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
- Pegg DE (1989). "Viability assays for preserved cells, tissues, and organs". CRYOBIOLOGY 26 (3): 212–231. PMID 2743785.
- Lindner B, Seydel U (1983). "Mass spectrometric analysis of drug-induced changes in Na+ and K+ contents of single bacterial cells". Journal of General Microbiology 129 (1): 51–55. doi:10.1099/00221287-129-1-51. PMID 633967.
- Pichugin Y, Fahy GM, Morin R (2006). "Cryopreservation of rat hippocampal slices by vitrification" (PDF). CRYOBIOLOGY 52 (2): 228–240. doi:10.1016/j.cryobiol.2005.11.006. PMID 16403489.
- Crutchfield A, Diller K, Brand J (1999). "Cryopreservation of Chlamydomonas reinhardtii (Chlorophyta)". EUROPEAN JOURNAL OF PHYCOLOGY 34 (1): 43–52. doi:10.1080/09670269910001736072.
- Wusteman MC, Pegg DE, Robinson MP, Wang LH, Fitch P (2002). "Vitrification media: toxicity, permeability, and dielectric properties". CRYOBIOLOGY 44 (1): 24–37. doi:10.1016/S0011-2240(02)00002-0. PMID 12061845.
- "Overview of Probes for Cell Viability, Cell Proliferation and Live-Cell Function—Section 15.1". Invitrogen. Life Technologies. 2010. Retrieved 2010-10-15.
- Henkelman S, Lagerberg JW, Graaff R, Rakhorst G, Van Oeveren W (2010). "The effects of cryopreservation on red blood cell rheologic properties". Transfusion 50 (11): 2393–2401. doi:10.1111/j.1537-2995.2010.02730.x. PMID 20561300.
- Southard JH (1989). "Viability assays in organ preservation". CRYOBIOLOGY 26 (3): 232–238. PMID 2663353.
- Lecoeur H (2002). "Nuclear apoptosis detection by flow cytometry: influence of endogenous endonucleases". Experimental Cell Research 277 (1): 1–14. doi:10.1006/excr.2002.5537. PMID 12061813.