As mentioned before, capillary blood contains interstitial and intracellular fluids. One constituent of the interstitial fluid is tissue thromboplastin, which is involved in the coagulation process. Also, platelets have a tendency to accumulate at the site of the wound as part of the clotting process. Therefore, capillary specimens need to be collected quickly to reduce the chance of clotting. Clinical and Laboratory Standards Institute (CLSI) recommends the order of draw for multiple capillary drawn tubes to be:
- Blood gas specimens
- Ethylenediaminetetraacetic acid (EDTA) specimens (lavender top)
- Other additive specimens
- Serum specimens
Microcontainer tubes need to be filled and mixed appropriately to prevent microclots from forming. A note should be made regarding capillary specimens. Although some manufacturers offer light blue microtubes, they should not be used for capillary specimens. They should only be used for venous blood.