Cell viability determines the proportion of live, functional cells within a total cell population. This ratio is foundational to nearly all areas of biomedical research, serving as a direct indicator of cell culture health and experimental integrity. Determining the percentage of living cells is a necessary first step in drug discovery, where researchers evaluate a compound’s toxicity before testing its effectiveness. In therapeutic development, particularly for cell-based medicines, this metric is a mandatory quality control step to ensure the final product is safe and potent.
Understanding Cell Viability
A cell is considered viable when it demonstrates biological characteristics that confirm its survival and function, not just its physical presence. Primary indicators of a viable cell are an intact plasma membrane, active metabolic processes, and the potential for replication. Non-viable cells have suffered irreversible damage, leading to a breakdown of these functions and eventual cell death.
Viability is expressed as a percentage, calculated by dividing the number of living cells by the total number of cells counted in a sample and multiplying by one hundred. This percentage is a more accurate representation of a sample’s quality than a simple total cell count alone. A high cell count with low viability indicates a sample full of dead or dying cells that will negatively affect experiments or therapeutic outcomes.
Common Methods for Assessing Viability
Scientists rely on different methods to assess viability, generally categorized by measuring membrane integrity or metabolic function. Membrane exclusion assays are among the simplest and most common, utilizing dyes like Trypan Blue that are repelled by the intact membranes of living cells. If a cell’s membrane is compromised—a hallmark of cell death—the dye enters the cell, staining it a distinct blue color under a microscope.
Metabolic assays measure the biochemical activity occurring inside the living cells. Methods like the MTT or Alamar Blue assays use specialized compounds that are chemically altered by enzymes found only in metabolically active cells, such as those in the mitochondria. The conversion of the assay compound produces a measurable signal, typically a color change or fluorescence, with intensity directly proportional to the number of viable cells.
Contextual Standards for Acceptable Viability
What constitutes a “good” viability percentage is not a fixed number but depends on the specific application and stage of work. For cells maintained in routine laboratory culture, a viability of 90% or higher is expected before the cells are used for experimentation or expansion. Maintaining this high standard ensures the integrity of the cell line and promotes consistent growth rates.
In the context of cryopreservation, where cells are frozen for long-term storage, the acceptable viability range is temporarily lower due to the stress of the process. Cells should be frozen only when their initial viability is above 90%. A post-thaw viability of 70% to 85% is considered acceptable because the surviving cells are expected to quickly proliferate and regain full health once returned to optimal growth conditions.
For high-throughput applications like preclinical drug screening, minimum viability thresholds are established to maintain data reliability. International standards for in vitro cytotoxicity testing require that control cells (those not exposed to the test compound) maintain a viability exceeding 80% to ensure the experiment is valid. Clinical-grade cell therapies intended for direct human infusion, such as CAR T-cells, demand the highest quality control, requiring viability percentages of 90% or higher before release. This requirement ensures the cell product retains maximum function and safety after manufacturing and prior to patient administration.
Environmental and Handling Factors Influencing Viability
Cell viability is highly sensitive to external conditions, and a drop in the percentage often points to correctable stressors in the environment or handling. Temperature fluctuations are a common culprit, as most mammalian cells thrive best at 37°C; even minor deviations can induce cellular stress responses. The chemical balance of the culture medium is also important, particularly the pH level, which must be maintained within a tight range (typically 7.2 to 7.4) to prevent disruption of enzyme activity.
Old or poorly buffered media can lead to waste accumulation and pH changes, causing viability to decline. Mechanical stress from harsh handling, such as forceful pipetting or high-speed centrifugation, can physically damage cell membranes and reduce viability. Contamination by microorganisms also poses a direct threat, as bacteria or fungi compete for nutrients and release toxic byproducts that quickly lead to widespread cell death.