Mercury (blood)

There is no level the body needs; UK labs print an upper limit for the general population of around 25 nmol/L (5 ug/L), people who eat a lot of fish run higher, and separate action levels apply to people exposed at work.

Mercury is a metal with no role in the body. The blood test measures total mercury, which in most people comes from the organic form in fish and shellfish, and reflects exposure over the past few weeks. Mercury from dental fillings, broken thermometers and some jobs contributes smaller amounts. High levels affect the nerves, kidneys and, in pregnancy, the developing baby.

It is requested when exposure is possible, for example in people who eat a lot of large predatory fish, in some jobs, after a spill or a broken thermometer, with some imported skin-lightening creams or traditional remedies, and when unexplained tremor, numbness, mood change or kidney problems are being looked into.

• Eating a lot of tuna, swordfish, shark, marlin or other large fish
• Work with mercury: dentistry, some manufacturing, gold recovery
• Skin-lightening creams and some traditional remedies containing mercury
• A recent spill or broken thermometer or bulb, if inhaled
• Many dental amalgam fillings, slightly
• Some old vaccines and antiseptics, historically

• Cutting back on large fish, after which the level halves in about two months
• A value below the cut-off is the expected finding
• Chelation treatment, in a hospital setting

• A fish-rich diet is the usual reason for a value above the cut-off in the UK
• A fish meal in the day or two before the test raises it
• Blood collected in a standard tube rather than a trace-element tube can be contaminated

A whole blood EDTA sample in a trace-element tube, no fasting needed. Fish intake in the previous week should be noted. Urine mercury is used instead when the exposure is to the inorganic or vapour form.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.