Capillary Blood Glucose: Using the Meter Properly and Knowing When Not to Trust It

Capillary blood glucose is the most frequently performed test in most hospitals and the one interns are least often formally taught. The device is simple. The reasons it misleads are not, and they matter, because decisions get made on this number within seconds of seeing it.

At a glance

Before you start

Check the meter has been quality-controlled according to your ward’s schedule, and that the strips are in date and have been stored in their own pot with the lid closed. Strips degrade on exposure to air and humidity, and a pot left open on a warm ward is a genuine source of error.

Choose the side of a fingertip rather than the pulp centre. The sides are less densely innervated, so it hurts less, and patients who are tested several times a day notice.

How it is done

1. Identify the patient and explain what you are doing.
2. Have the patient wash their hands with soap and water and dry them thoroughly. This is the single highest-value step in the whole procedure and the one most often skipped.
3. Wash your own hands and put on gloves.
4. Warm the hand if it is cold — a poorly perfused finger yields a poor sample and needs more squeezing, which introduces the second error.
5. Let the hand hang below the level of the heart for a few seconds.
6. Lance the side of the fingertip in one decisive movement.
7. Wipe away the first drop.
8. Allow the next drop to form on its own. Apply gentle pressure along the finger if needed, but do not milk it repeatedly.
9. Apply the drop to the strip in the way the specific meter requires — edge-fill and top-fill devices are not used the same way.
10. Apply pressure with gauze until bleeding stops.
11. Dispose of the lancet immediately into a sharps bin.
12. Document the result, the time, and any relevant context such as whether the patient had eaten.

Why hand washing matters more than anything else

If there is glucose on the skin, the meter measures it. Fruit, juice, a sugary drink, a dextrose infusion running into that arm, or a nurse’s glove that touched a spillage will all produce a falsely high reading, sometimes dramatically so. Alcohol wipes are not a substitute: they do not reliably remove sugar residue, and injecting through undried alcohol stings.

The classic error runs like this — a patient eats a piece of fruit, someone tests them without washing, the reading comes back high, and insulin is given to a patient whose actual glucose was normal or low. Soap and water prevents this entirely.

When a capillary reading should not be trusted

The test depends on adequate peripheral perfusion. Where that is absent, the reading may not reflect arterial or venous glucose at all. Be sceptical in shock of any cause, severe dehydration, marked peripheral vasoconstriction, hypothermia, and significant peripheral oedema. In those situations a laboratory sample is the correct test.

Be similarly sceptical whenever the reading and the patient disagree. A patient who is sweating, confused and tremulous with a reassuring number, or an entirely well patient with an alarming one, is telling you something about the sample.

What to do when the reading is low

If the patient has symptoms of hypoglycaemia, treat according to your hospital’s hypoglycaemia protocol immediately. Do not wait for laboratory confirmation, and do not repeat the capillary test first. Confirmation can follow treatment; brain injury from delay cannot be undone. Then look for the cause, because a hypoglycaemic episode with no explanation is itself a finding — think about missed meals, changed renal function, a dose given at the wrong time, or a drug interaction.

What seniors actually ask

• Did the patient wash their hands?
• Which arm is the drip in, and which hand did you test?
• Does this number fit the patient in front of you?
• What was the trend, not just this reading?
• If you did not believe it, did you send a laboratory sample?

Last-minute checklist

• Hands washed with soap and water, and dried — not wiped with alcohol
• Test the hand that does not have the infusion in it
• First drop wiped away, second drop used
• No repeated milking of the finger
• Strips in date and stored closed
• Number that does not fit the patient means a laboratory sample
• Symptomatic hypoglycaemia is treated before it is confirmed

References

• Your hospital’s hypoglycaemia and blood glucose monitoring protocols
• Manufacturer’s instructions for the specific meter in use on your ward