Transepithelial/Endothelial Electrical Resistance (TEER) measurement qualitatively measures cellular monolayer health and quantitatively evaluates cellular confluence by measuring the electrical resistance across a cell monolayer. It is commonly used to assess the integrity and permeability of cellular barriers, such as epithelial and endothelial cell layers grown onto permeable membrane cell culture inserts or transwell plates. TEER can be used as the quality analysis (QA) and quality control (QC) parameter for cellular studies, such as cell therapy. TEER measurement being very simple and fast makes it a suitable screening tool for drug discovery. The use of TEER measurement to verify barrier formation and tissue function has been growing in the emerging fields of in vitro 3-D tissue models, organoids, and Organ-on-Chips. What is TEER?.

TEER is typically measured using a probe with a pair two pairs of electrodes placed on either side of the cellular monolayer, one pair in the apical compartment (above the cells) and the other pair in the basolateral compartment (below the cells). An electronic instrument such as, automated EVOM™ Auto or portable EVOM™ Manual, applies a very small magnitude (≤10 µA) fixed electrical current with switching polarity across the electrodes and measures the resulting change in the voltage. The resistance value is calculated by the instrument using Ohm's Law (R = V/I), where R is resistance, V is voltage, and I is current. TEER is the resistance multiplied by the cell growth area, and TEER values are presented with the units of ohms times square centimeters (Ω-cm²).

Since EVOM™ technology uses a small magnitude electrical current with switching polarity, TEER measurement does not cause any charging of the sample and does not have any significant physiological effect. Also, TEER measurement does not require any chemical or biological labeling. TEER can be measured in biological conductive solutions, such as cell culture media or buffer. Thus, TEER measurement by EVOM™ is a label-free and non-invasive evaluation of cellular samples. The same samples can be used for other biological studies, such as immunocytochemistry and western blots. TEER measurement studies are often complemented with additional permeability studies with tracer molecules like lucifer yellow and FITC-dextran to verify the change of epithelial and endothelial barrier function.

TEER values of confluent cellular monolayers can vary depending on the cell type. Monolayers of certain cell types (e.g., cell type A), which normally show low TEER values, generally have relatively leaky tight junctions. Monolayers of other cell types (e.g., cell type B) show high TEER values, and these cell types are known to have tight junctions. Ions and molecules are known to pass rather easily across leaky cellular layers as compared to tighter cellular layers. The presence of more transcellular ion channels on cells can further allow easier flow of ions or electrical current through the transcellular pathway, which can additionally lower TEER values.

Cell type A allows greater amounts of current and ions to pass between the cells and yields a low TEER value. With its tighter junctions, cell monolayers of type B cells will show a higher TEER value. While both monolayers are confluent, the TEER resistance values can be markedly different based on the nature of the cells themselves.