Clonogenic Assay Calculators

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The clonogenic assay (colony formation assay) is the gold-standard method for measuring the reproductive integrity of individual cells — their ability to proliferate indefinitely and form a visible colony after treatment with radiation, cytotoxic drugs, or other agents. A colony is defined as a group of ≥50 cells derived from a single progenitor cell. The assay directly measures clonogenic survival — the most biologically relevant endpoint in radiation biology and cancer pharmacology — unlike other cytotoxicity assays (MTT, trypan blue) that only measure short-term viability.

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Clonogenic Assay Protocol

  1. Treat cells with the agent (radiation dose, drug concentration, etc.)
  2. Trypsinize and count cells
  3. Plate a known number of cells in complete growth medium at low density (cells must not touch)
  4. Incubate 10–14 days (depending on cell doubling time; enough time for visible colonies)
  5. Fix and stain with methanol + crystal violet (or Coomassie)
  6. Count colonies with ≥50 cells (by eye, automated counter, or microscope)

Surviving Fraction Calculation

PE (plating efficiency) = colonies / cells plated in untreated control

SF = colonies / (cells plated × PE)

Example: untreated: 50 colonies from 100 cells → PE = 50% = 0.50. Treated (5 Gy): 8 colonies from 100 cells → SF = 8 / (100 × 0.50) = 0.16 (16% surviving fraction).

Dose-Response Curves

Plot SF (log scale) vs. radiation dose (linear) or drug concentration → generates the cell survival curve. Linear-quadratic model: SF = e^(−αD − βD²); α/β ratio characterizes radiosensitivity. IC₅₀ (drug dose reducing SF to 50%) or LD50 from pharmacology applications.

Advantages and Limitations

Advantage: measures true proliferative capacity (not just short-term viability). Limitation: time-intensive (2+ weeks); requires cells that can form discrete colonies; high plating efficiency required for accuracy.

Glossary

Clonogenic Assay
A method measuring the proportion of cells capable of unlimited proliferation after treatment; SF = colonies/(cells plated × PE); gold standard for radiation biology and anticancer drug evaluation.
Plating Efficiency (PE)
PE = colonies in untreated control / cells plated; measures baseline colony-forming ability; used to normalize treated samples; varies widely (10–80%) between cell lines and conditions.
Surviving Fraction (SF)
SF = colonies treated / (cells plated × PE); the proportion of treated cells retaining unlimited proliferative potential; plotted vs. dose on a log-linear scale to generate cell survival curves.

Frequently Asked Questions

A clonogenic assay measures the proportion of treated cells that retain unlimited proliferative capacity — the ability to divide repeatedly and form a visible colony (≥50 cells from one progenitor). This is the most clinically relevant endpoint in radiation biology and anticancer drug testing because: a cell that appears alive immediately after treatment (e.g., by trypan blue exclusion or MTT) may still have lethal DNA damage that prevents further proliferation (mitotic cell death). Only the clonogenic assay distinguishes truly surviving cells from these 'living dead' cells with reproductive failure. Surviving fraction (SF) = cells that form colonies / expected colonies if no treatment had occurred.

Plating efficiency (PE) = colonies formed / cells plated in untreated control = baseline colony-forming ability. SF = colonies in treated sample / (cells plated × PE). Why PE is needed: not all cells plated form visible colonies even without treatment; PE accounts for this baseline limitation. Example: Control: plate 200 cells; count 80 colonies → PE = 80/200 = 0.40 (40%). Treated (2 Gy): plate 200 cells; count 52 colonies → SF = 52/(200 × 0.40) = 52/80 = 0.65 (65% survival). At 5 Gy: plate 200 cells; count 12 colonies → SF = 12/80 = 0.15 (15% survival). Report SF values with error bars (SD from replicates) to show reproducibility.

MTT assay: measures mitochondrial enzyme activity (reduction of MTT to purple formazan) as a proxy for metabolic activity and cell number. Results in hours to days. Cannot distinguish dead cells from living cells with reproductive failure. Suitable for high-throughput drug screening (IC₅₀ determination). Clonogenic assay: measures ability to form colonies (unlimited replication) over 10–14 days. Distinguishes cells with intact proliferative capacity from those with reproductive death (lethal DNA damage). Provides biologically most relevant endpoint for radiation biology and evaluating cytotoxic anticancer agents. Which to use: MTT for rapid screening and IC₅₀ determination; clonogenic for detailed cell kill characterization, radiation sensitizer testing, and publishable radiobiology data.

Critical factors: Plating density: too dense → colonies merge (impossible to count); too sparse → unreliable counting statistics; aim for 50–150 colonies per dish. Cell suspension quality: complete single-cell suspension (no clumps — one clump = multiple colonies from multiple cells → artificially increases count). Incubation time: must be sufficient for at least 6 doublings to form ≥50 cell colonies; shorter incubation → fewer, smaller colonies → undercounting. Colony definition: consistently apply the ≥50 cell threshold; use automated imaging for objective counting. Plating medium: cells must be plated in their optimal medium with appropriate serum; medium changes during incubation may be needed for long assays. Cell line variability: PE varies widely (10–80%) between cell lines; always normalize to untreated PE for SF calculation.