Human Immunodeficiency Virus (HIV) testing plays a significant role in diagnosing the infection and monitoring public health. These tests typically involve analyzing blood samples to detect either the virus itself, its genetic material, or the antibodies produced by the body in response to the infection.
The Common Blood Collection Tube for HIV Testing
For common HIV antibody and antigen testing, blood is typically collected into tubes designed to yield serum. The most frequently used are red-top tubes or gold-top tubes, also known as serum separator tubes (SSTs). Red-top tubes are plain, meaning they do not contain anticoagulants, allowing the blood to clot naturally. Gold-top tubes, or SSTs, contain a clot activator and a gel separator.
Many HIV tests, particularly fourth-generation antigen/antibody combination tests, rely on detecting antibodies or the p24 antigen in serum or plasma. The gel in gold-top tubes creates a barrier between the serum and the clotted blood cells after centrifugation, helping to maintain sample integrity and prevent contamination of the serum by cellular components.
The Significance of Tube Colors
The color of a blood collection tube stopper serves as a standardized indicator of the additives present inside the tube. These additives ensure the blood sample is preserved in a manner suitable for specific laboratory analyses. Different tests require different blood components, such as whole blood, plasma, or serum, and the additives prevent clotting or preserve certain substances.
For instance, red-top tubes either have no additive or contain a clot activator, enabling the blood to clot and yield serum. Gold-top tubes, a type of serum separator tube (SST), typically contain a clot activator and a gel that separates serum from blood cells after centrifugation. In contrast, tubes with different colored tops contain anticoagulants to prevent clotting. A lavender or purple-top tube contains EDTA (ethylenediaminetetraacetic acid), which binds calcium to prevent coagulation, making it suitable for whole blood analysis or obtaining plasma. Green-top tubes contain heparin, another anticoagulant, which is used for tests requiring plasma.
Handling the Blood Sample After Collection
After blood is collected, specific handling procedures prepare the sample for laboratory analysis. If the tube contains an additive, such as a clot activator, it is gently inverted several times (typically 5-10 times) to ensure proper mixing of the blood with the additive. For tubes intended to yield serum, like red-top or gold-top tubes, the blood is then allowed to clot at room temperature. This clotting process usually takes about 30 to 60 minutes for red-top tubes and potentially less for gold-top tubes due to the clot activator.
After the blood has clotted, the tube is placed into a centrifuge. Centrifugation spins the tube at high speeds, typically between 1,000 to 2,000 times gravity (g) for about 10 to 15 minutes, which separates the heavier cellular components from the lighter liquid portion. For serum tubes, this process yields a clear, yellowish serum layer above the clotted red blood cells, often with a gel barrier in SSTs that acts as a stable separation layer. The separated serum is then carefully removed and used for the HIV test.