Mercury (Blood)

Diagnostic Use

Mercury exists in elemental, inorganic and organic forms, all of which may be toxic. The toxic manifestation depends on the form of exposure.

Whole blood testing should be performed:

1. In very recently exposed patients with suspected acute toxicity. This method is less accurate after redistribution of mercury through the tissues.
2. Patients exposed to organic forms of mercury, since this is eliminated primarily by the faecal route (and less so in the urine).

24 hour urine testing should be measured in patients with suspected chronic (medium to long term) exposure to elemental or inorganic forms of mercury.

Toxic effects of mercury:  Irreversible neurological damage (central and peripheral nervous system) is the most important, but other organs are also affected e.g. renal tubular damage. Toxic effects have been clearly demonstrated at blood mercury levels >1000 nmol/L,  and levels >500 nmol/L should be regarded as indicating high risk.

The clinical effects of levels between 100 and 500 nmol/L are uncertain. Obvious toxicity in adults has not been reported at these levels. However the foetal brain is more sensitive to mercury, and subtle effects on cognitive and fine motor performance in children have shown correlations with cord blood mercury levels in this range in some studies.

Non-specific symptoms such as memory loss, cognitive decline, or chronic fatigue syndrome are not a sufficient indication for measuring blood or urine mercury. There is no evidence that mercury has any causal relationship to autism spectrum disorder.

The half-life in blood is 42 days for inorganic mercury and 45-70 days for organic mercury.

Interpretation

Blood and urine mercury tests measure the total mercury level (regardless of form) and does not attempt to quantify organic vs inorganic fractions.

Organic mercury is usually the major fraction of blood mercury. Fish intake is the strongest determinant of blood mercury levels in people not occupationally exposed to mercury. Mercury levels are highest in large predatory fish at the top of the food chain (e.g. tuna, swordfish, shark).

Dental amalgam makes a minor contribution to body mercury levels.   Claims that amalgam fillings cause mercury toxicity are not supported by good evidence.

In women of childbearing age

It is desirable that women of child bearing age have low blood mercury level. Although a “safe” level during pregnancy is not well established some health authorities suggest <40 nmol/L.  The toxic effects of high antenatal mercury exposure to newborns has been well described e.g. from Japan.   However, at low level especially between 40 and 100 nmol/L, there is no clear relationship with toxicity.

Fish is the main source of mercury in most people.  It is advisable to avoid or have limited intake of fish species known to have high mercury content when planning for, or during, pregnancy. On the other hand, that should be balanced against the health benefits that fish species with low mercury content can bring as part of diet.   Information on recommended servings for specific fish species can be found at https://www.mpi.govt.nz/food-safety/food-safety-for-consumers/food-and-pregnancy/list-of-safe-food-in-pregnancy/ .

Reference Intervals

Units: nmol/L

Normal:    0 – 50

Blood mercury levels <50 nmol/L are seen in people who do not have a high fish intake.

Levels up to 100 nmol/L are not uncommon in people who eat fish frequently, especially large predatory types of fish e.g. shark and swordfish.

Toxic effects (neurological damage) are possible with blood mercury levels greater than 250 nmol/L, but the blood mercury level and symptoms do not correlate well, especially in the range between 100-500 nmol/L. A spectrum of increasing risk probably applies in this range.

Conversion :

ppb x 5 = nmol/L

ug/L x 5 = nmol/L