Which color-topped evacuated tube is used for collecting blood for a CBC test?

The lavender-topped evacuated tube is specifically designed for collecting blood for a complete blood count (CBC) test. This tube contains EDTA (ethylene diamine tetraacetic acid), an anticoagulant that prevents the blood from clotting. This is crucial for a CBC, as the test measures various components of blood, including red blood cells, white blood cells, and platelets. The preservation of these components in their natural state is critical, which is why EDTA is used.

In comparison, the other color-topped tubes serve different purposes; for example, the green-topped tube typically contains heparin, which is also an anticoagulant but is more suited for tests that measure plasma rather than whole blood components. The red-topped tube may be used for serum tests and does not contain any anticoagulants, which would lead to clotting of the blood. The blue-topped tube, containing sodium citrate, is primarily used for coagulation studies and is not appropriate for a CBC. Thus, the lavender tube is the correct choice for collecting specimens for a CBC test.