NMDA receptors are in neuronal membrane, not free proteins in the peripheral blood. It is possible that to detect their metabolits (?) or antibodies against to NMDA receptors.
I don't think so in density of the receptors. But there is a correlation of NMDA receptor subunits in human peripheral blood lymphocytes and opioid addiction (Eur J Pharmacol. 2010 Jul 25;638(1-3):29-32).