Medical Device Special Studies


Sterilisation Validation Programs: ISO 11137: VDmax

Sterilisation is designated by standard bodies as “special” in that the outcome of the process cannot be fully verified. Medical Devices can be claimed “Sterile” when a Sterility Assurance Level (SAL) of 10-6 can be guaranteed. This means that the probability to find a non-sterile sample is 1 in a million. With currently available techniques, it is not feasible to measure directly the attainment of a SAL much below 10-2



VD-max starts from the assumption, Distribution of Resistances (SDR).The SDR is a hypothetical distribution of micro-organisms. The SDR is regarded as a severe challenge to the sterilisation process. At 25 kGy a SAL of 10-6 is obtained for a bioburden of 1000 with the SDR.

The idea proposed by VDmax is to verify if the resistance against radiation sterilisation of the microbial flora in situ is higher than the SDR. This is performed by sterilising the product at a lower dose where a SAL10-1 would be obtained for the SDR.

If at least 9 out of 10 products, sterilised at VDmax, are sterile the microbial flora on the product is equal or less resistant as the SDR. It can thus be concluded that a sterilisation dose of 25 kGy is substantiated.

A complete VDmax procedure consists out of the following tests:

  • Bioburden validation
  • Bioburden determination and Calculation of the VDmax dose
  • Bacteriostasis/Fungistasis
  • Sterility test: Verification Dose Experiment


Bioburden Validation

Exhaustive Recovery (repetitive treatment)

The bioburden of non-sterile devices is determined repeatedly. The recovery of the first extraction is divided by the sum of the recoveries of each repeated bioburden determination to determine the recovery percentage.

Spore Inoculation

An artificial bioburden of 30-100 CFU is introduced on sterile test items. After the inoculum has been allowed to dry the bioburden of the inoculated devices is determined. The recovery percentage can be determined by dividing the inoculated amount of CFU by the recovered amount of CFU.

Bioburden Estimation

Samples are first extracted in an isotonic extraction fluid. Extracts and fluid test items are vacuum-filtered through a membrane filter with a pore size of 0.22 µM. Filters of that size will retain micro-organisms present in a solution.

The filter is aseptically transferred to an agar plate and incubated at the appropriate conditions.

Following the appropriate incubation time, colonies are counted.

To establish an accurate picture of the microbial contamination in the production environment, the bioburden of at least 3 batches and 10 samples per batch has to be determined.

The verification dose can be established using the highest batch average if, one or more batch average is equal to or greater than two times the overall average bioburden estimate, or using the overall average bioburden estimate if each of the batch averages are less than two times the overall average bioburden estimate.

Bacteriostasis Fungistasis

Prior to performing a sterility test, the absence of inhibitory substances in the growth medium, originating from the test item should be verified. The presence of such interfering substances could result in false negative results during the sterility test.

Sterile test items will be introduced into the growth medium. Subsequently, 100 CFU of either Bacillus spizizenii, Pseudomonas aeruginosa, Clostridium sporogenes, Candida albicans, Aspergillus brasiliensis and Staphylococcus aureus will be introduced in the incubation medium.

The absence of bacteriostatic and fungistatic effects is proven if the growth in medium + test item is comparable to the growth in the medium that contains no test item for all 6 micro-organisms.

If not, measures such as diluting the test item, adding polyvalent kations or polysorbate, etc should be taken to overcome the inhibitory effects.


The test items are transferred to microbial growth medium and incubated for 14 days.

For the VDmax procedure 10 samples of a lot sterilised at the verification dose (see sub. 0) are tested for sterility.

If at least 9 out of 10 samples are sterile, a sterilisation dose of 25 kGy is substantiated.

Periodic Dose audit

The dose release validation program must be audited for frequently produced products. For products produced less frequently than quarterly, each production lot is validated for release. On a quarterly base, 10 bioburdens on non-sterile samples and 10 sterility tests on test items sterilised at the verification dose should be performed.


Anti-microbial Efficacy - Time Kill Kinetic Assays

In these studies the anti-microbial efficacy of test items (wound-dressings, implants,...) containing anti-microbial agents is studied over time. Because coetaneous wounds are open to the environment and wound beds are such favourable environments for bacterial growth, heavy bacterial colonization is commonly associated with acute and chronic wounds.

 The presence of bacteria significantly increases the likelihood of overt, clinical wound infection. Bacterial colonization and infection can interfere with the wound healing process because many organisms produce a variety of toxins, proteases, and pro-inflammatory molecules, which in turn may promote an altered host response indicative of an excessive inflammatory response.

Therefore, prevention and treatment of bacterial colonization or infection is a major concern in wound care.

Traditionally, topical antiseptic agents such as silver sulfadiazine (SSD), silver nitrate (AgNO3), and mafenide acetate have been widely used in a variety of situations, including burn units. More recently, several new dressings with antimicrobial activity have been developed and used clinically.

 Antiseptics are anti-infective substances that, after topical administration, destroy or inhibit the growth of microorganisms in or on living tissue (skin, mucous membrane and wound). Antiseptics are applied externally and, to prevent the development of biocide resistance, they are used at concentrations considerably higher than minimal bactericidal concentrations (MBCs). Ideally, antiseptics should have a broad microbicidal spectrum and potent germicidal activity with rapid onset and long-lasting effect. Antiseptic preparations should not be toxic to host tissues/cells and in line with the concept of biocompatibility of medical products, as far as possible, they should not impair healing processes.

The relationship between the antibacterial activity and the biocompatibility of an antiseptic agent can be expressed as biocompatibility index (BI). The BI is the ratio of the highest concentration at which the antiseptic agent is not cytotoxic and the microbicidal effect producing at least a 3 log reduction.

Test items are introduced in a medium simulating the wound environment. Different contaminants are transferred individually to the medium at a known concentration and the test samples are incubated at 37°C to simulate the body temperature.

On different time points the amount of viable micro-organism is determined and the reduction compared to the control test item containing no anti-microbial agents.

To study how long the anti-microbial effect of the test items lasts the test items can be spiked at a second time point.  

Graphical presentation of the viability of microorganisms during a 5 day Time Kill Kinetic Assay for different anti-microbial wound dressings and one negative control containing no anti-microbial agents (without re-spiking) 


Microbial Integrity/Ingress studies

The aim of this test is to evaluate the microbial sealing capacity of a test item. The test items are exposed to a single or multiple microbial contaminations of at least 106 CFU/mL of different strains.

Subsequently, the test items are tested for sterility


Cleaning Validation of Reusable Medical Devices

Reusable medical devices are devices that health care providers can reuse to diagnose and treat multiple patients. Examples of reusable medical devices include surgical forceps, endoscopes and stethoscopes.

When used on different patients, reusable devices become soiled and contaminated with microorganisms. To avoid any risk of infection by a contaminated device, reusable devices undergo "reprocessing," a detailed, multistep process to clean and disinfect or sterilize them. When the labeling instructions for reprocessing are completely and correctly followed, reprocessing results in a medical device that can be safely used for more than one patient.

Adequate reprocessing of reusable medical devices is vital to protecting patient safety.

Inadequate reprocessing between patients can result in the retention of blood, tissue and other biological debris (soil) in certain types of reusable medical devices. This debris can allow microbes to survive the disinfection or sterilization process, which could then lead to Healthcare-Associated Infections (HAIs). Inadequate reprocessing can also result in other adverse patient outcomes, such as tissue irritation from residual reprocessing materials, like chemical disinfectants.

 The FDA places the primary responsibility for developing and validating methods for effective reprocessing of a reusable medical device on the manufacturer of the device. The manufacturer is expected to validate that the device can be cleaned and disinfected or sterilized adequately to allow the device to be reused. As outlined by the FDA, the manufacturer must test and validate any labelling claims of fitness for reuse that are provided in the instructions for the handling, cleaning, disinfection, packaging, and sterilization of medical devices in a healthcare facility.

Depending on the use, the device can be classified into non-critical, semi-critical and critical. Depending on the classification the device should undergo a low or high-level disinfection or sterilisation (see table).

The manufacturer of a medical device must validate the effectiveness of the reprocessing method with the device and validate the compatibility of the device with the reprocessing method. A validated approach should be mentioned in the instructions for use of the reusable device. 


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