Serum is the clear, yellowish liquid that remains after blood clots and the clot is removed. It contains proteins, electrolytes, hormones, and other dissolved substances, but unlike whole blood, it has no red or white blood cells and no clotting proteins. The term also has a completely different meaning in skincare, where “serum” refers to a lightweight topical product, so context matters.
Blood Serum vs. Plasma
People often use “serum” and “plasma” interchangeably, but they’re not the same thing. The key difference comes down to clotting. Plasma is the liquid portion of blood collected before it clots, preserved with an anticoagulant. Serum is what you get after blood is allowed to clot naturally and the clot (along with the cells and clotting proteins trapped in it) is spun out in a centrifuge.
The practical result: serum lacks fibrinogen and several other clotting factors that plasma retains. Everything else, including proteins like albumin and globulins, electrolytes, hormones, antibodies, and waste products, is present in both. This distinction matters in medicine because some lab tests require serum specifically, while others need plasma.
What Serum Contains
About 90% of serum is water. The rest is a complex mix of dissolved substances that reflects what’s happening throughout your body at any given moment.
Proteins make up the largest non-water component. Albumin accounts for the majority of serum protein, with normal levels falling between 3.5 and 5.5 grams per deciliter in adults. The remaining proteins are collectively called globulins, which break down into several subgroups: alpha-1, alpha-2, beta, and gamma globulins. Gamma globulins include your antibodies, which is why serum has historically been used to transfer immunity from one person to another (a practice called serum therapy).
Serum also carries electrolytes that keep your cells functioning properly. In a healthy adult, serum sodium typically runs between 135 and 145 mEq/L, potassium between 3.5 and 5 mEq/L, and calcium between 8.5 and 10.3 mg/dL. These numbers are tightly regulated, and even small shifts can signal problems with your kidneys, heart, or other organs. Beyond proteins and electrolytes, serum carries hormones, glucose, dissolved gases, enzymes, and metabolic waste products like urea and creatinine.
What Serum Does in Your Body
Albumin, the most abundant serum protein, plays an outsized role. It acts as a molecular transport system, binding to and carrying hormones, fatty acids, drugs, and even toxic substances through the bloodstream. Nearly all known medications bind to albumin to some degree, which directly affects how those drugs are distributed and how long they stay active in your body.
Albumin also maintains what’s called osmotic pressure, the force that keeps fluid inside your blood vessels rather than leaking into surrounding tissues. When albumin levels drop significantly (from liver disease, kidney damage, or malnutrition), fluid can seep out of the bloodstream and accumulate in tissues, causing visible swelling. Albumin also functions as an antioxidant, helping neutralize reactive molecules that can damage cells.
The globulin proteins handle other critical jobs. Gamma globulins (antibodies) are central to immune defense. Other globulins transport metals like iron and copper, or carry fat-soluble vitamins through the watery environment of your blood.
How Serum Is Collected in a Lab
Producing serum from a blood draw follows a straightforward process. A blood sample is collected in a tube without any anticoagulant and left at room temperature for 30 to 60 minutes, giving it time to clot naturally. The tube is then spun in a centrifuge at moderate speed for about 10 minutes. This separates the dense clot and blood cells to the bottom, leaving the clear serum on top. A technician pipettes off the serum layer and either tests it immediately or freezes it at extremely low temperatures (around negative 70 to 80 degrees Celsius) for later analysis.
Why Doctors Order Serum Tests
A blood serum sample is one of the most commonly used materials in diagnostic medicine. When your doctor orders a “blood panel” or “metabolic panel,” they’re typically measuring substances dissolved in your serum. Electrolyte levels reveal how well your kidneys are balancing fluids. Albumin and total protein levels can flag liver disease, kidney problems, or nutritional deficiencies. Hormone levels measured in serum help diagnose thyroid disorders, diabetes, and reproductive conditions. Antibody levels in serum confirm whether you’ve been exposed to a specific infection or responded to a vaccine.
Because serum reflects so many body systems at once, a single sample can generate dozens of useful measurements. That’s why it remains the foundation of routine blood work.
Serum in Skincare: A Different Meaning
If you searched “what is serum” while shopping for skincare products, you’re looking at something entirely unrelated to blood. In the cosmetics world, a serum is a lightweight, quickly absorbed liquid designed to deliver a concentrated dose of active ingredients to your skin. Skincare serums are either oil-based or water-based and are thinner than moisturizers or creams.
Unlike a moisturizer, a skincare serum isn’t primarily meant to hydrate by forming a barrier on your skin’s surface. Instead, it’s formulated to penetrate quickly and deliver ingredients like vitamin C, retinol, hyaluronic acid, or niacinamide at higher concentrations than you’d find in a typical lotion. You apply a serum before your moisturizer, not as a replacement for one. The name borrows from the biological term loosely, suggesting a concentrated, purified liquid, but the similarity ends there.