When blood samples are taken for medical analysis, a distinct yellow liquid often separates from the red components in the test tube. This clear, or sometimes slightly cloudy, yellowish layer is a normal part of preparing blood for laboratory examination.
The Identity of the Yellow Liquid
The yellow liquid observed in blood test tubes is primarily either plasma or serum, both liquid components of blood. Whole blood consists of about 55% plasma, which is mostly water (90-92%) and contains dissolved proteins, salts, hormones, nutrients, and waste products. Plasma suspends red blood cells, white blood cells, and platelets, transporting them throughout the body.
Serum is similar to plasma but lacks clotting factors, specifically fibrinogen. It is the fluid that remains after blood has clotted, with cellular components and clotting proteins removed. Both plasma and serum contain various proteins like albumin and globulins, essential for maintaining osmotic pressure and fighting infections.
The Process of Separation
Obtaining this yellow liquid from whole blood typically involves centrifugation. After a blood sample is collected, it is placed into a centrifuge, a machine that spins the tubes at high speeds. This creates a centrifugal force, causing denser components like red blood cells to settle at the bottom. Lighter components, including plasma or serum, remain at the top.
For plasma separation, an anticoagulant is added to prevent clotting. This allows plasma, which still contains clotting factors, to be separated by centrifugation. In contrast, to obtain serum, no anticoagulant is added, and the blood is allowed to clot naturally before centrifugation, separating the serum from the clot and cellular material.
Why the Liquid Appears Yellow
The normal yellow tint of plasma and serum is primarily due to bilirubin, a yellow pigment formed as a byproduct of the breakdown of old red blood cells. This pigment is normally processed by the liver and excreted from the body. Other natural substances, such as carotenoids from dietary intake (like those found in carrots), can also contribute to the yellow color.
The shade of yellow can vary based on individual factors like hydration and recent diet. However, an unusually deep yellow, orange, or even green color can indicate underlying health conditions. For instance, a very dark yellow or brownish tint might suggest elevated bilirubin levels, often associated with jaundice or liver dysfunction. A cloudy or milky white appearance, known as lipemia, can occur due to high levels of lipids (fats) in the blood. Reddish-orange discoloration can be caused by hemolysis, the breakdown of red blood cells.
Its Importance in Medical Tests
The yellow liquid, whether plasma or serum, is an invaluable sample for a wide array of medical tests because it contains numerous dissolved substances that offer insights into a patient’s health. Plasma and serum are used to assess organ function, such as kidney and liver health, by measuring specific protein and enzyme levels. They also provide information on electrolyte balance, hormone levels, and the presence of infections or certain diseases.
Analyzing these components helps diagnose conditions, monitor disease progression, and evaluate treatment effectiveness. For example, tests performed on plasma can reveal clotting disorders due to the presence of fibrinogen, while serum is often preferred for biochemistry tests as it lacks clotting factors that could interfere with certain analyses. The diagnostic utility of this yellow liquid is fundamental to modern medicine.