A blood sugar reading of 138 mg/dL is above the normal range, but what it means depends entirely on when you took it. If that number came from a fasting test (no food for at least 8 hours), it falls into the diabetes range. If it came an hour or two after a meal, 138 is actually within normal limits. That distinction changes everything about how seriously you should take this number.
What 138 Means as a Fasting Reading
The American Diabetes Association defines a normal fasting blood sugar as anything below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes, and 126 mg/dL or higher points toward diabetes. A fasting reading of 138 mg/dL clears that diabetes threshold by 12 points.
That said, a single fasting reading of 138 doesn’t lock in a diabetes diagnosis. Doctors typically require an elevated result on more than one testing occasion before diagnosing diabetes. Your body can temporarily push fasting blood sugar higher for a variety of reasons: a bad night of sleep, dehydration, illness, significant stress, or even skipping meals the day before. Caffeine sensitivity, certain nasal sprays, and gum disease can also nudge blood sugar upward. If your reading came from a home glucose meter rather than a lab draw, there’s also a margin of error to account for.
Still, a fasting number of 138 is worth taking seriously. It’s not borderline. It’s well past the prediabetes zone, so getting a follow-up lab test is the logical next step.
What 138 Means After Eating
If you checked your blood sugar within two hours of a meal, 138 mg/dL is normal. Blood sugar naturally rises after eating as your body breaks down carbohydrates into glucose. The standard cutoff for a normal reading two hours after eating is below 140 mg/dL, so 138 falls just inside that line.
If you tested less than two hours after a carb-heavy meal, your blood sugar may still have been on its way up or hadn’t fully come back down yet. A reading of 138 in that window tells you very little on its own. The more useful question is where your blood sugar sits in a fasted state or what your average looks like over time.
What 138 Means During Pregnancy
Pregnant women are screened for gestational diabetes with a glucose challenge test, usually between weeks 24 and 28. You drink a sugary solution, and your blood is drawn one hour later. Most clinics consider a result below 140 mg/dL to be standard, which would make 138 a passing score. However, some clinics and labs use a lower cutoff of 130 mg/dL. Under that stricter threshold, 138 would trigger a follow-up three-hour glucose tolerance test. If you’re pregnant and got this number, check which cutoff your provider uses.
Why One Reading Isn’t the Full Picture
Blood sugar fluctuates throughout the day. It tends to be lowest in the morning before eating and harder to control as the day goes on. A hormone surge in the early morning hours (sometimes called the dawn phenomenon) can push fasting numbers higher than expected, even in people without diabetes. Stress hormones from pain, poor sleep, or emotional strain raise blood sugar too. The CDC notes that even a single night of inadequate sleep can make your body use insulin less efficiently, and dehydration concentrates the glucose already in your bloodstream, producing a higher reading.
This is why doctors don’t diagnose diabetes from one test. A single elevated number could reflect a rough night, a stressful week, or a meter that read slightly high. Two or more elevated readings, ideally from a lab rather than a home device, provide much more reliable information.
Follow-Up Tests That Clarify the Picture
If a fasting reading of 138 has you concerned, the most useful next step is an A1C test. Rather than capturing a single moment, A1C reflects your average blood sugar over the previous two to three months. An A1C of 6.5% or higher indicates diabetes, and 5.7% to 6.4% falls in the prediabetes range. For reference, an A1C of about 6.5% corresponds to an average blood sugar near 140 mg/dL, so if 138 is truly your average rather than a one-time spike, it would place you right at that threshold.
The other common option is an oral glucose tolerance test. You fast overnight, have your blood drawn, drink a standardized sugar solution, then have your blood drawn again two hours later. A two-hour result below 140 mg/dL is normal, 140 to 199 is prediabetes, and 200 or above indicates diabetes.
One important detail from the National Institutes of Health: these different tests don’t always agree. Someone can have a normal fasting glucose but an A1C in the prediabetes range, or vice versa. If one test comes back normal but the suspicion is still there, it’s reasonable to confirm with a different type of test or retest sooner than the usual yearly interval.
Would You Feel Symptoms at 138?
Probably not. Most people with diabetes don’t notice physical symptoms until blood sugar reaches 250 mg/dL or higher. The classic signs of high blood sugar, like excessive thirst, frequent urination, blurred vision, and fatigue, generally don’t show up at 138. People who haven’t been diagnosed with diabetes may notice symptoms at somewhat lower levels than those who are already managing the condition, but 138 is still well below the range where most people feel anything unusual.
This is part of what makes moderately elevated blood sugar tricky. You can walk around with readings in the 130s and 140s for months or years without obvious warning signs, while the elevated glucose gradually affects blood vessels and nerves. That’s why screening matters more than symptoms at this level.
Factors That Can Push Blood Sugar to 138
Beyond diabetes and prediabetes, several everyday factors can temporarily land you at 138 mg/dL:
- Sleep deprivation: Even one poor night reduces your body’s ability to use insulin effectively.
- Dehydration: Less water in your system means the same amount of glucose is more concentrated in your blood.
- Stress or pain: Physical stress, including something as simple as a sunburn, triggers cortisol release that raises blood sugar.
- Caffeine: Some people’s blood sugar is particularly sensitive to coffee, even without added sugar.
- Skipping meals: Going without breakfast can cause higher blood sugar spikes after lunch and dinner.
- Time of day: Blood sugar regulation gets less efficient later in the day, so an evening reading may run higher than a morning one after the same food.
If any of these applied when you tested, repeating the test under better conditions (well-rested, hydrated, fasted for 8 to 12 hours) gives a more accurate baseline. If the number stays above 126 in a true fasting state, that’s a result worth discussing with a healthcare provider and confirming with an A1C or glucose tolerance test.