Collecting the blood sample

For effective anticoagulation, it is of course essential that the heparin (coated on the inside of the capillary tube) is distributed quickly and evenly through the whole blood sample by thorough but gentle mixing. The recommended mixing technique for capillary samples is by using a flea and magnet, moving through the sample steadily, avoiding vigorous mixing as this may destroy red blood cells (haemolysis), which can have the effect of reducing pH, pCO2 and pO2 values.

Inadequate or delayed mixing can result in inadequate anticoagulation and the formation of fibrin clots in the sample that might block the analyser and prevent analysis. Inadequate anticoagulation due to inadequate mixing of the sample remains a common reason for the rejection of blood gas samples.

For the capillary blood sample to be comparable to an arterial blood sample the blood must be collected without contamination of oxygen or other gases from the room air. In other words, the environment should be anaerobic.

The sample should have little contact with the room air, as the high concentration of oxygen in the atmosphere will affect the true oxygen reading. The requirement that blood is not exposed to air determines that any air-bubbles trapped in the blood-filled capillary tube must be expelled immediately the sample is collected, and that the capillary tube is then capped for the duration of period between collection and analysis. Blood is aspirated directly from capillary tube to the analyzer in order to preserve anaerobic conditions.

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