A viability assay is an assay to determine the ability of organs, cells or tissues to maintain or recover viability. Viability can be distinguished from the all-or-nothing states of life and death by use of a quantifiable index between 0 and 1 (or 0 and 100%). Viability can assay mechanical activity, motility (spermatozoa or granulocytes), contraction (muscle tissue or cells), mitotic activity, etc.
For example, examining the ratio of potassium to sodium in cells can serve as an index of viability. If the cells do not have high intracellular potassium and low intracellular sodium, then (1) the cell membrane may not be intact, and/or (2) the sodium-potassium pump may not be operating well  As with many kinds of viability assays, quantitative measures of physiological function do not indicate whether damage repair and recovery is possible. An assay of the ability of a cell line to adhere and divide may be more indicative of incipient damage than membrane integrity.  Fluorescent-based assays do not require large sample sizes. Viability assays are used to assess the success of cryopreservation techniques, the toxicity of substances, or the effectiveness of substances in mitigating effects of toxic substances.
Classification of viability assays
- Cytolysis or membrane leakage assays: This category includes the lactate dehydrogenase assay, a stable enzyme common in all cells which can be readily detected when cell membranes are no longer intact. Examples:
- Mitochondrial activity or caspase assays: Resazurin and Formazan (MTT/XTT) can assay for various stages in the apoptosis process that foreshadow cell death.
- Functional assays: Assays of cell function will be highly specific to the types of cells being assayed. For example, motility is a widely used assay of sperm cell function. Fertility can be used to assay gamete survival, in general. Red blood cells have been assayed in terms of deformability, osmotic fragility, hemolysis, ATP level, and hemoglobin content. For transplantable whole organs the ultimate assay is the ability to sustain life after transplantation, an assay which is not helpful in preventing transplantation of non-functional organs.
- Genomic and protoemic assays: Cells can be assayed for activation of stress pathways using DNA microarrays and protein chips.
List of common viability assays
- ATP test
- Calcein AM
- Clonogenic assay
- Ethidium homodimer assay
- Evans blue
- Fluorescein diacetate hydrolysis/Propidium iodide staining (FDA/PI staining)
- Flow cytometry
- Formazan-based assays (MTT/XTT)
- Green fluorescent protein
- Lactate dehydrogenase (LDH)
- Methyl violet
- Propidium iodide, DNA stain that can differentiate necrotic, apoptotic and normal cells.
- Trypan Blue, a living-cell exclusion dye (dye only crosses cell membranes of dead cells)
- TUNEL assay
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