Cytotoxicity testing can be used to test a sample to determine how much it can damage, or even cause the death of, human cells. Standards and legislation require manufacturers to demonstrate that their devices are not cytotoxic.
With a good test setup, it is possible to detect whether a sample is 100% cytotoxic, i.e., highly cytotoxic, or 0% cytotoxic, i.e., not cytotoxic. In contrast, an unsuitable test set-up can lead to the cytotoxicity not being determined correctly.
And this can lead to the non-detection of cytotoxic substances that may have irritating, sensitizing or, in the worst case, even carcinogenic/mutagenic/reprotoxic (CMR) properties. With fatal consequences for patients, users and third parties.
Therefore, carefully designing the tests is vital. This article explains how to do this and gives you 7 power tips that will help you select a laboratory.
EN ISO 10993-1 defines cytotoxicity as follows:
“Cytotoxicity tests employing cell culture techniques can be used to determine the cell death (e.g. cell lysis), the inhibition of cell growth, colony formation, and other effects on cells caused by medical devices, materials and/or their extracts. If testing is performed, it shall be conducted in accordance with ISO 10993-5.”
Source: EN ISO 10993-1, 126.96.36.199
EN ISO 10993-5 also defines the limits and, therefore, when a device/material/extract has cytotoxic effects. The standard gives two criteria for determining whether a substance should be classified as cytotoxic:
“Reduction of cell viability by more than 30% is considered a cytotoxic effect.”
“The achievement of a numerical grade greater than 2, based on Tables 1 and 2, is considered a cytotoxic effect.”
Source: EN ISO 10993-5, section 8.5
That there are two approaches to determining whether something is cytotoxic is logical given the two ways to analyzing cytotoxicity tests: quantitative evaluation and qualitative evaluation.
Cell changes, growth inhibition
Cell death, inhibition of cell growth, cell proliferation, colony formation
Assay/staining, etc. (MTT, XTT, BCA, etc.)
Amount of protein, release of enzymes, reduction of vital dye…
Presentation of results
> Grade 2
Table 1: Difference between the qualitative and quantitative evaluation (measurement)
While the quantitative evaluation usually relies on assays and evaluation by photometer, the qualitative evaluation is purely visual using a microscope. In a qualitative microscopic evaluation, the cells are assessed visually and the sample is graded.
State of the culture
… no cell lysis, no reduction of cell growth
<= 20% of cells are round, loosely attached... only slight growth inhibition observable
<= 50% of the cells are round, …no extensive cell lysis, <= 50% growth inhibition observable
<= 70% of the cell layers contain rounded cells or are lysed… > 50% growth inhibition observable
Nearly complete or complete destruction of the cell layers
Table 2: Qualitative grading of cytotoxicity
Under the microscope, the sample might look like this:
The cytotoxicity test is not a hard pass or fail test. Even cytotoxic results can be acceptable for certain materials and applications.
Regulations like the MDR require proof of the biocompatibility of all materials that come into direct or indirect contact with patients.
The cytotoxicity test is THE standard test for biocompatibility according to EN ISO 10993-1.
The article on biocompatibility and ISO 10993-1 will give you an overview of the topic and introduce you to additional regulatory requirements.
Cytotoxicity testing is essential for every medical device category, contact duration and type of contact. In addition, it also used to test final cleaning as part of the validation (ISO 19227). It has been a standard test for batch releases for years.
This is due to the cytotoxicity test’s strong sensitivity, which makes it ideal as a screening test.
A cytotoxicity test is a screening test that can detect a wide range of production tools. monomers, metal ions, plasticizers, cleaning agents and disinfectants, etc. But not all in toxicologically relevant concentrations!
However, the cytotoxicity results themselves do not allow conclusions to be drawn about the cause of the effect. In addition, a passed cytotoxicity test is not a guarantee of the absence of critical residues or that problematic substances are not released by the device/material. The reason for this is that the test is a very sensitive screening test overall (i.e., it detects a lot of different substances well even at low concentrations) but is limited and no longer works if the concentration is too low, even though it may still be highly toxicologically relevant.
Cytotoxicity testing alone is not sufficient to demonstrate biocompatibility or the purity of medical devices. Additional analyses adapted to the device, its use and the question should always be performed.
A cytotoxicity test essentially consists of 3 steps.
The first step is the extraction. For this, the device is brought into contact with an extraction medium and extracted in an inert vessel over a specified period of time, at a specified temperature and with an extraction ratio adapted to the device. Details can be found in EN ISO 10993-12.
The extraction medium can vary widely but is usually a cell culture medium with serum added, sometimes also with DMSO (dimethyl sulfoxide) added as a migration enhancer.
After extraction, the extract is incubated together with the cells. There are several ways of doing this: the cells can be pre-incubated (pre-grown) with no extract or be grown together with the cells.
The duration of incubation is variable: it is never less than 24 h and can last up to several days. The cell line L929 (mouse fibroblasts) is the established standard for the test. In principle, other cell lines can also be used.
The cells then incubate (grow) in the presence of the extract until subconfluency (confluency = cell density) is reached.
“approximately 80% confluency, i.e., the end of the logarithmic phase of growth”
Source: EN ISO 10993-5, 3.6
After the incubation to a subconfluent (80% confluency) cell lawn, the influence of the extract on cytotoxicity is evaluated. This is done either qualitatively or quantitatively, as described above.
You must now be wondering what influence all the variables can have on the test and, therefore, on your test result.
The answer is: you can never get a bad test result from a very good device. A device that does not release or contain on its surface cytotoxic substances will never become cytotoxic as a result of the test parameters being changed.
But you can lower the sensitivity of the test by using certain combinations of test parameters so that the cytotoxicity of a bad device is reduced from 100% to 0%. We have seen numerous examples of this in the last 10 years.
Specify all test parameters so that test results are fully comparable in the event of retesting and avoid commissioning too many test laboratories for one device. If you have approved several laboratories as suppliers, make sure that the test parameters are as comparable as possible.
Do not mask poor results with tests designed to have low sensitivities. Cytotoxic results always have a very serious cause and should be looked into as a matter of urgency. Otherwise, the results will eventually catch up with you. In the case of product contamination, in particular, it is not uncommon for this to increase over time if the cause is not eliminated.
The testing of extracts described above is by far the most common test setup. For the manufacturer, knowledge of the above interrelationships is enough.
Please feel free to contact the Johner Institute’s experts (e.g., by email) if you would like to know more about the procedure and the performance of other special test variants (testing by direct contact, agar diffusion and filter diffusion).
The title of EN ISO 10993-5 is “Biological evaluation of medical devices — Part 5: Tests for in vitro cytotoxicity.” At 46 pages long, the standard is relatively manageable. Much of the standard (Annexes A-D) is devoted extensively to how to implement specific test setups and is primarily of interest for laboratories or for troubleshooting.
The sections relevant to the normal manufacturer finish on page 21.
In addition to the requirements of EN ISO 10993-5, manufacturers must also take into account the requirements of EN ISO 10993-1 and EN ISO 10993-12 for the tests.
Unfortunately, different laboratories or even the same employee in a laboratory can select different parameters for your device when re-testing it. For example, a change in the extraction ratio from 3 cm²/ml to 6 cm²/ml can result in the cytotoxicity being twice as high and vice versa. The same also applies for various other variables (see the figure above).
Since each laboratory has a slightly different test setup, you shouldn’t expect to get exactly the same results from two different laboratories. A second test in the event of poor results may, therefore, not be helpful.
So don’t be surprised by contradictory results. If your laboratory is accredited or GLP-certified, you should generally trust the laboratory’s results.
Nevertheless, even laboratories can make mistakes. Suboptimal device extraction can have a particularly big effect.
Outliers also regularly occur in the measurement results. If you have tested several devices in parallel, “mix-ups” are unfortunately not uncommon.
You almost certainly check your devices for batch uniformity or at least at certain intervals for changes. So, you should urgently introduce monitoring to identify trends at an early stage.
For example, if you have a titanium product and cytotoxicity levels consistently around 15%, there may already be a major problem, even if the test is officially considered to be passed. This is because even low concentrations of residues can be above the relevant toxicological limits, even if these substances are almost not cytotoxic in the test.
Cytotoxic does not mean that you have not passed the test. There are no passes or fails in cytotoxicity testing.
With some materials, you will always get cytotoxic results. That doesn’t have to be a problem. It is well known, for example, that polyurethane is cytotoxic, and so are a lot of other materials.
A failed test does not mean that your device is not biocompatible. In such cases, a more detailed toxicological evaluation that goes beyond the material characterization required by EN ISO 10993-1 (N.B.: material certificates are not sufficient) is required.
Please email the Johner Institute’s experts, if you have any questions on this.
The cytotoxicity test alone is not sufficient to demonstrate purity or biocompatibility.
Do not assume that the cytotoxicity test will sufficiently detect every substance. For this reason, the test alone is not sufficient to prove the purity of medical devices or the absence of leachable substances.
Unsurprisingly, the cheapest test is not always the best. Of course, there are some very good laboratories with very low prices. However, paying low prices often only gets you a standard test that is not tailored to your device.
As always, you can tell if a laboratory is good or not by how quickly and actively they communication with you. Always ask about additional costs for:
Sometimes cheap offers turn out to be a black box with considerable additional costs of up to 50%.
How you select your laboratory determines your test results at least as much as your device itself. And a passed cytotoxicity test does not mean that no substances of concern are released. By making sure your test parameters are tailored to your device and that they are correctly specified to the laboratory, you can achieve optimal comparability and ensure that you do not over or underestimate the cytotoxic effects of your device.
We will be happy to help you select a laboratory, determine the test parameters, commission the tests and interpret the results if your device does not pass the test. Contact us by email or via our free micro-consulting service.