Cell Viability Calculators

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Cell viability is the proportion of living, functionally intact cells in a population, expressed as a percentage: viability % = (live cells / total cells) × 100. It is a critical quality measure for cell culture experiments, cell therapy products, and cytotoxicity assays. Different methods measure different aspects of cell death: membrane integrity (trypan blue, propidium iodide), metabolic activity (MTT, resazurin), caspase activation (apoptosis markers), or clonogenic potential. Choosing the right assay depends on the application, required throughput, and the type of cell death expected.

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Trypan Blue Exclusion

Most common manual method. Live cells (intact membranes) exclude trypan blue → clear. Dead cells (compromised membranes) take up blue dye → blue/stained. Count with hemocytometer. Viability % = (clear cells / total cells) × 100. Limitations: subjectivity; cannot distinguish early apoptotic from live; trypan blue is mildly toxic if exposure is prolonged. Target viability for experiments: ≥ 90% (most cell biology); ≥ 95% for transfection; ≥ 85% minimum for most assays.

MTT Assay (Colorimetric Metabolic Activity)

MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) is reduced by mitochondrial enzymes in metabolically active cells to a purple formazan product. Absorbance measured at 570 nm. Relative viability = (A_treated/A_control) × 100%. Does not give absolute cell count; measures metabolic activity (proxies for viability). Related reagents: MTS, WST-1, resazurin (Alamar Blue) — all colorimetric metabolic activity assays.

Flow Cytometry Viability

PI (propidium iodide): penetrates compromised membranes; intercalates into DNA; fluorescent → dead cells are PI-positive. DAPI: similar principle to PI. Live/Dead fixable dyes: react with amines; dead cells stain brighter (more amine-reactive sites exposed). Annexin V/PI: Annexin V binds phosphatidylserine (flipped to outer membrane in early apoptosis, PI-negative); late apoptotic/necrotic cells are PI-positive.

Glossary

Cell Viability
The proportion of living cells in a culture: viability % = (live cells/total cells) × 100; ≥90% required for most experiments; measured by trypan blue, MTT, or flow cytometry.
Trypan Blue Exclusion
A cell counting method where live cells with intact membranes exclude the blue dye (clear) while dead cells take it up (blue); viability = clear/(clear+blue) × 100%.
MTT Assay
Colorimetric cell viability assay measuring mitochondrial reduction of MTT to purple formazan; absorbance at 570 nm; gives relative viability (treated/control × 100%); used for drug dose-response screening.

Frequently Asked Questions

Cell viability = percentage of living cells: viability % = (live cells / total cells) × 100. 'Live' is defined differently by different assays: trypan blue exclusion: live = membrane-intact (dye excluded); MTT assay: live = metabolically active (reduces tetrazolium dye); flow cytometry: live = PI-negative and FSC/SSC profile consistent with intact cells. Example: hemocytometer count shows 250 clear cells + 12 blue cells: total = 262; live = 250; viability = (250/262) × 100 = 95.4%. Viable cell density = viability% × total cell density.

Trypan blue exclusion: directly counts live and dead cells based on membrane integrity; gives absolute cell numbers; performed at a single time point; fast (< 5 min); requires hemocytometer or automated cell counter; cell count is independent of cell size or metabolism. MTT (colorimetric): measures metabolic activity of the cell population as a whole; gives relative values (treated vs. control); suitable for high-throughput (96-well plates); cannot distinguish whether reduced absorbance is due to fewer cells or less active cells; takes 2–4 hours; better for drug dose-response screening. Use trypan blue for routine culture monitoring and absolute counts; MTT for drug screening and comparative viability studies.

Annexin V/PI dual staining by flow cytometry discriminates: early apoptosis = Annexin V+/PI− (phosphatidylserine flipped to outer membrane, membrane still intact); late apoptosis = Annexin V+/PI+ (membrane integrity lost); necrosis = Annexin V−/PI+ (membrane rupture without apoptotic PS flip); live = Annexin V−/PI−. Caspase activity assays (CellEvent Caspase-3/7 green) specifically detect initiator and effector caspases — biochemical hallmarks of apoptosis. Morphology: apoptosis = cell shrinkage, nuclear condensation, membrane blebbing; necrosis = cell swelling, organelle disintegration, plasma membrane rupture. Differentiating apoptosis from necrosis matters for mechanistic understanding of cell death pathways.

Minimum acceptable viability depends on the experiment: general cell culture maintenance: ≥ 85% viability (flag for subculture if lower). Routine assays and biochemistry: ≥ 90%. Transfection/electroporation: ≥ 90–95% (stressed cells transfect poorly). Flow cytometry: ≥ 80% (dead cells cause non-specific staining and background). Cell therapy/clinical products: FDA requires ≥ 70% post-thaw for many products; ≥ 80% is preferred for infusion. Stem cell protocols: ≥ 90–95% (less robust cells). Low viability causes: delayed culture establishment; poor assay performance; non-specific drug responses; false interpretation of treatment effects. Always record and report cell viability at seeding.