Blood Lead Levels are usually measured in micrograms per deciliter.

Floor dust levels are measured in parts per square foot / micrograms per square foot.

Soil levels are measured in parts per million (density). 

Levels in toys are measured either in terms of the surface film levels or the parts per million (taking the whole toy into consideration.)

The dust swab (such as LeadCheck) kits you buy in the hardware store are designed/ intended only to be used on painted surfaces (like houses) not toys.  They just give you a positive (pink) or negative (yellow) result. They may not give you an accurate result if you use them for any purpose other than house/ building paint.

XRFs (X-Ray Flourescence Spectrometers / Analyzers) were initially designed to determine levels in a thin film of paint.  Newer XRF technology also measure lead levels in the density of a small amount of any solid material or the levels on the surface of a small toy (parts per million.)  A good resource for understanding how XRFs work can be found here: http://www.niton.com/portable-XRF-technology/how-xrf-works.aspx?sflang=en

For me (as a parent, not being a scientist) this has been one of the hardest parts of the equation to understand completely.  Patrick Lehne with the Western Regional Lead Training Center put together these sheets for my website/ to help educate you... (& me!) (Click on each of the three pages for a full size PDF you can print or download.)