Yes, I did a kryptopyrrole test,which had come back negative.. but today the guy talking about porphyrin test said it's actually quite different..so did my doc.
I think they are sort of in the same family at least though.
And the test at Phillip Auguste Laboratoire, is kind of the gold standard for this test and sure enough one of the markers....for precoproporphyrin, is a marker specifically for heavy metal toxicity too, or Hg as you mentioned.
Here's a short link to read if interested.. http://www.ncbi.nlm.nih.gov/pubmed/16782144