AHA! I found it!

It's the fractionated porphyrin urine test. I guess I'm getting dislexic.

Mercury vapor passes through the blood rapidly(half-life in blood is 10 seconds(370)) and accumulates in other parts of the body such as the brain, kidneys, liver, thyroid gland, pituitary gland, etc. Thus blood test measures mostly recent exposure and mostly organic mercury. Kidneys have a lot of hydroxyl(SH) groups which mercury binds to causing accumulation in the kidneys, and inhibiting excretion(503). As damage occurs to kidneys over time, mercury is less efficiently eliminated (11,36,57,183,216,260,503), so urine tests are not reliable for body burden after long term exposure. Some researchers suggest hair offers a better indicator of mercury body burden than blood or urine(279,21ab,66), though still not totally reliable and may be a better indicator for organic mercury than inorganic. But hair mercury levels have been found to be inversely related with mercury toxicity effects and body burden in those most affected by mercury due to having low detoxification ability for mercury(86). Hair tests are useful since they provide information on other toxic metal exposures and essential mineral imbalances(229). It is documented that essential mineral deficiencies and imbalances given a normal diet are a strong indication of mercury toxicity, due to mercury's causing cell membrane permeability changes, absorption problems, and enzyme blockages(229,600). A challenge test using a chelator like DMPS or DMSA is a more reliable test for mercury body burden(290,360,273) and the fractionated porphyrin test is a more reliable test for mercury toxicity effects(260).

http://www.home.earthlink.net/~berniew1/damspr17.html

An FDA approved test for mercury is the fractionated porphyrin urine test- high precoproporphyrin and/or coproporphyrin indicates metabolic damage from toxics, usually mercury. Other high porphyrins indicate damage from other toxics.

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