µg/gram is the same as PPM
µg = microgram
PPM - parts per million
I don't know why chelation urine tests are done (especially when using chelation agents such as DMPS or DMSA as I had terrible side effects not from the agent itself (500mgs of DMPS to be specific) but from the moblized mercury that dumps into other areas of your body. ) when you can be checked first and foremost by submitting a random urine sample for a porphyrin test. On pg 93 of his book "It's All In Your Mouth", he states that "the urine porphyrin test is perhaps the best indicator of heavy metal toxicity."
This from "Urine Porphyrin Profile Test for Mercury" as per - and please read - http://www.mercurypoisoned.com/new/urine_porphyrin_test.html
"Porphyrin (poor' fur rin) Profile Testing
Dental Truth Magazine
Spring-Summer 2007 edition

by Leo Cashman, Executive Director of DAMS

This heavy metal toxicity test is being used for non-invasive, safe, and accurate mercury toxicity testing. It is available from several labs in the US and a lab in Paris, France. Rather than looking for mercury in the urine directly, this test looks for effects of mercury, namely an elevation of certain proteins called porphyrins that are showing up in the urine. Normally porphyrins are processed in the body, through a series of chemical reactions, into heme, which is made into hemoglobin or which is used in the making of energy in the ATP cycle. But mercury poisons the enzymes that are needed in the process of making the heme, causing a buildup in the body of excess levels porphyrins. The excessive porphyrins are excreted in the urine. Mercury in our body can thus be detected by measuring the amount of porphyrins in the urine and looking for the distinctive profile of these porphyrins caused by mercury poisoning. The porphyrin known as "precoproporphyrin," is particularly elevated by the action of mercury.

Porphyrin testing is done at several US labs: Metametrix Clinical Laboratory, 800-221-4640; Great Plains Lab, 888-347-2781; and Laboratory Corporation of America, www.labcorp.com."

"Dr. Boyd Haley presented a paper to the FDA Advisory panel during the 2006 meeting on mercury fillings and neurological diseases, showing why he thinks provocative or challenge urine tests are not so accurate. The provacative tests only measure how much mercury an individual is excreting into their urine. There is no way to measure the body burden of mercury in a person, except through an autopsy."

As per "Testing For Mercury & Other Heavy Metals" - please read - http://www.beatcfsandfms.org/html/CheckForHg.html

"Do a Porphyrin Test

The best way to see mercury that was absorbed into the cells in the past is a Porphyrin Analysis test. This involves sending a urine sample to a lab. If Coproporphyria III is high, this indicates mercury had been there. For an example of such a test, please click here. For more information on these kinds of tests, please contact Mayo Medical Labs, TEL 800-533-1710. For Professor James Wood's evidence that Porphyrin Analysis can detect past mercury exposures, please click here REFERENCE #5. To get this test, you will need to work with a Doc that has an account with a lab that does this test (e.g. Mayo Medical Labs) and who is willing to run this test. Your local Doc may not be willing to do this unless you print out the Professor James Woods study and place it in front of your Doc and tell him you'll pay for it (most Doc's know nothing about all this). If your Doc does not work with you, it is recommended that you work through an Internet Doc that specializes in CFS/FMS. For a list of these, click here. Signs of high Copro II in severe cases would be: reddish colored urine, and pain one inch to the right of the belly button (ileocal value). The Porphyrin test will see both inorganic (Amalgam) and organic (Fish) mercury that is inside the liver cells. It will also show the levels of several enzymes used to manufacture blood. These enzymes can be disabled from things like Lead, Arsenic, Alcohol, Cadmium, Nickel, H2S (hydrogen sulfide), and other poisonous chemicals."

If you read Hal Higgins book, there is a whole chapter on 'Diagnosing Mercury Toxicity' and he describes all the chemical indicators through blood, urine and hair samples that can indicate mercury toxicity.
I think and feel if toxicity is suspect, then other tests can and should be run first before a heavy metal tox screen with a chelating agent is introduced to a patient.
Kindest regards
D