The pad tests are fairly accurate. I have actually compared them side by side with urine, fecal and hair analysis, with very similar results.
The limitation on the pad analysis stems from the fact that even though it is a good qualitative test, it does not reflect quantities, which means you will get a positive whether a toxin is found in small amounts or in high levels.
The pad test will give you an idea of what is being removed by the pads. Should you want a quantitative result, hair analysis would be my test of choice for heavy metals.