Interpretation of Trace Element Assays

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Interpretation ofTrace Element Assays

• Tee L. Guidotti, MD, MPH, DABT
• The George Washington University
• Medical Center

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Trace Elements

• Includes metals and metalloids (As, Se)
• Some of concern with respect to neurological
  development Pb, Hg, As
• Exposure situation
• No specific exposure
• Plausible exposure
• Hazardous waste sites and residential exposure
• Why was the test done?

3
Disease Trace Element

• Toxicity and poisoning
• Many disorders associated with trace element
  excess are inborn errors and relate to carrier
  proteins
• Wilsons disease, Cu
• Relatively few disorders are associated with
  excessive intake
• Transient excessive intake usually does not
  produce toxicity because of excretion

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Laboratory Analysis

• State of the art is ICP-MS
• 12 14 elements
• Wide range of accuracy
• Low minimum level of detection
• Body fluid measurements good!
• Blood or plasma
• Spot urine (with or without creatinine
  correction)
• 24-hour urine (always preferred)
• Hair measurement bad!
• Bad signs on website testimonials, advertising,
  counseling by internet, value judgments, lists of
  symptoms

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Interpretation of Results

• Usual units
• In US ?g/L, ng/L, ?g/dL (Pb only)
• Rest of world ?mol/L, nmol/L
• Few metals close to toxic levels
• Pb, Hg, Mn
• For most, wide gap (gt 2 orders of magnitude)
• High, mid-level, low are misleading
• Know the comparison population!

From Globalrph website
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Population DistributionUsual Laboratory Analytes

• Never zero or nondetect
• Tightly controlled range, normal distribution

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Interpretation of Lab Results

• No normal range for trace elements
• Curve for metabolically and homeostatically
  important chem tests is normal
• Curve for trace elements is log-normal
• Trace elements therefore show different
  characteristics in distribution
• Mode is low, may be close to zero
• Long tail to curve, simply representing
  opportunity for exposure
• Cannot apply usual statistical tests
• Laboratory normal values do not apply!

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Population DistributionTrace Elements
Unexposed
Exposed

• Frequent non-detects or very low levels
• Long tail but still below toxicity threshold
• Persons with opportunity to be exposed have own
  log-N distribution

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Example Mercury

• Metal mercury
• Inhalation principal route of exposure
• Tremor, motor disorders
• Erethism, acrodynia
• Inorganic primarily affects the kidneys
• Organomercury compounds are principally toxic to
  the CNS, now rarely encountered
• sensory effects on vision
• motor problems with gait disturbance, and ataxia
• Minamata Bay congenital mercury neurotoxicity
• Dental amalgams

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Interpretation of Case

• Usual upper limit of normal is 30 ?g/ml in
  blood
• Pregnant woman, age 23
• Blood level of 32 ?g/ml
• Interpreted as elevated
• Considering abortion
• What to advise?
• Tradeoffs omega-3 ffas, etc.
• Toxic exposure or population risk factor?

Not the real patient, of course.
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Lessons to be Learned

• Dont order a test unless you know what to do
  with the results.
• Dont confuse one laboratories population norms
  with toxicity levels.
• Avoid hair analysis.
• Think in terms of population distribution, not
  clinical testing.
• Always correlate with clinical findings.