A Liver Function Test (LFT) is a common blood panel utilized by healthcare providers to gain insight into the health and functional status of the liver. The collection tube’s color is standardized across laboratories and directly relates to the substances contained within it, ensuring the integrity of the sample for analysis. This color-coding system helps phlebotomists and lab technicians correctly process the sample.
Identifying the Standard Tube for LFTs
The standard blood collection container for a Liver Function Test is the Serum Separator Tube (SST), recognized by its Gold stopper or sometimes a Tiger Top (red/gray mottled). This tube is selected because LFT components are measured in the serum, the liquid portion of blood remaining after clotting has occurred. The SST is designed to facilitate the separation of serum from the heavier cellular components of the blood sample.
The preference for the SST over a plain Red top (which contains no additive) is due to the presence of a clot activator inside the SST that speeds up the natural clotting process. Once the blood has clotted, the tube is spun in a centrifuge. This process allows the gel barrier to separate the serum, making it readily available for laboratory analysis.
What the Liver Function Test Measures
A standard LFT panel measures several key components, providing an assessment of liver status, including markers related to damage, protein synthesis, and waste processing. Common measurements include the liver enzymes Alanine Transaminase (ALT) and Aspartate Aminotransferase (AST). Elevated levels of these enzymes in the bloodstream indicate injury or inflammation to the liver cells, as the enzymes leak out from the damaged tissue.
The panel also assesses the liver’s synthetic functions by measuring proteins such as Albumin and Total Protein. Albumin is a major protein manufactured by the liver; a decreased concentration suggests a decline in the organ’s protein-producing capacity. Another component measured is Bilirubin, a yellowish waste product generated when red blood cells break down.
The liver processes Bilirubin for elimination. Measuring both Total and Direct Bilirubin helps determine if issues relate to the liver’s uptake, processing, or excretion of this waste product. Alkaline Phosphatase (ALP) and Gamma-Glutamyl Transferase (GGT) are also measured. Elevations in these enzymes can suggest problems with the bile ducts, which transport bile away from the liver.
Why Tube Additives Dictate Color Choice
The color of the tube stopper signifies the type of chemical additive contained within the tube, which is necessary for the stability and separation of the sample required for testing. For LFTs, the Gold or Tiger Top SST contains two main components: a clot activator and a thixotropic serum separator gel. The clot activator, often microscopic silica particles, initiates the coagulation cascade, accelerating the time it takes for the blood to clot.
Once the blood has fully clotted, the tube is placed in a centrifuge, which separates the components based on density. The separator gel, intermediate in density between the clotted blood cells and the serum, moves during centrifugation to form a stable, physical barrier. This protective layer prevents the serum from contacting the blood cells, ensuring the serum remains pure for accurate chemical analysis. A Lavender-top tube, by contrast, contains an anticoagulant like EDTA, making it unsuitable for LFTs that require serum.
Patient Preparation for an LFT
To ensure accurate test results, patients are often required to follow specific instructions before the blood draw, particularly regarding food and drink intake. Fasting is a common requirement, typically involving abstaining from all food and beverages other than water for 8 to 12 hours prior to the procedure. Fasting is necessary because certain components of a comprehensive panel, like glucose or triglycerides, are affected by recent meals, which could skew the test interpretation.
Patients should communicate all medications, supplements, and recent alcohol consumption to their healthcare provider, as these factors can influence liver enzyme levels. Alcohol intake, even within 24 hours of the test, can impact results. Certain prescription or over-the-counter drugs are also known to affect liver function markers. Strenuous exercise should be avoided immediately before the test, as it can cause temporary elevations in some liver enzymes.