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Lead is considered to be a toxic, heavy metal. It has no known metabolic role to play in the human body. In contrast, an excessive body burden of lead will produce a range of symptoms ranging from upset stomach (colic), to lethargy, frank signs of encephalopathy and ultimately death. This article will provide a description of lead toxicity and an overview of the blood test for lead.
Lead is a soft, malleable, dense metal that has been used for a wide range of industrial purposes since antiquity. The Romans used lead and it has been used in artefacts for at least 5000 years. Lead has been used in cosmetic preparations to whiten the complexion; as a black eye-liner (Ghee) and an ingredient in some Asian and Middle Eastern patent medicines used to treat a wide array of ailments (including colic!).
In modern times, the metal was use extensively for plumbing purposes and as water piping. The metal was also used in white paint as a pigment and until recently, tetra-ethyl lead was used as an additive to petrol as an “anti-knocking” agent to promote smooth combustion (and lubricate internal parts of the engine). Lead is also a component of electrical solder and was used in early canning as a means to solder shut cans. This use led to some cases of inadvertent lead poisoning when the foodstuff placed into the can was mildly acidic (fruit preserves), due to the leaching of lead from the solder. Owing to its density and the fact that it is opaque to X-rays, lead is used in a variety of shielding roles in the nuclear industry and within nuclear medicine applications.
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Consequences of Lead Exposure
At very high levels, lead causes abortions; lower levels can lead to miscarriages and low birth weight and also elevates the risk of mental retardation of the infant. There have been studies which suggest that significant exposure in men (>50 µg/100 ml) may lead to decreased sperm count and motility (mainly in groups occupationally exposed to lead).
Low level exposure to lead (≤20 µg/100 ml, particularly in utero) has been associated with subtle neurological effects, but this remains contentious. Concern that low levels of lead exposure could cause mental retardation in infants led to the eventual ban on the use of leaded petrol in Europe. In adults who are occupationally exposed, impairment of cognitive function has been seen at blood lead concentrations above 40 µg/100 ml. Chronic lead poisoning may result in kidney (renal tubular) damage although this may not be clinically significant and there is some evidence to suggest that it may decrease immune function.
In the UK, the normal exposure range in the general population is 0.2 to 1.2 µmol/l (4.1 to 24.9 µg/100 ml) and fatal toxicity has been seen in children with blood lead concentrations above 3.0 µmol/l.
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Measuring Lead Concentration In Whole Blood
The established method for blood lead determination is Micro-sampling Flame Atomic Absorption Spectroscopy (MSFAAS) which was devised by H. Trevor Delves and is known more generally as the Delves Cup method. A brief overview of the blood test for lead using the Delves Cup will be given.
A small amount of whole human blood is placed in a small platinum crucible; hydrogen peroxide is added and the cup is transferred to a hot plate to dry. A matrix matched calibration curve is prepared from bovine blood by the addition of known quantities of lead standard. The cups are placed into a platinum wire loop and swung into the flame of an AAS which determines the concentration of lead in the sample on the basis of the absorption of light at the characteristic wavelength of lead ground to excited state atomic transfer. Concentration is determined directly from the Beer-Lambert law. Usually, a silica atom trap is place in the flame to enhance sensitivity by trapping ground-state lead atoms in the measurement beam for slightly longer.
Again (see the companion article on the determination of copper in blood), the widespread availability of ICP-AES in clinical laboratories means that a blood sample may be analysed quasi-simultaneously for a number of elements. Similarly, ICP-MS is also continuing to gain ground in routine laboratories as it is an extremely sensitive and versatile technique.
On a personal note, I had the great pleasure to work with Dr Delves as his research assistant on a project to study the origins (sources) of lead in UK children’s blood by ICP-MS in the early 1980s. ICP-MS is capable of isotopic analysis and can be used to “fingerprint” lead sources on the basis of their isotopic composition. Lead has four stable isotopes; three of which are the daughters of uranium and thorium decay. Consequently, lead isotopic ratios vary from ore deposit to ore deposit around the world based on their age, geological factors and the concentrations of thorium and uranium in the parent strata. These variations are conserved in artefacts produced from these deposits.
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In-depth review: lead toxicity; D. A. Gidlow; Occupational Medicine 2004, 54:76-81, http://occmed.oxfordjournals.org/cgi/reprint/54/2/76.pdf