Open flow microperfusion (OFM) is a novel, tissue-specific sampling technique that is based on push pull perfusion through a patented membrane-free probe. OFM probes have macroscopic openings that allow the perfusate to mix directly with surrounding interstitial fluid (ISF). OFM samples contain all substances regardless of their physico-chemical properties (i.e. lipophilicity) or molecular size (i.e. peptides, antibodies and even cells!).