Capillary blood glucose testing
- Procedure
A fingerstick test measures glucose in a drop of blood from the side of a finger, with a single-use lancet and a clean meter.
What it is
Capillary blood glucose testing measures blood glucose in seconds from a drop obtained by pricking a finger. It is often done before meals and before giving insulin or other diabetes medicines, and when there are symptoms of hypoglycaemia. Its accuracy depends on technique: clean, dry skin, a puncture at the right site, no squeezing and wiping away the first drop. Its safety depends on infection control: lancets and fingerstick devices are never shared, and the meter is cleaned between patients.
Indications
- As ordered: often before meals and before giving insulin or other diabetes medicines.
- Symptoms of hypoglycaemia: sweating, shaking, palpitations, hunger, dizziness, irritability or confusion.
- Patients on enteral or parenteral nutrition, or on medicines that change glucose, as ordered.
- A follow-up check after treating hypoglycaemia, per protocol.
Contraindications and precautions
- Do not prick a swollen, injured, infected or scarred finger, or the same site twice.
- Do not use the arm with a fistula or the hand with the IV if another option exists.
- Never reuse a lancet or share a fingerstick device: hepatitis B outbreaks have been traced to this.
- Check that the strips match the meter, are in date and are stored sealed, away from moisture and heat.
- A result that does not fit the patient's condition is repeated and, per protocol, confirmed with a venous sample.
Equipment
- A meter assigned to the patient, or one that has been cleaned and disinfected, with quality control up to date.
- Compatible, in-date test strips.
- Sterile single-use lancet, preferably with a safety mechanism.
- Alcohol swab, dry gauze and gloves.
- Sharps container within reach.
Step by step (and why)
- Check the order and identify the patient with two identifiers — the result may change the insulin dose.
- Clean your hands, explain the procedure and put on gloves — this protects the patient and you from blood.
- Prepare the meter and a compatible, in-date strip — a wrong or expired strip gives false values.
- Have the patient's hand washed and dried (with warm water if cold) and choose the side of the middle or ring finger — clean, warm skin bleeds better.
- Clean with alcohol and let it air-dry — wet alcohol dilutes the drop and stings.
- Prick with one quick, firm movement — one puncture is enough and blood flows well.
- Wipe away the first drop and use the next, without squeezing hard — the first and squeezed drops carry tissue fluid.
- Touch the strip to the drop, then press the site with gauze — this fills the strip and stops the bleeding.
- Discard the lancet in the sharps container and clean the meter — this prevents needlesticks and cross-infection.
- Record the value, the time and its relation to meals, and act on the order — without context treatment cannot be adjusted.
Complications
- Pain and bruising at the puncture site.
- Local infection from non-aseptic technique or reusing the site.
- Scarring and loss of sensation with repeated punctures of the same finger.
- Skin damage from adhesive strips in older adults and children.
- Transmission of hepatitis B or other infections when lancets or fingerstick devices are shared.
Next step: when to call for help
- If the value is below 70 milligrams per decilitre, apply the facility hypoglycaemia protocol, report it and repeat the check as directed.
- Call at once if the value is below 54, if the patient is confused, drowsy or seizing, or cannot swallow.
- Call if the value is above the threshold in the order or protocol, or if there are symptoms of hyperglycaemia.
- Call if the result does not fit the patient's condition, or if the site keeps bleeding after several minutes of pressure.
Test yourself
Think of the answer, then tap the question to see it.
Why is the first drop of blood wiped away in a fingerstick test?
Because it may be contaminated with tissue fluid or skin debris and change the result.
Where on the finger is the puncture made, and on which fingers?
On the side of the pad of the middle or ring finger; the thumb and index finger are avoided for calluses and the little finger for its thin tissue.
Why is the finger not squeezed hard?
Because it dilutes the sample with tissue fluid and promotes haemolysis, which gives an unreliable result.
Which infection-control rules apply to the meter and the lancet?
The lancet is single-use and the fingerstick device is for one patient only; a shared meter is cleaned and disinfected after every use.
What capillary glucose value is considered hypoglycaemia?
Below 70 milligrams per decilitre; below 54 is severe hypoglycaemia.
Connections
Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.
Sources: WHO guidelines on drawing blood: best practices in phlebotomy (chapter 7, Capillary sampling) (World Health Organization, 2010); Considerations for Blood Glucose Monitoring and Insulin Administration (Centers for Disease Control and Prevention, 2024); Low Blood Sugar (Hypoglycemia) (Centers for Disease Control and Prevention, 2024); Monitoring Your Blood Sugar (Centers for Disease Control and Prevention, 2024).
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Representative model, for educational purposes only.