Blood serum is the clear, yellowish fluid that remains after blood clots and the clot is removed. It contains water (about 90% by volume), proteins, electrolytes, hormones, enzymes, antibodies, nutrients like glucose and fats, and metabolic waste products. Serum is essentially the same as plasma, with one key difference: the clotting proteins, especially fibrinogen, have been removed during the clotting process.
How Serum Differs From Plasma
Both serum and plasma are the liquid portion of blood, separated from red and white blood cells. To get plasma, a lab adds an anticoagulant to freshly drawn blood to prevent clotting, then spins it in a centrifuge. To get serum, the blood is allowed to clot naturally first. The centrifuge then separates the clot, the blood cells, and the related clotting factors from the liquid that remains. That liquid is serum.
The practical result: serum lacks fibrinogen and several other clotting factors that plasma retains. Everything else, including proteins, electrolytes, sugars, fats, and hormones, is present in both. Most routine blood tests use serum because removing clotting proteins simplifies the analysis.
Proteins: The Largest Solid Component
Normal serum contains 6 to 8 grams of protein per deciliter. More than half of that is albumin, which alone accounts for 3.5 to 5.0 g/dL. The rest falls under the broad category of globulins.
Albumin does a surprising amount of work for a single protein. It generates roughly 70% of the osmotic pressure that keeps fluid inside your blood vessels rather than leaking into surrounding tissues. It also acts as a transport shuttle, carrying fatty acids, hormones, vitamins, metals, bilirubin, and many medications through the bloodstream. Its slight negative electrical charge helps balance the positive ions dissolved in blood.
Globulins are a diverse group of hundreds of proteins, typically sorted into four families based on how they move through an electrical field in the lab:
- Alpha-1 globulins include proteins that protect tissues from inflammatory damage.
- Alpha-2 globulins include large scavenger proteins and haptoglobin, which captures free hemoglobin released from worn-out red blood cells.
- Beta globulins include transferrin (the main iron-carrying protein) and several complement proteins that help the immune system destroy bacteria.
- Gamma globulins are primarily antibodies, also called immunoglobulins.
Antibodies in Serum
The antibodies circulating in serum are a major reason it’s so useful in medical testing. Three types dominate. IgG is by far the most abundant, averaging around 1,300 mg/dL in older adolescents and adults. It provides long-term immunity against infections you’ve already encountered or been vaccinated against. IgA, at roughly 200 mg/dL, protects mucosal surfaces like the gut and respiratory tract. IgM, around 140 mg/dL, is the first antibody your immune system produces when it encounters a new threat.
Two other types, IgE and IgD, circulate in much smaller amounts. IgE plays a central role in allergic reactions and defense against parasites.
Electrolytes and Minerals
Dissolved salts and minerals keep your nerves firing, muscles contracting, and body fluids in balance. The major electrolytes in serum, along with their typical ranges, are:
- Sodium: 135 to 145 mmol/L, the primary controller of fluid balance
- Potassium: 3.6 to 5.5 mmol/L, critical for heart rhythm and muscle function
- Chloride: 97 to 105 mmol/L, works alongside sodium to regulate fluid
- Calcium: 8.8 to 10.7 mg/dL, essential for bones, nerve signaling, and blood clotting
- Bicarbonate: 22 to 29 mmol/L, the body’s main acid buffer
These values are tightly regulated. Even small shifts outside these ranges can cause symptoms. Potassium that drifts too high or too low, for example, can trigger dangerous heart rhythms.
Sugars, Fats, and Other Nutrients
Serum carries the fuel your cells need. Fasting blood glucose normally sits between 70 and 100 mg/dL, while a random (non-fasting) reading should stay below 140 mg/dL. These numbers are the basis for diagnosing diabetes and prediabetes.
Lipids travel through serum packaged inside protein carriers. Total cholesterol below 200 mg/dL is considered normal, and triglycerides below 150 mg/dL. Serum also contains small amounts of amino acids, vitamins (including B12, with a reference range recently updated to 110 to 940 ng/L), and trace elements like iron and zinc.
Enzymes
Enzymes are proteins that speed up chemical reactions inside cells. Small amounts leak into the bloodstream continuously, and measuring them in serum helps doctors detect organ damage. When cells are injured, they release larger quantities of their enzymes into circulation.
Two liver enzymes are among the most commonly tested. ALT is considered the gold standard for detecting liver damage. AST is also released from damaged liver cells, though it comes from heart and muscle tissue too. Normal levels for both typically fall between 5 and 40 units per liter. Elevated levels can signal anything from a medication side effect to hepatitis.
Other enzymes routinely measured in serum include amylase (elevated in pancreatic inflammation), creatine kinase (a marker of muscle or heart damage), alkaline phosphatase (linked to bone and liver conditions), and lactate dehydrogenase (a general marker of tissue injury found across many organs).
Hormones
Serum contains virtually every hormone circulating in the body, making it the standard sample for hormone testing. Thyroid hormones, cortisol, insulin, testosterone, estrogen, and growth hormone are all present and measurable. Their concentrations are tiny compared to proteins or electrolytes, often measured in nanograms or even picograms per milliliter, but they regulate everything from metabolism and mood to reproduction and stress response.
Metabolic Waste Products
Your body continuously breaks down proteins, muscles, and old red blood cells, and the byproducts end up in serum before the kidneys filter them out. Blood urea nitrogen (BUN), normally 7 to 18 mg/dL, reflects how well the kidneys clear nitrogen waste from protein metabolism. Creatinine, a breakdown product of muscle activity, serves as another kidney function marker. Bilirubin, the yellowish pigment produced when old red blood cells are recycled, is processed by the liver and excreted in bile. Elevated bilirubin causes the yellowing of skin and eyes known as jaundice. Uric acid, a byproduct of DNA breakdown, can accumulate and trigger gout when levels climb too high.
Physical Properties of Serum
Serum is slightly alkaline, with blood pH held between 7.35 and 7.45 at normal body temperature. Dropping below 7.3 triggers acidosis, a potentially dangerous condition. The bicarbonate buffering system is one of the main mechanisms keeping pH in this narrow window. Blood as a whole has a density of about 1,060 kg per cubic meter, slightly heavier than water, and serum itself is a bit lighter since it lacks the cells.
How Serum Is Prepared in the Lab
When your blood is drawn into a tube without anticoagulant, it clots within minutes. The tube is then placed in a centrifuge and spun at 1,000 to 2,000 times the force of gravity for about 10 minutes, usually in a refrigerated machine to preserve sensitive components. The solid clot and all trapped blood cells settle to the bottom. The clear fluid on top is serum, ready to be tested for any of the components described above.