Capillary blood samples

Blood can also be taken from the earlobe or a digit for occasional ABG samples; this technique is considered less invasive and painful than an arterial stab. ✔ A capillary tube is required to collect the sample ✔ If the earlobe is used it needs to be warmed and a vasodilator cream applied ✔ This ensures a rapid flow of blood through the capillary ✔ If a digit is used, vasodilator cream does not need to be applied ✔ A small nick with a pointed scalpel or lancet will cause a large spot of blood to ooze out, which can be collected in the tube ✔ The tube is then sealed and taken immediately to the blood gas analyser ✔ Care is needed to ensure that air bubbles are not present in the sample ✔ The PaCO2 corresponds well with those obtained from arterial samples, but the accuracy of the PaO2 depends on good technique in arterialization of the earlobe or digit. General points ✔ The procedure should be explained to the patient; they should be warned that it may be painful ✔ The patient should be advised to breathe normally throughout the test; if the patient is hyperventilating the results may be inaccurate ✔ Certain conditions may interfere with the test results, for example, venous blood in the sample may lower PaO2 and elevate PaCO2 levels ✔ Air bubbles in the sample result in gas equilibration between the air and the arterial blood, lowering the PaCO2 and increasing the PaCO2 ✔ Wait at least 20 mins before drawing blood for an ABG after initiating, changing or discontinuing oxygen therapy ✔ Document the amount of oxygen the patient is receiving when the sample is drawn ✔ If the patient is pyrexial when the sample is drawn, the blood gas analyser may need to be adjusted, as most are set to analyse samples at 37° C ✔ The puncture site should be observed for signs of bleeding, or circulatory impairment, such as numbness, pain, tingling or swelling