CORRECT TECHNIQUE FOR CAPILLARY BLOOD SAMPLING
Both the accuracy and reliability of hemoglobin measurements can be affected by pre-analytical errors. To obtain correct results, especially when taking capillary blood samples (fingertips), it is essential to follow a standardized procedure, and the person taking the sample must have received adequate training to avoid inaccurate hemoglobin results.
Please find below a list of the possible sources of error when making the sample:
Choice of the lancet
The lancet should make a puncture which is sufficiently deein order to ensure adequate blood flow (lancets with a length between 1.85 to 2.25 mm are recommended, depending on the thickness of the skin). At Hemoglobin.eu we recommend that the adequate thickness of the lancets should be 21G, which assures us a range of blood volume of 75 to 125 microliters.
Selection of the puncture site
To ensure that we achieve a constant blood flow and at the same time to prevent the patient from feeling pain, we must use the middle finger or the third finger (3rd and 4th fingers). The most appropriate finger for that purpose is located in the non-dominant hand, since they usually have fewer calluses and are less susceptible to pain compared to the index finger or thumb. We should also avoid taking the blood sample from the thumb because of its pulse (arterial presence). The little finger is not ideal for sampling, since the distance between the surface of the skin and the bone is too small. The puncture should be made slightly offset from the central and fleshy part of the fingertip, near the side.
Cleaning and disinfection
After cleaning and disinfecting the puncture site, it must be completely dried. Any remaining disinfectant solution may dilute the blood sample and cause false low readings.
The person taking the sample should hold the patient's finger. The finger must be massaged before the puncture to activate blood circulation in the area. Maintaining a light pressure at the time of the puncture helps achieve a good penetration of the lancet. After the puncture, the finger can be massaged to further promote blood circulation, but it should be done very gently.
The first 2-3 drops of blood should be discarded by wiping them with clean gauze. We should avoid the weakening of the flow and the coagulation of the blood as it can cause incorrect results. Good capillary flow is usually achieved 30-45 seconds after the puncture. The cuvette should be filled completely with the third or fourth drop of blood and said drop should have sufficient volume to fill the cuvette completely. Incomplete filling or air bubbles produce erroneous results. The finger should not be pressed too hard, as this may favor the passage of tissue fluid into the blood and thus cause false low readings.