Urine, serum, plasma, whole blood, red blood cells, feces, hair, fingernails—the list of biological samples used for element testing is long. But choosing the right sample type is one of the most critical decisions in producing results that are clinically meaningful and scientifically valid.
Key questions laboratories must address include:
- Which biological sample best reflects exposure or status for each element?
- Can results be meaningfully compared to published research?
- Can an element appear elevated in one sample type but normal in another?
- Do results reflect recent intake, long-term body burden, acute toxicity, chronic exposure, deficiency, or normal homeostatic regulation?
These questions are central to the development and validation of reliable essential and toxic element profiles.
Why a Single Sample Type Is Not Enough
Most commercially available element panels analyze 20–30 elements using a single sample type—most often urine or serum. While this may appear convenient, it is scientifically flawed.
Each element behaves differently in the body:
- It is absorbed differently
- It is stored in different tissues
- It is excreted at different rates
- It has different clinical interpretations depending on the sample tested
As a result, a value that is meaningful for one element may be misleading—or entirely meaningless—for another when using the same sample type.
For this reason, ZRT Laboratory does not rely on large, single-sample panels. Instead, elements are grouped and tested using the sample type that best reflects their biology and clinical relevance.
Sample Types Used and Why
Urine (Dried Urine)
Elements tested:
Iodine, Bromine, Selenium, Arsenic, Cadmium, Mercury
(+ Creatinine for dilution correction)
Urine is particularly useful for assessing recent intake or excretion patterns, especially for elements that are rapidly cleared from the bloodstream.
Whole Blood (Dried Blood Spot)
Elements tested:
Zinc, Copper, Zinc/Copper Ratio, Magnesium, Selenium, Cadmium, Lead, Mercury
Whole blood reflects both intracellular and extracellular compartments and is often the best indicator of long-term status or exposure to elements that bind to red blood cells.
Element-by-Element Rationale
Iodine
Urine is the gold standard for iodine assessment. Over 90% of dietary iodine is excreted in urine, and nearly all population and clinical studies rely on urinary iodine to assess deficiency or excess. Serum iodine is occasionally used for acute exposure screening but has limited clinical utility.
Bromine
Bromine is primarily excreted in urine, making urinary testing the best indicator of recent dietary intake.
Selenium
- Urine: Best for recent dietary intake (50–70% excreted in urine)
- Whole blood: Better reflects long-term selenium status than serum
- Serum: Represents current status but at lower concentrations (~80% of whole blood)
Arsenic
Urinary arsenic is the most reliable indicator of recent intake, with ~80% excreted within three days. Blood and serum arsenic are cleared within hours and are only useful for detecting very recent or extreme exposure.
Cadmium
- Urine: Best indicator of long-term exposure and body burden (half-life ~30 years)
- Whole blood: Reflects exposure within the past 1–2 months
- Serum: Poor indicator due to cadmium’s strong binding to red blood cells
Lead
Whole blood is the gold standard for lead testing:
- ~95% of lead binds to red blood cells
- Serum contains only ~1% of whole-blood lead concentration
Urine is not useful except in occupational monitoring or chelation contexts.
Mercury
- Urine: Best for inorganic and elemental mercury (kidney burden)
-
Whole blood: Best for organic mercury (methyl/ethyl mercury), such as from fish consumption
Serum should not be used for mercury analysis.
Zinc and Copper
Zinc and copper are functional antagonists, making the zinc/copper ratio clinically important.
- Whole blood: Better reflects intracellular and long-term status
- Serum: Commonly used but less reflective of tissue levels
- Urine: Reflects recent intake only and does not correlate well with tissue status
Magnesium
Magnesium assessment is particularly challenging:
- <1% of total body magnesium is found in blood
- Serum magnesium is tightly regulated and often appears normal even in deficiency
- Whole blood magnesium better reflects intracellular stores and long-term status than serum or urine
Real-World Examples
- A patient regularly eats mercury-contaminated fish. Testing would potentially show low urinary and serum mercury, while whole blood tests would be high for mercury. This is because a majority of the mercury in fish tissue is methylmercury which can only be detected in whole blood samples.
- A patient continuously drinks water contaminated with arsenic from a well. Testing would potentially show low whole blood and serum arsenic and high urinary arsenic. This is because arsenic is cleared rapidly in blood but is excreted over multiple days in urine.
- A patient ceased smoking cigarettes (high source of cadmium) 6 months ago, but was a habitual smoker for 20 years. Whole blood and serum would potentially show low cadmium levels while urine tests high for cadmium. This is because whole blood represents recent cadmium intake and serum is a poor indicator of cadmium burden, while urine indicates long term cadmium exposure.
As you can see, proper sample type matters when testing toxic and essential elements. In certain cases testing two sample types will provide a better picture of total exposure.
The Takeaway
Element testing is not a one-size-fits-all process. The choice of sample type directly determines the clinical value of the result. In many cases, testing more than one sample type provides the most complete and accurate picture of exposure, status, and risk.
When it comes to essential and toxic elements, what you test matters—but how you test it matters even more.