Arsenic Fractionation, 24 Hour, Urine
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Arsenic (As) exists in a number of different forms; some are toxic, while others are not. The toxic forms are the inorganic species of As(+3) (As-III), As(+5) (As-V), and their partially detoxified metabolites, monomethylarsine and dimethylarsine. As-III is more toxic than As-V and both are more toxic than mono- and dimethylarsine. The biologic half-life of inorganic arsenic is 4 to 6 hours, while the biologic half-life of the methylated metabolites is 20 to 30 hours. Target organs of As-III-induced effects are the heart, gastrointestinal tract, skin and other epithelial tissues, kidney, and nervous system.
Inorganic arsenic is carcinogenic to humans. Symptoms of chronic poisoning, called arseniasis, are mostly insidious and nonspecific. The gastrointestinal tract, skin, and central nervous system are usually involved. Nausea, epigastric pain, colic abdominal pain, diarrhea, and paresthesias of the hands and feet can occur.
Nontoxic, organic forms of arsenic are present in many foods. Arsenobetaine and arsenocholine are the 2 most common forms of organic arsenic found in food. The most common foods that contain significant concentrations of organic arsenic are shellfish and other predators in the seafood chain (cod, haddock, etc). Some meats such as chicken that have been fed on seafood remnants may also contain the organic forms of arsenic.
Following ingestion of arsenobetaine and arsenocholine, these compounds undergo rapid renal clearance to become concentrated in the urine. Organic arsenic is completely excreted within 1 to 2 days after ingestion and there are no residual toxic metabolites. The biologic half-life of organic arsenic is 4 to 6 hours.
Diagnosis of arsenic intoxication
The quantitative reference range for fractionated arsenic applies only to the inorganic forms. Concentrations > or =25 mcg inorganic arsenic per specimen are considered toxic.
There is no limit to the normal range for the organic forms of arsenic, since they are not toxic and normally present after consumption of certain food types. For example, a typical finding in a urine specimen with total 24-hour excretion of arsenic of 350 mcg/24 hours is >95% is present as the organic species from a dietary source, and <5% present as the inorganic species. This would be interpreted as indicating the elevated total arsenic was due to ingestion of the nontoxic form of arsenic, usually found in food.
A normal value for blood arsenic does not exclude a finding of an elevated urine inorganic arsenic, due to the very short half-life of blood arsenic.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Consumption of seafood before collection of a urine specimen for arsenic testing is likely to result in a report of an elevated concentration of arsenic, which can be clinically misleading.
High concentrations of gadolinium and iodine are known to interfere with most metals tests. If either gadolinium- or iodine-containing contrast media has been administered, a specimen should not be collected for 96 hours.
Nitric acid cannot be added to either the collection or aliquot container because nitric acid will cause assay failure. All specimens are tested for the presence of nitric acid prior to analysis and cannot be analyzed if nitric acid is present.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Reference values apply to all ages.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Caldwell KL, Jones RL, Verdon CP, et al: Levels of urinary total and speciated arsenic in the US population: National Health and Nutrition Examination Survey 2003-2004. J Expo Sci Environ Epidemiol 2009;19:59-68