is present
👉 Confirmed via eye examination
If any of the above are observed, diabetes is diagnosed with a single test.
What is diabetic retinopathy?
Damage to the retina of the eye caused by diabetes. If high blood glucose levels persist for a long time, the capillaries spreading across the retina are damaged, which can lead to blindness if it progresses.
[Cases requiring a re-test on a different day]
・When only the blood glucose level falls into the diabetic rangebutno typical symptoms of diabetes or retinopathy are observed・
When only HbA1c falls into the diabetic range
In these cases, a re-test will be performed on a different day. If the results of the re-test meet any of the following three criteria, a diagnosis of diabetes can be made.
If the re-test results meet any of the following three criteria, a diagnosis of diabetes can be made.
In the re-test,both blood glucose and HbA1c are in the diabetic range
In the re-test,only blood glucose is in the diabetic range
In the initial test, only blood glucose level is in the diabetic range → In the retest, only HbA1c is in the diabetic range
On the other hand, if either of the following two conditions is met during the retest, a retest of blood glucose and HbA1c will be conducted within 3 to 6 months as a 'suspected case of diabetes'.
・Neither is in the diabetic range
・Only HbA1c is in the diabetic range in both the initial test and the retest
Conclusion
In this article, I explained the items necessary for diagnosing diabetes and the diagnostic process.
Diabetes often has no subjective symptoms, but even without symptoms, it may be diagnosed based on blood test results such as blood glucose and HbA1c, or the presence of diabetic retinopathy.
I believe blood glucose and HbA1c are items checked during health checkup blood tests, so if you are concerned about diabetes, you may want to refer to the diagnostic criteria values or look back to see if you have any applicable subjective symptoms.
However, just because you do not meet the diagnostic criteria for diabetes does not mean you are completely fine! There is also something called 'pre-diabetes' where you do not reach the diagnostic criteria for diabetes, but your blood glucose levels are higher than normal.
Pre-diabetes will be explained in another article.
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If you are interested, please be sure to read my self-introduction article and past articles as well.
References
(1) The Japan Diabetes Society (2024) Diabetes Treatment Guide 2024. Bunkodo (2) Fujiko Hijikata (2025) Complete Illustrated Guide: Diabetes Nursing & Blood Glucose Control. Chohinsha (3) Ministry of Health, Labour and Welfare. HbA1c. Last updated June 20, 2023. https://kennet.mhlw.go.jp/information/information/dictionary/metabolic/ym-066 (Accessed October 13, 2025) (4) Ministry of Health, Labour and Welfare. Diabetic Retinopathy. Last updated August 26, 2024. https://kennet.mhlw.go.jp/information/information/dictionary/metabolic/ym-061 (Accessed October 13, 2025)