by Sadie
QUESTION: Hi, my blood sugar levels are normally around 90 mg/dL, but lately I got a reading of 121 mg/dL. Am I doing something wrong?
A blood sugar reading of 121 mg/dL (6.7 mmol/L) does not necessarily mean that you are doing anything wrong. The most important question is when the test was taken.
If 121 mg/dL was a true fasting laboratory result after at least 8 hours without calories, it falls within the prediabetes range of 100–125 mg/dL. If it was measured after eating or at a random time, it cannot be interpreted using fasting ranges.
One reading of 121 mg/dL does not diagnose prediabetes or diabetes. If fasting readings remain higher than your usual 90 mg/dL, I would discuss a laboratory fasting glucose and HbA1c test with your doctor.
Answer by Dr. Albana Greca, MD, MMedSc
Hi Sadie,
I understand why a change from your usual blood sugar of around 90 mg/dL to 121 mg/dL would catch your attention. However, I would not judge this number until I know whether you were fasting, whether you tested at home or in a laboratory, and whether 121 was a single reading or part of a repeated pattern.
A fasting plasma glucose test is taken after at least 8 hours without calories. For nonpregnant adults who have not been diagnosed with diabetes:
Therefore, a fasting result of 121 mg/dL is higher than your previous readings and falls within the prediabetes range. It does not mean that you definitely have prediabetes based on this one measurement. A laboratory test and, when appropriate, repeat testing are important.
The difference between these ranges is explained more fully in our guide to normal fasting blood sugar levels.
If you had recently eaten, a reading of 121 mg/dL has a different meaning. Blood glucose normally rises after food, so a post-meal or random result should not be compared directly with a fasting result of 90 mg/dL.
I would also not tell you that a random glucose of 121 mg/dL proves that you are completely healthy. Random glucose alone is not normally used to rule out prediabetes. A random plasma glucose of 200 mg/dL or higher together with classic symptoms of high blood sugar can be used to diagnose diabetes, but lower random readings do not provide the same diagnostic information as fasting glucose or HbA1c.
Understanding whether a blood sugar reading is normal therefore always requires knowing when the measurement was taken.
Blood glucose naturally changes from day to day. A temporary increase can occur because of:
Morning readings can also be affected by hormones that stimulate the liver to release glucose before waking. For this reason, one higher result should be interpreted as part of a pattern rather than in isolation.
I would not panic over one reading of 121 mg/dL, but I also would not ignore repeated fasting readings in this range.
If your fasting glucose repeatedly measures between 100 and 125 mg/dL, your doctor may check an HbA1c. An HbA1c of 5.7%–6.4% is within the prediabetes range, while 6.5% or higher meets a laboratory criterion for diabetes when confirmed as required.
A blood sugar level chart can also help you distinguish fasting, after-meal, and diabetes treatment targets instead of comparing different types of readings as though they mean the same thing.
If you already have diabetes, the interpretation is different. For many nonpregnant adults with diabetes, a common premeal glucose target is approximately 80–130 mg/dL, although your personal target may be different.
In that situation, a fasting reading of 121 mg/dL may actually be within your treatment target. Your HbA1c, other daily readings, medicines, risk of hypoglycemia, age, and overall health are more useful than one number alone.
Sadie, the change from around 90 to 121 mg/dL is worth watching, but it does not by itself mean that you have diabetes or that you have done something wrong. First establish whether these are true fasting readings. If fasting values repeatedly remain above 100 mg/dL, discuss a laboratory fasting glucose and HbA1c with your doctor.
The most useful information is not one isolated number, but the pattern of your readings over time.
Medical review: Dr. Ruden Cakoni, MD, Endocrinologist
Last reviewed: August 2026.
Educational safety note: This answer is for general diabetes education only and does not replace personal medical advice, diagnosis, or treatment. Do not start, stop, or change medicines, supplements, diet, or exercise plans without discussing them with your healthcare provider.
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