No. I multiplied 1 nM by Avogadro's number. Do you have any reason to believe that free IgG is not uniformly distributed at steady-state in circulation? ... Because that's what they're testing for.
Many (most) of the things we're interested in measuring in blood are bound to some sort large macro-molecular assembly... be it cells, lipid vesicles, large protein agglomerates, etc.
In the clinic the most common testing is metabolic paneling (levels of various salts, urea, glucose, creatine, etc.), serology (for circulating antibodies), or cell counts (which they weren't doing, as far as I know).
