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General information for researchers and lab staff, not medical advice.
Short answer
Cytotoxicity is the quality of being toxic to cells: a substance, material or agent that kills cells or damages them enough to impair their function. In the lab it is measured with in vitro cell assays, and for medical devices FDA lists in vitro cytotoxicity among the biocompatibility endpoints to consider.
Definition and scope
“Cytotoxic” describes anything that damages or kills cells, whether a chemical, an extract from a device material, or an immune cell. The same word covers very different situations: a cytotoxic effect is unwanted when you are evaluating the safety of a material, and intended when the goal is to kill target cells. Phrasing such as cellular cytotoxicity or cell cytotoxicity refers to the same idea measured in cultured cells. Cytotoxicity is distinct from proliferation or viability readouts, although many assays report one as the inverse of the other; see cell proliferation assays for that side.
What are cytotoxicity tests in vitro?
In vitro cytotoxicity tests expose cultured mammalian cells to a test substance or material and measure a quantitative readout of cell damage or death. For devices and materials, FDA’s guidance on ISO 10993-1 (issued September 8, 2023) describes how this is done in practice:
- Test articles are extracted at 37 °C for 24 to 72 hours in a vehicle that can extract both polar and nonpolar constituents, for example mammalian cell culture medium such as MEM with 5-10% serum.
- For novel materials, FDA says to consider both direct-contact and elution methods; a direct-contact study per ISO 10993-5 may be needed.
- FDA recommends that cytotoxicity be evaluated with a quantitative method, not confluence estimations.
- If a material is inherently cytotoxic, dilutions of the test solution may be needed to find the level at which cytotoxicity no longer occurs, weighed against duration of contact and clinical need. For dental acid etchants, devices containing a cytostatic or cytotoxic agent, and uncured polymer resins, comparative cytotoxicity testing against a legally marketed device may be needed.
Sample preparation for extracts is covered in ISO 10993-12, and the wider regulatory picture in FDA biocompatibility guidance.
What is the ISO standard for cytotoxicity tests?
The cytotoxicity part of the ISO 10993 series is ISO 10993-5, the standard FDA’s guidance refers to for direct-contact cytotoxicity studies. The guidance also says test reports should state compliance with, or deviations from, ISO 10993-5 and ISO 10993-12. For the other parts of the series see ISO standards for medical devices. The standard itself is published by ISO; its exact test methods and acceptance criteria are in the purchased text.
Cytotoxicity assay families and what each reads
A cell cytotoxicity assay is chosen for what it reads out, and the choice can change the answer. The rows below compare three common readouts.
| Assay readout | What it measures | Caveat |
|---|---|---|
| MTT (tetrazolium reduction) | Commonly used to assess cytotoxicity through intracellular tetrazolium reduction | Non-specific intracellular reduction can underestimate cytotoxicity; in primary glioblastoma cells, ethanol at 5% was markedly underestimated, and the authors recommend pairing MTT with other assays for primary cells (Jo et al., 2015) |
| LDH release | Plasma membrane damage and cytotoxicity | Used alongside MTT by Jo et al. as the complementary readout |
| Trypan blue dye exclusion | Plasma membrane integrity only | Early apoptotic cells may still exclude the dye |
For method detail and pitfalls see MTT assay: measurement and pitfalls, CCK-8 and ATP assays.
Where it matters for labs and device decisions
Two points matter in practice. First, a single-readout result can mislead: because MTT can underestimate cytotoxicity in some primary cells, a membrane-integrity readout such as LDH release gives an independent check. Second, for device submissions, the extraction conditions, quantitative readout and reporting against ISO 10993-5 and ISO 10993-12 are the points FDA’s guidance addresses, so they belong in the test plan and report.
Is cytotoxic good or bad?
It depends on intent. For a device material, a cytotoxic response is an adverse finding that testing is designed to detect. For agents designed to kill cells, such as cytotoxic or cytostatic agents, cell killing is the intended mechanism. The word on its own is neither a safety verdict nor a quality claim.
What is cytotoxic chemo?
“Cytotoxic chemo” is a general term for chemotherapy medicines that work by killing or damaging cells. This page gives no treatment, dosing or handling instructions; those come from a prescriber, the product labelling and the institution’s hazardous-drug policy.
Common misunderstandings
- Viability and cytotoxicity are the same measurement. They are related but read different things; a metabolic assay and a membrane-integrity assay can disagree.
- One passing assay proves non-toxicity. The MTT evidence above shows why a second readout is used.
- Visual confluence is enough. For in vitro mammalian cell assays FDA recommends a quantitative method.
FAQ
What does cytotoxicity mean?
It means toxicity to cells: the ability of a substance, material or agent to damage or kill them.
What is a cytotoxicity assay?
A laboratory test that exposes cultured cells to a test article and quantifies cell damage or death, for example by MTT reduction, LDH release or dye exclusion.
Which standard governs device cytotoxicity testing?
ISO 10993-5, referenced in FDA’s 2023 guidance on ISO 10993-1, together with ISO 10993-12 for sample preparation.
Sources
- FDA, Use of International Standard ISO 10993-1, Biological evaluation of medical devices – Part 1, issued September 8, 2023, https://www.fda.gov/media/142959/download; checked 2026-11-07.
- Jo HY et al., The Unreliability of MTT Assay in the Cytotoxic Test of Primary Cultured Glioblastoma Cells, Exp Neurobiol 2015;24(3):235-245, https://pmc.ncbi.nlm.nih.gov/articles/PMC4580751/; checked 2026-11-07.
