Blood Sugar 158 mg/dl — Too High? Fasting & After Eating
Where 158 mg/dl sits — adjust to your situation
158 mg/dl as a fasting reading
Diabetes range
A fasting reading of 126 mg/dl or higher falls into the diabetes range. Diagnosis requires a repeat test on another day — take this result to your doctor.
Fasting = no food for at least 8 hours, typically first thing in the morning. More on fasting readings.
158 mg/dl after eating
Prediabetes range
Between 140 and 199 mg/dl two hours after a meal suggests impaired glucose tolerance, the post-meal form of prediabetes.
Guidelines measure 2 hours after the start of a meal. More on post-meal readings.
On prednisone and seeing 158 in the afternoon
Steroid tablets such as prednisone, prednisolone and dexamethasone are used for asthma flares, lung disease, joint and skin inflammation, some cancers and many other conditions. They are very effective, and they raise blood sugar. A reading of 158 in the afternoon or evening while on a course of prednisone is a typical example, and it can happen in people who have never had a high reading before.
The pattern is distinctive. A single morning dose of prednisone usually has its largest effect on glucose from around midday into the evening. By the next morning, the effect has partly worn off, so the fasting reading can look normal or only slightly raised.
Why a fasting check can miss steroid highs
Steroids make the liver release more glucose and make muscle less responsive to insulin. With a once-daily morning tablet, those effects build through the day. Someone who only checks first thing in the morning may see a reassuring 105 and never notice the 158 or higher that came the afternoon before.
For that reason, care teams usually suggest checking before lunch, before the evening meal, or two hours after lunch while on steroids. Those checks show the peak effect. If you already have diabetes, your team may ask for several checks a day during a course and may adjust medicine temporarily, often increasing insulin or adding it at the time of day when steroids have their peak effect.
How much does the form of steroid matter?
- Tablets and IV steroids have the largest effect. Higher doses and longer courses mean bigger rises.
- Joint or soft-tissue injections can raise glucose for a few days, sometimes longer in people with diabetes.
- Inhaled steroids for asthma and COPD at usual doses have little effect on blood sugar for most people.
- Skin creams and nasal sprays at usual doses rarely matter.
When the course ends or the dose is reduced, glucose usually falls back toward its previous level within days. In some people, a steroid course reveals diabetes or prediabetes that was developing anyway; a follow-up test after the course can sort that out.
While on steroids, watch for signs that glucose is climbing well beyond the 150s: strong thirst, passing large amounts of urine, a dry mouth, blurred vision or unusual tiredness. These suggest much higher levels and are a reason to call the prescriber the same day, even if you have no history of diabetes.
Do not stop steroids because of the numbers
Never stop a steroid on your own. After more than a short course, the body’s own cortisol production is suppressed, and stopping suddenly can cause a dangerous drop in cortisol. The condition the steroid was treating may also flare. If your glucose readings worry you, contact the prescriber or your diabetes team. The steroid dose, timing or other treatments can be adjusted safely. Other medicines can raise glucose too; the what affects blood sugar page lists some.
Measured right after exercise?
There is no official range for readings taken during or just after a workout, so the two classifications above do not apply to them. How exercise moves blood sugar.
How 158 mg/dl compares to a typical day
Typical day without diabetes; your reading is the horizontal line — higher than a typical day reaches two hours after a meal; brief one-hour peaks this high are common.
What to do with a reading of 158 mg/dl
Book a doctor's appointment to confirm with a lab test (fasting glucose or A1C). If you are already diagnosed, this reading belongs in your log for your care team.
What actually moves the number: How to lower blood sugar, Foods that steady your glucose and Exercise and blood sugar.
Save this reading
Stored only in your browser on this device. See the privacy policy for what analytics records when you allow it. Build a history you can chart, print for your doctor, or export.
Questions about 158 mg/dl
Is 158 mg/dl blood sugar normal?
It depends on when you measured. Fasting, 158 mg/dl is diabetes range (normal fasting range: 70–99 mg/dl). Two hours after a meal it is prediabetes range (normal post-meal range: 70–139 mg/dl).
What is 158 mg/dl in mmol/L?
158 mg/dl equals 8.8 mmol/L. Divide mg/dl by 18 to convert (used in the UK, Canada, Australia and most of Europe).
Does one reading of 158 mg/dl mean I have diabetes?
No single home reading is a diagnosis. Guidelines require a second abnormal lab result, either a different test from the same blood draw or a repeat on another day (fasting glucose, an oral tolerance test, or A1C). But a reading like this is exactly the reason those tests exist — take it to your doctor.
How accurate is a home reading of 158 mg/dl?
Home meters are allowed an error of ±15 mg/dl below 100 mg/dl and ±15 % from 100 up (ISO 15197), so a displayed 158 can correspond to a true value anywhere from 134 to 182 mg/dl. Fasting, the entire window stays inside the diabetes range band — this classification is robust even to meter error. After a meal, the window touches the 140 mg/dl boundary.
What A1C would 158 mg/dl correspond to?
If 158 mg/dl were your average glucose over three months, it would work out to an A1C of about 7.1 % — at or above the 6.5 % diabetes threshold. A single reading is not an average — but the conversion shows what this level would mean as a habit. How the two measures relate: A1C and blood sugar.
I don’t have diabetes. Should I check my sugar during a prednisone course?
For a short, low-dose course, most people do not need to. For high doses, long courses, or if you have prediabetes, a strong family history or symptoms such as heavy thirst, ask your doctor whether a few afternoon checks would be sensible.
Why are my readings normal in the morning but high by dinner?
That is the classic pattern of a morning steroid dose. The effect peaks hours after taking it and fades overnight. Readings taken before the evening meal are usually the best way to see how much the steroid is raising your glucose.
BloodSugarEasy — blood sugar reading report
Reading: 158 mg/dl (8.8 mmol/L)
Fasting classification: Diabetes range · After-meal classification: Prediabetes range
Reference: ADA criteria, reviewed 2026-11-03 · https://bloodsugareasy.com/blood-sugar-158/
Printed: