The Blood Test for Mercury Toxicity

Mercury is a heavy metal which has no known biological function in human health. Like other heavy metals, mercury poisoning will cause a range of symptoms ranging from abdominal pain through to severe disturbances of the central nervous system and death.

In the Western world, frank (accidental) mercury poisoning is very rare; however, everybody will have traces of mercury within their bodies. The predominant source of mercury exposure is dietary, stemming from the consumption of fish and seafood which contains mercury in the form of methylmercury. The concentration of methylmercury in seafood does not usually give any cause for concern, in general. However, consumption of heavily contaminated seafood in the past has led to tragic consequences, most notably in the Minimata Bay tragedy due to the fact that mercury can bioaccumulate within the food chain.

Tragically, mercury poisoning and poverty sometimes go hand in hand. Since mercury will form an amalgam with gold, the element is added from tailings (river sediments etc) where gold has been found. The gold extracted in this way is liberated by heating off the mercury which is then inhaled by the gold prospector, causing significant health problems.

Owing to the fact that mercury is toxic, there has been considerable concern, amongst the public, that mercury amalgam fillings could represent a source of mercury poisoning. Whilst it is certainly true that scientists can immediately detect mercury on the breath of individuals with mercury amalgam fillings, there is no compelling scientific evidence to suggest that such fillings pose a health risk. Indeed, removal of amalgam fillings has been shown to cause a spike in blood mercury concentrations due to the physical disruption of the filling. Dental practitioners are not considered to be at any significant additional risk due to their occupational exposure to mercury and are not required to undertake any monitoring. Therefore, the motivation for replacing such fillings should be aesthetic rather than due to concerns over mercury toxicity.

In general, doctors will order a blood mercury test in order to rule out mercury as a cause of a neurological disorder. Frank mercury poisoning in non-occupationally exposed individuals is rare, but it will cause a range of neurological disorders ranging from tremor to sensory and balance disturbances and mental health problems such as memory loss and mood swings. Many other conditions can cause some or all of these symptoms, so ruling out mercury poisoning is often a simple diagnostic step should there be any reason to assume that the patient could have been exposed to excessive amounts of mercury.

The blood test for mercury toxicity is used to measure the quantity of mercury in a patient’s blood, but it does not identify the chemical form (speciation) of the mercury present. The exact nature of a chemical species determines its toxicity (or nutritive benefit). In the case of mercury, organic forms, such as methylmercury are more toxic than inorganic mercury since they are better able to cross the blood-brain barrier.

The blood test for mercury toxicity is usually based on atomic fluorescence spectroscopy which is a highly sensitive, element specific technique. Sample preparation may require digestion (destruction and solubilisation) of the blood sample using a microwave digestion system and reagents need to be added to reduce mercury to Hg⁰ such that it can be detected. The sample is then irradiated with specific wavelength of light which causes ground state electrons to be promoted into higher orbitals, when these electrons relax back to the ground state, they will give off a characteristic wavelength of light which is measured; this process is known as fluorescence.

In the general population, blood mercury concentration typically ranges from 0.1 to10 µg/l. Patients with blood mercury levels above 15 µg/l should be referred to a clinician for follow-up study (this is the biological exposure index threshold value) although they will be asymptomatic at this level of exposure.

1. Health Effects of Mercury, Environmental Protection Agency: http://www.epa.gov/hg/effects.htm
2. Mercury Safety Products: http://www.mercurysafety.co.uk/hlthinfo.htm
3. Health Canada: http://www.hc-sc.gc.ca/ewh-semt/pubs/contaminants/mercur/q7-q11-eng.php