RBC Selenium Test

Most selenium tests measure the mineral floating in your blood plasma or serum, a snapshot that changes within days based on what you ate recently and whether your body is fighting an infection. RBC selenium is different. Because selenium gets locked into red blood cells when they are built in your bone marrow, and those cells live for about 120 days, this test gives you a two- to three-month average of your selenium status. That longer window makes it far more reliable for answering the question that actually matters: does your body have enough selenium to keep your antioxidant systems running?

This distinction becomes especially important when inflammation is present. During any acute illness, surgery, or infection, plasma selenium drops dramatically as part of your body's inflammatory response, even if your actual selenium stores are fine. RBC selenium stays steady through all of that. If you are tracking selenium because of a gut condition, a restricted diet, or heavy training, this is the measurement that tells you the truth.

What Selenium Does in Your Body

Selenium works through a family of about 25 specialized proteins called selenoproteins. Your body builds these by inserting a selenium-containing amino acid (selenocysteine) into the protein during assembly. The most important of these proteins act as antioxidant enzymes, neutralizing hydrogen peroxide and other reactive molecules that would otherwise damage your cell membranes and DNA.

Inside red blood cells specifically, the dominant selenoprotein is an enzyme called glutathione peroxidase 1 (GPx1). Red blood cells are constantly exposed to oxidative stress (damage from the reactive byproducts of carrying iron and oxygen) and GPx1 is their primary line of defense. When selenium is scarce, GPx1 activity drops, and red blood cells become more fragile and prone to breaking apart prematurely.

Selenoproteins also play roles outside the blood. They help regulate thyroid hormone conversion, support immune function, protect sperm quality, and maintain normal function of the heart, liver, and brain. A shortfall in selenium ripples across multiple organ systems.

Heart Disease and Cardiovascular Risk

The connection between selenium status and cardiovascular risk is one of the strongest in the research. A 2025 meta-analysis pooling 20 cohort studies found that for each standard-deviation increase in selenium levels (measured in blood or serum, not RBC specifically), the risk of dying from cardiovascular disease dropped by about 11%. A separate 2020 meta-analysis of over 25,000 individuals found that people with the lowest circulating selenium were roughly 35% more likely to die from cardiovascular causes than those with the highest levels.

In people with heart failure, the protective effect appears even stronger. A study of over 15,000 NHANES participants found that those in the highest blood selenium group had a 67% lower risk of cardiovascular death compared to the lowest group. Among individuals with type 2 diabetes (about 3,200 participants followed for an average of 12.6 years), those in the top quarter of serum selenium had a 34% lower risk of dying from heart disease.

Severe selenium deficiency can directly damage the heart. Keshan disease, a form of heart muscle disease first described in selenium-poor regions of China, causes the heart to enlarge and weaken, with mortality rates historically reaching 80% in affected children and women of childbearing age. This condition involves loss of antioxidant protection in heart muscle cells, increased vulnerability to viral infection of the heart, and progressive scarring that leads to heart failure.

Cancer Risk

The relationship between selenium and cancer is more nuanced, and most of the evidence comes from serum or plasma selenium rather than RBC selenium specifically. The 2025 meta-analysis found a 15% lower risk of cancer death per standard-deviation increase in selenium biomarkers. But the benefit appears to depend on where you start.

A large study from Linxian, China, where baseline selenium was low, found that people in the top quarter of serum selenium had a 44% lower risk of esophageal cancer compared to the bottom quarter. For colorectal cancer, a European study of nearly 1,000 cases found a protective effect in women specifically: each 25 mcg/L increase in serum selenium was linked to a 17% lower risk. In men, the association was weaker and did not reach statistical significance.

The evidence suggests that selenium supplementation reduces cancer risk primarily in people who start with low selenium. In populations where selenium is already adequate (plasma selenium above about 122 mcg/L, which includes much of the U.S.), additional selenium does not appear to reduce cancer risk and may increase risk of certain cancers.

Mortality

The mortality data reinforces the pattern seen in cardiovascular and cancer data. The 2025 meta-analysis found a 13% lower risk of dying from any cause per standard-deviation increase in selenium. A U.S. study of over 13,000 adults tracked for up to 16 years showed that rising serum selenium was associated with falling mortality risk up to about 135 mcg/L, after which the benefit plateaued and may have reversed at very high levels.

This U-shaped relationship is one of the most consistent findings in selenium research: too little is clearly harmful, but too much may be as well. Your goal is to land in the middle, not to push your levels as high as possible.

Diabetes: A Complicated Picture

Unlike cardiovascular disease, where higher selenium is almost uniformly protective, the relationship with type 2 diabetes is more complex. A case-control study nested in the Jinchang Cohort (China) found that people in the third and fourth quarters of serum selenium had 62% and 79% higher odds of developing diabetes compared to the lowest quarter. A large dose-response meta-analysis of 34 studies confirmed this pattern: above about 80 mcg per day of dietary selenium, diabetes risk begins to rise, and at a blood selenium of 160 mcg/L compared to 90 mcg/L, the risk roughly doubles.

This is one of the clearest examples of the U-shaped curve in action. If you are supplementing selenium to protect your heart, you need to watch that you are not overshooting into a range where metabolic risk increases. Tracking your actual level, rather than guessing based on supplement doses, is the only way to thread this needle.

Reference Ranges

Published reference ranges for RBC selenium specifically are limited compared to serum or whole blood selenium. Most clinical labs report results in micrograms per liter (mcg/L). Your sex, age, and geographic location all influence where your level falls, so comparing your results within the same lab over time is more meaningful than matching a single cutpoint.

These values come from published research using various assays. Your lab may use different methods and cutpoints. The most actionable approach is to establish your own baseline and track changes over time rather than relying on any single threshold.

For serum selenium (a related but different measurement), research suggests that levels between 130 and 150 mcg/L are associated with the lowest all-cause and cancer mortality. Below about 105 mcg/L in whole blood is considered borderline low. The protein selenoprotein P, a major selenium transport molecule in the blood, reaches its maximum concentration (saturation) at a serum selenium of about 146 mcg/L, which many researchers consider a functional marker of optimal selenium status.

Who Has Lower Selenium

Several groups consistently show lower levels. People over 60 tend to have lower RBC selenium even when their serum selenium looks stable, suggesting age-related shifts in how the body distributes selenium. Women generally have lower levels than men. People with chronic kidney disease have significantly reduced selenium in both red blood cells and serum. Black and Hispanic Americans tend to have somewhat lower serum selenium than non-Hispanic White or Asian Americans, even after adjusting for diet and other factors.

Geography matters enormously. Soil selenium content varies widely, and this directly affects the food supply. Parts of Europe, sub-Saharan Africa, and inland China have selenium-poor soil, and populations in these regions are at meaningfully higher risk of deficiency. In the U.S. and Canada, dietary selenium is generally adequate, but individuals with restricted diets, malabsorption, or heavy exercise demands can still fall short.

Tracking Your Trend

A single RBC selenium reading is more informative than a single plasma reading because it averages your status over months rather than days. But even a stable measurement benefits from context. Your goal is to see where your level sits relative to the protective range (roughly 130 to 150 mcg/L in serum, with equivalent RBC thresholds depending on your lab's assay), whether it is trending up or down, and whether any interventions you are making are actually working.

Because RBC selenium takes three to four months to fully reflect a change in intake, retesting sooner than that will not show you the effect of a new supplement or dietary shift. Get a baseline reading, make any changes you plan to make, and retest at four months. After that, annual monitoring is reasonable if your level is in range. If you are correcting a deficiency, retest every four to six months until you reach your target and hold it through two consecutive readings.

Within-subject variability for selenium biomarkers runs about 7.7%, meaning your result can fluctuate by that amount even without any real change. A shift of more than about 20 to 24% between two readings is likely real and worth investigating.

When Results Can Be Misleading

The biggest advantage of RBC selenium is what does not confuse it. Unlike plasma selenium, RBC selenium stays stable during acute illness, surgery, and infection. In one study of critically ill children with highly elevated inflammatory markers (median CRP of 126 mg/L), plasma selenium was low while RBC selenium remained normal. The correlation between RBC and plasma selenium, which is normally very strong (r = 0.95), drops to just 0.30 during systemic inflammation. If you have recently been sick, had surgery, or have an active inflammatory condition, a plasma selenium test will probably underestimate your true status. RBC selenium will not.

Intense exercise can cause a brief dip in serum selenium that normalizes within 30 to 60 minutes. This transient shift is unlikely to affect RBC selenium meaningfully, but if your lab draws blood for both RBC and serum selenium, exercising right before the draw could make the serum value look falsely low.

A few medications can alter selenium distribution. High-dose corticosteroids (around 20 to 60 mg of prednisolone daily) can lower plasma selenium, though the effect on RBC selenium is less well characterized. Metformin reduces selenium by suppressing the liver's production of selenoprotein P, the main transport protein for selenium in the blood. If you take either of these, mention it when interpreting your results.