An easy, quick if somewhat rough and ready method of screening prospective blood donors to ensure that have a sufficiently high Hb to tolerate a blood donation safely is to introduce a drop of blood to a tube of copper sulphate solution. If the Hb is greater that a nominal level, the drop will have a greater specific gravity than the copper sulphate solution, and drop to the bottom of the tube. If the specific gravity is less than the copper sulphate solution, this means that the Hb is below the nominal level and the donor could be deferred.
Just thought you might be interested, following Maggies post :)
http://www.ehow.com/h...g-copper-sulfate.html