Capillary blood obtained for laboratory testing is significantly different from the usual venous blood tested. It is a mixture of venous, arterial, and capillary blood as well as interstitial and cellular fluids. Because of this mixture, capillary blood more closely resembles arterial rather than venous blood, and the reference values will differ. Analytes such as potassium, calcium, and total protein are lower in capillary specimens, where glucose is typically higher than in venous blood. Therefore, it is extremely important to inform the person performing the testing that the blood is capillary and not venous. This can be done by noting the type of blood on the requisition slip, the collection tube, or in the LIS system.
