Trace elements, also known as heavy metals, are found in the human body in very small concentrations, ranging from parts per million to parts per trillion. Some trace elements (eg, chromium, cobalt, copper, iron, manganese, molybdenum, selenium, zinc) are essential for biological processes in humans. Deficiencies in these elements can have adverse clinical manifestations that are reversed with supplementation. In excessive amounts, some of these metals can be harmful.
Other metals, such as aluminum (Al), arsenic (As), beryllium (Be), lead (Pb), and mercury (Hg) have no known biological functions and are toxic. Heavy metal poisoning can occur from occupational and environmental exposures, food and medications, or lead-based paints.
For all of these metals, clinical laboratories perform trace element tests on biological samples, either to assess the nutritional status of patients or to detect toxicity.
Sample collection is essential
The validity of the trace element results is highly dependent on taking the necessary steps to collect the sample properly. To avoid contamination, laboratories list acceptable collection devices and procedures. The best practice for blood collection is to use certified trace metal free tubes (ie royal blue cap, available with and without anticoagulant) or lead free tubes (ie tan cap).
Some laboratories may accept samples in metal-free tubes if preferred tubes are not available, such as in a shortage. A recent example is the use of EDTA lavender stoppered tubes for lead testing and the addition of a disclaimer to the result to alert that the container was not free of metals. Tube contamination and erroneous results due to this practice have been reported numerous times. The Centers for Disease Control and Prevention (CDC) (www.cdc.gov/nceh/lead/lab) offers educational tools for proper collection of venous and capillary samples.
For sample collection from children, elevated lead from a capillary collection is suspected to be contaminated and a second analysis is performed on a venous sample. A sample should not be collected in patients receiving gadolinium (Gd), iodine (I), or barium (Ba)-containing contrast material within 96 hours. Metal-based contrast agents interfere with the analysis of trace metals.
Selection of trace element analysis methods
The method of choice for trace element analysis is inductively coupled plasma mass spectrometry (ICP-MS), due to its sensitivity and multi-element quantification. This method uses plasma heated to temperatures up to 10,000 K to ionize the sample and specific isotopes that are detected using mass spectrometry.
Some clinical laboratories also perform elemental analysis by atomic absorption spectrophotometry (AAS). In the AAS graphite furnace, when a sample is heated with a flame, the elements absorb light at a certain wavelength that is detected by a spectrophotometer. For context, participants in a recent proficiency test survey conducted by the College of American Pathologists primarily tested lead by ICP-MS (54%), followed by AAS (39%). Other heavy metals were universally reported using ICP-MS.
Despite the advantages of ICP-MS, this method has some shortcomings. ICP-MS is more expensive, requires more technical expertise for method development, and is affected by spectral interferences (eg, isobars, polyatomic or doubly charged species with the same m/z ratio as the element of interest). With modern instruments, we can overcome most spectral interferences, since ICP-MS instruments are equipped with a Collision or Reaction Cell (CRC). Depending on the instrument, interferences can be eliminated by promoting a reaction between a reactive gas (eg, O2 or H2) and the interferent or analyte, or by combining a nonreactive gas, such as helium, with kinetic energy discrimination, which it will discriminate against large polyatomic species, among other approaches.
Technological advances have continued to eliminate isobaric interference in ICP-MS. More recently, ICP tandem mass spectrometers (ICP-MS/MS or ICP-QQQ) have become commercially available. The first quadrupole (Q1) pre-filters ions with a certain m/z ratio to enter the cell, and the other quadrupole (Q2) detects the target m/z ratio after the CRC reaction. Balcaen and colleagues have reviewed this technique in detail (Anal Chim Acta 2015; doi: 10.1016/j.aca.2015.08.053).
A current application of ICP-MS/MS in clinical laboratories is the precise determination of selenium, which is not affected by gadolinium-based imaging contrasts. Gadolinium is doubly charged (156Gd2+) under ICP conditions and interferes with selenium assays using the 78Se isotope. To accurately quantify Se in the presence of Gd, Q1 filters the m/z ratio of 78, O2 reacts with Se in the CRC, and Q2 detects the m/z ratio of 94 for 78Se16O+.
ICP-MS/MS technology is still relatively new and not widely used in clinical laboratories. However, its robustness in removing interferences in biological and other complex matrices, while maintaining sensitivity for detecting trace elements, holds promise for clinical applications.
Jessica Colon-Franco, PhD, DABCC, FAACC, is section chief of clinical chemistry and medical director of specialty chemistry at Cleveland Clinic Laboratories in Cleveland, Ohio.+Email: [email protected]