A lipid panel, often called a lipid profile, is a standard blood test that measures the fats, or lipids, in the bloodstream, including total cholesterol, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides. This test provides physicians with a snapshot of a patient’s cardiovascular health risk. To accurately measure these components, the blood sample must be collected in a specialized tube that ensures the separation of the liquid portion of the blood from the cellular components. The most common collection vessel for the lipid panel is the Gold-top tube, also known as a Serum Separator Tube (SST).
The Essential Tube for Lipid Analysis
The primary tube used for a lipid panel is the Gold-top or Red/Gray mottled top tube, which contains a gel barrier and a clot activator. These tubes, designated as Serum Separator Tubes (SSTs), are preferred because they yield serum, the liquid portion of the blood that remains after clotting. Serum is necessary for most chemistry tests, including the lipid profile, as it contains the components to be analyzed.
An alternative collection method involves using a Green-top tube, which contains the anticoagulant heparin. This tube yields plasma, the liquid portion of blood that is prevented from clotting by the additive. While serum is generally the preferred specimen, the Plasma Separator Tube (PST), often a light Green-top with a gel barrier, may be used in settings like point-of-care testing where rapid results are needed.
Understanding the Role of Additives
The SST’s effectiveness stems from its two main internal components: a clot activator and an inert separation gel. The clot activator, typically made of silica particles, rapidly accelerates the natural clotting process, ensuring the blood sample fully coagulates within a short period, commonly around 30 minutes.
Once the blood is clotted and the tube is spun in a centrifuge, the specialized gel barrier migrates between the heavy blood clot and the lighter serum. This creates a stable physical barrier, isolating the serum from the cells and preventing contamination that could alter the test results. In contrast, the Green-top tube contains lithium heparin, which works as an anticoagulant by inhibiting clotting factors, allowing the separation of plasma without coagulation.
Pre-Test Requirements for Accurate Results
Patient preparation is a major factor influencing the accuracy of lipid panel results. Traditionally, patients were instructed to fast for 9 to 12 hours before the blood draw, consuming only water. This preparation was necessary primarily to clear chylomicrons, which are large lipoprotein particles formed after a meal that carry dietary fat. These particles significantly elevate triglyceride levels, which can lead to an inaccurate measurement of the body’s baseline lipid status.
However, current clinical guidelines increasingly support the use of non-fasting lipid profiles for routine cardiovascular risk assessment. Non-fasting samples have minimal changes in total and HDL cholesterol, and only a small increase in triglycerides, often around 20%. Non-fasting testing is convenient and avoids the risk of hypoglycemia in diabetic patients. A fasting sample may still be required if a patient’s non-fasting triglycerides exceed 400 mg/dL, because the non-fasting value can interfere with the calculation of LDL cholesterol by the standard Friedewald formula.
Handling the Sample Post-Collection
Once the blood is collected into an SST, proper handling is essential to ensure sample integrity before analysis. The tube must be gently inverted five to eight times immediately after the draw to thoroughly mix the blood with the clot activator and the gel. Vigorous shaking is avoided as it can cause hemolysis, the rupturing of red blood cells, which releases contents that interfere with the chemistry analysis.
The sample must be allowed to clot fully before being placed into a centrifuge, typically taking 20 to 30 minutes at room temperature. Centrifugation separates the serum from the clot, with the gel barrier forming a stable seal between the two layers. The serum or plasma should be separated from the cells within two hours of collection to maintain optimal sample quality. The laboratory also visually inspects the serum for lipemia, a cloudy or milky appearance caused by high levels of triglycerides, which may signal a preparation error or severely elevated lipid levels.