If your blood sugar is rising or falling when you have not eaten, exercised, or taken bolus insulin, the most likely issue is your basal insulin. A fasting basal test is a simple but powerful way to determine whether your background insulin is holding your blood sugar steady.
In this guide, you will learn how to properly test your basal insulin and identify whether adjustments are needed.
A fasting basal test helps you isolate and evaluate your long-acting insulin. This could be a daily injection such as Lantus, Levemir, or Tresiba, or a basal rate programmed into an insulin pump.
The goal is to determine whether your blood sugar stays stable during periods when food, bolus insulin, and exercise are not influencing it. If your blood sugar remains flat, your basal insulin is likely set correctly. If it rises or falls consistently, your dose or timing may need to be adjusted.
Select one time period to test. Focus on only one window per day so you can clearly identify patterns. Choose from the following:
Overnight
Morning (6 am to 12 pm)
Afternoon (12 pm to 6 pm)
Evening (6 pm to midnight)
To test your basal accurately, you must avoid variables that affect blood sugar. During the test window:
Do not eat
Do not take any bolus insulin
Drink only water
Avoid caffeine and exercise
This ensures that only your basal insulin is influencing your blood sugar.
Begin the test when your blood sugar is in your target range, ideally between 90 and 126 mg/dL (or 5.0 to 7.0 mmol/L), and is not actively rising or falling. Make sure there is no active insulin or recent food in your system.
If your blood sugar is too high or low at the start, delay the test until you are back in range and stable.
Use your CGM or take fingerstick readings every hour during the 4 to 6 hour window. Your blood sugar should remain within about 30 mg/dL (or 1.7 mmol/L) of where it started. A small amount of variation is normal, but the key is that it should not steadily climb or drop.
Keep a simple log of the following:
Start time
Blood sugar readings
Notes about stress, poor sleep, hormonal changes, or anything else that might affect results
This will help you see patterns clearly and decide if changes are needed.
If your blood sugar drops consistently during the test, your basal insulin may be too strong during that time block. If your blood sugar rises consistently, your basal may be too weak.
Adjustments should be made gradually and carefully. If you are using a pump, small hourly changes can make a big difference. If you are on injections, timing and dose may need to be adjusted.
Work with a diabetes coach or your healthcare team to adjust your settings safely.
A well-set basal insulin dose is the foundation of blood sugar stability. It reduces the need for corrections, improves time in range, and helps you feel more in control.
You should not need to constantly chase highs or lows if your basal insulin is working correctly. Getting this right can improve everything from sleep quality to workout performance.
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