Checking blood sugar involves pricking your finger with a small needle, placing a drop of blood on a test strip, and reading the result on a portable meter. The whole process takes under a minute once you get the hang of it. There are also wearable sensors that track your levels continuously without finger pricks. Here’s how both methods work, what can throw off your results, and how to make sense of the numbers.
Finger-Stick Testing Step by Step
A blood glucose meter (glucometer) is the most common way to check blood sugar at home. You’ll need the meter itself, a lancing device with lancets, and test strips that match your meter. Once you have your supplies, the process looks like this:
- Wash your hands with soap and warm water, then dry them completely. Hand sanitizer isn’t a substitute here because residue on your skin can skew the reading.
- Insert a fresh test strip into the meter so it’s ready to receive blood.
- Load a lancet into the lancing device and press it against the side of your fingertip. The sides hurt less than the pad of your finger.
- Squeeze gently from the base of your finger toward the tip until a small drop of blood forms.
- Touch the drop to the edge of the test strip. The strip will wick the blood in automatically.
- Wait a few seconds for the meter to display your result.
- Record the number along with the time, what you ate, and anything else that might explain an unusual reading.
If you’re having trouble getting enough blood, try warming your hands first. Cold fingers restrict blood flow to the fingertips. Running them under warm water or shaking your hands for a few seconds before washing can help.
Continuous Glucose Monitors
A continuous glucose monitor, or CGM, is a small sensor you wear on your body that tracks glucose around the clock. The sensor sits just under the skin, typically on the back of your upper arm or your abdomen, and measures glucose in the fluid between your cells (not directly in your blood). A transmitter on the sensor sends readings wirelessly to your phone or a separate receiver every few minutes.
You apply the sensor yourself using a one-touch applicator. A tiny needle inserts the sensor under the skin, then the needle retracts and only the flexible sensor filament stays in place. Most sensors last 7 to 15 days before you replace them. One implantable version, placed by a healthcare provider, lasts several months.
Because CGMs measure the fluid around your cells rather than blood directly, there’s a built-in time lag. Glucose reaches your bloodstream first, then leaks into the surrounding fluid. This delay is roughly 10 minutes on average. That means during rapid changes, like right after a meal or during exercise, a CGM reading may not perfectly match a finger-stick result. For most daily decisions, the difference is small enough not to matter.
Testing From Sites Other Than Your Fingertip
Some meters allow you to draw blood from your forearm, upper arm, the base of your thumb, or your thigh. These alternative sites can be less painful than fingertips because the skin there has fewer nerve endings. However, blood sugar changes show up at these sites more slowly than at your fingertips.
Stick with a fingertip test if you suspect your blood sugar is low, if you don’t usually feel symptoms when it drops, or if the alternative-site result doesn’t match how you’re feeling. Not every meter supports alternative-site testing, so check your device’s instructions before trying it.
What Can Throw Off Your Reading
A glucometer is only as reliable as the conditions you test under. Several common factors can produce an inaccurate number:
- Dirty hands. Sugar residue from food is the classic culprit. Even a trace of fruit juice on your fingertip can make a normal reading look dangerously high. Soap and water, fully dried, is the fix.
- Expired or improperly stored test strips. Strips are sensitive to heat and humidity. Once you open a vial, strips generally remain stable for about 15 weeks when stored at room temperature (around 77°F / 25°C). Don’t leave them in a hot car or a steamy bathroom, and always check the expiration date.
- Dehydration or anemia. Changes in the concentration of red blood cells in your blood can shift readings in either direction. If you’re significantly dehydrated or have untreated anemia, your results may be less reliable.
- Alcohol wipes. If you use an alcohol swab instead of soap and water, let the site dry completely before pricking. Residual alcohol can dilute the blood sample.
What the Numbers Mean
Blood sugar is measured in milligrams per deciliter (mg/dL) in the United States. The American Diabetes Association’s targets for most non-pregnant adults with diabetes are:
- Before meals: 80 to 130 mg/dL
- 1 to 2 hours after the start of a meal: below 180 mg/dL
These are general goals. Your own targets may be tighter or looser depending on your age, how long you’ve had diabetes, and other health conditions. The post-meal check should happen one to two hours after you start eating, since that’s when blood sugar typically peaks.
If you don’t have diabetes and you’re checking out of curiosity or concern, a fasting reading (first thing in the morning, before eating) below 100 mg/dL is considered normal. Between 100 and 125 mg/dL falls into the prediabetes range. A fasting result of 126 mg/dL or higher on two separate occasions points toward diabetes.
How Spot Checks Relate to A1C
Your daily meter readings capture individual moments. An A1C blood test, done at a lab or your doctor’s office, reflects your average blood sugar over the previous two to three months. The two are connected by a straightforward conversion: an A1C of 7% corresponds to an estimated average glucose of about 154 mg/dL, while an A1C of 8% translates to roughly 183 mg/dL. Each half-percentage-point increase in A1C adds about 14 mg/dL to the estimated average. Comparing your daily readings to your A1C can reveal patterns you might miss from individual checks alone, like overnight highs you sleep through.
Reducing Pain From Finger Pricks
Lancets come in different gauges, and thinner needles are generally more comfortable. Most modern lancets range from 28 to 33 gauge (higher numbers mean thinner needles). Research comparing different needle diameters found that at shallower puncture depths, pain was about the same regardless of needle size. Pain increased more with deeper punctures than with thicker needles. So the penetration depth setting on your lancing device matters as much as the lancet gauge. Start at the shallowest depth that gives you enough blood and increase only if needed.
Rotating fingers also helps. Using the same spot repeatedly can cause the skin to thicken and become more sensitive over time. You have ten fingers and can prick either side of each fingertip, giving you twenty usable sites to rotate through.
Disposing of Lancets Safely
Used lancets are considered sharps and shouldn’t go loose into your household trash. Place them immediately into a sharps disposal container, which is a puncture-resistant plastic container you can buy at most pharmacies. A thick plastic laundry detergent bottle with a screw cap works in a pinch. Fill the container no more than three-quarters full, then seal it.
How you get rid of the sealed container depends on where you live. Options include drop-off sites at pharmacies, hospitals, or fire stations; household hazardous waste collection programs; and mail-back services. You can find location-specific disposal options by calling 1-800-643-1643 or checking with your local health department.