Testing for sexually transmitted infections (STIs), often called an STD panel, involves blood tests designed to screen for infections like HIV, Syphilis, and Hepatitis B and C. The blood collection tubes used are color-coded to indicate the specific chemical additives they contain. These additives, such as clot activators or anticoagulants, are necessary to preserve the blood sample in the state required for accurate laboratory analysis. The color of the stopper ensures the phlebotomist selects the tube with the correct contents, which is fundamental for obtaining a reliable test result.
The Standard Blood Collection Tube for STD Screening
Routine blood-based STD screening relies on serology, which detects antibodies or antigens within the serum component of the blood. Serum is the liquid remaining after blood has been allowed to clot and cellular components have been removed. To generate serum efficiently, the standard tube used is the Serum Separator Tube (SST), identified by a Gold or sometimes a “Tiger Top” stopper.
This tube contains a clot activator, typically silica-based, which accelerates the natural clotting process. It also contains a thixotropic gel barrier at the bottom. After the blood clots and the tube is spun in a centrifuge, the gel moves upward to form a stable, physical barrier between the blood cells and the newly formed serum layer. This barrier allows the laboratory to easily separate the serum for testing, which is required for common HIV antibody and Syphilis RPR tests.
When Different Tubes Are Required for Specific Tests
While most screening uses serum, specialized STD tests require either whole blood or plasma, the liquid portion of blood that still contains clotting factors. These are typically molecular assays, such as Polymerase Chain Reaction (PCR) testing, used for monitoring HIV viral load or detecting genetic material. Because clotting must be prevented for these tests, a tube containing an anticoagulant is required.
The tube primarily used to prevent coagulation is the Lavender or Purple-top tube, which contains Ethylenediaminetetraacetic acid (EDTA). EDTA is a potent anticoagulant that works by binding to calcium ions, halting the biochemical cascade that leads to clotting. This results in a plasma sample after centrifugation, or preserved whole blood for specific analyses.
Why Proper Sample Handling Is Essential
The integrity of the test result depends on selecting the correct tube and meticulous handling of the specimen following the blood draw. Tubes containing additives, such as EDTA, must be gently inverted eight to ten times to ensure complete mixing. Insufficient mixing of an anticoagulant can lead to small clots forming, rendering the sample unsuitable for analysis and resulting in laboratory rejection.
For serum tubes, proper handling requires allowing the blood to clot fully at room temperature, which can take up to 60 minutes, before the sample is spun down. Time and temperature are constraints, as some samples must be processed within a narrow window, such as 24 hours, to prevent analyte breakdown. If immediate testing is not possible, the specimen must be stored or shipped under specific temperature conditions, often refrigerated or frozen, to maintain chemical stability.