Wound Dressing Microbial Ingress Test

Microbial Ingress Testing is the evaluation of the ability of a medical device, such as a wound dressing, to resist or inhibit the transfer of infectious microorganisms under repeated simulated use conditions. Many wound dressings have features that could potentially allow microorganisms a point of entry directly into a patient or into sterile fluids during the course of clinical or home use, increasing the risk of infection. This is especially true with repeated use, product misuse, or in the event of product failure. Microbial ingress testing can be used to evaluate these risks and to help manufacturers design effective products and/or recommend appropriate practices.          

Wound Dressing Background

Wound dressings are considered Class II medical devices. They can be used as primary dressings for acute wounds from injury or surgery, as a means to hold primary dressings in place, or as wound packing. Typically these provide a sterile physical barrier to external contamination and they may or may not contain antimicrobial agents.

Wounds are an ideal environment for bacterial colonization and biofilm formation, providing both a growth surface and an ample supply of nutrients. It has been shown that multiple types of pathogenic biofilm producing bacteria such as S. aureus and P. aeruginosa can grow unchallenged within the wound dressing environment, but that antimicrobial wound dressings can limit, but may not eradicate, this bacterial growth.2

According to an FDA Guidance Document for Wound Dressings3 in addition to demonstrating the ability to provide a physical barrier from a broad spectrum of microorganisms (barrier testing), manufacturers of wound dressings are recommended to perform quantitative biochemical testing, following ASTM E2315-03 or an equivalent method to specifically address the identified risk of infection.

In the case of wound dressings that contain antimicrobial agents, Microbial Ingress Testing can help support the manufacturers claim that the antimicrobial agent provides an additional clinical benefit. The FDA recommends performance testing to demonstrate the antimicrobial effectiveness of the antimicrobial agent on the treated medical device.4


Wound Dressing Microbial Ingress Test summary

The Microbial Ingress Test is a simulated use test that mimics the use of a device in a clinical sittingFor wound dressings, both Barrier Testing and Quantitative Testing are recommended.

Barrier Testing is completed to insure that the wound dressing provides an adequate microbial barrier as described on the product label. The testing protocol should address the specific wound dressing functionality, should cover a broad spectrum of microorganisms, and should be performed both when the wound dressing is initially manufactured and at the device expiration date. The FDA guidance3 indicates,

The testing should include initial inoculum concentrations of 1 x 106 cfu/ml and an exposure time of at least 48 hours. The acceptance criteria should be the suppression of growth (of the inoculum species) within the dressing itself and on the underlying plate, at various time points representing an assessment of the barrier function of the dressing”.

Quantitate testing such as ASTM E2315-03, or an equivalent method, is used for the assessment of antimicrobial activity. These methods measure the change of a defined population of microorganisms within a specific time as tested against antimicrobial test materials in vitro (i.e. time-kill procedure) typically with tests demonstrating at least a 4 log reduction of the pathogenic species. Specifically5,

The test material or a dilution of the test material is brought into contact with a known population of microorganisms for a specified period of time at a specified temperature. The activity of the test material is quenched at specified sampling intervals (for example, 30 s, 60 s, or any range covering several minutes or hours) with an appropriate neutralization technique. The test material is neutralized at the sampling time and the surviving microorganisms enumerated. The percent or log10 reduction, or both, from either an initial microbial population, or test blank is calculated.”

A detailed study protocol can be prepared for the Wound Dressing Microbial Ingress test using sponsor provided testing parameters. Testing parameters may include the number or type of wound dressings to be tested, the number and length sampling cycles, and other conditions required in order to meet the device [510(k)] premarket approval or other requirements. Typically two gram negative and two gram positive organisms are chosen to demonstrate the ability of the test device to prevent ingress of infectious microorganisms. The protocol is reviewed and approved by the sponsor prior to initiating the testing. In the summary report, the study procedures are fully described, including the preparation, quantities and types of challenge organisms, the contamination or inoculation methods and sites, the sample size, the environmental conditions, the positive and negative controls used, along with any applicable validation methods.


strenGths of Wound Dressing Microbial INgress Testing

These quantitative time-kill tests are relatively inexpensive and the test parameters are easy to control in the laboratory setting. Completion of the microbial ingress testing will demonstrate the ability of the wound dressing to prevent the transmission of microbial contamination when the agreed upon test procedures are followed. Microbial Ingress Testing allows the sponsor to report the microbial barrier properties of their devices with confidence and can be submitted as part of the requirements suggested by the FDA guidance documents.3


weaknesses of Wound Dressing Microbial INgress Testing

As with any test method that simulates actual use, it is challenging to take all “real life” factors into account, and the test results will have limitations due to the assumptions used for the testing protocol. Microchem will work with the sponsor to identify appropriate procedures and test conditions for testing protocols in order to best replicate use during actual patient procedures.


why microchem?

Since 1988 Microchem Laboratory has served industry needs as an EPA and FDA GLP-compliant testing organization with a wide array of product and customer tailored testing services for environmental surface disinfectants, high-level disinfectants, sanitizers, medical devices, antimicrobial surfaces, personal care products, and antimicrobial devices. Our skilled, highly experienced microbiologists and chemists are ready to collaborate with you to evaluate the performance of your antimicrobial devices with regards to microbial ingress. Microchem can help speed your project to completion.

Our microbial ingress studies are conducted following the guidance provided by the FDA1 in accordance with Good Laboratory Practice Standards (GLPs) stipulated by U.S. FDA 21 CFR 58. In addition, studies are conducted in accordance with Microchem Laboratory’s Quality Management System and undergo a full quality assurance review. All studies by Microchem Laboratories are conducted in accordance with general terms and conditions as posted on www.MicrochemLab.com/terms.



  1. Photo content released into the public domain by its author Kei Jo at English Wikipedia. https://en.wikipedia.org/wiki/Dressing_(medical)
  2. Testing wound dressings using an in vitro wound model. J. Wound Care. 2010 June; 19(6):220-226. C. Lipp et al. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2923929/pdf/nihms226366.pdf
  3. Guidance for Industry and FDA Staff – Class II Special Controls Guidance Document: Wound Dressing with Poly(diallyl dimethyl ammonium chloride) (pDADMAC) Additive. October 6, 2009. U.S. Food and Drug Administration. http://www.fda.gov/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm186571.htm#6
  4. Draft Guidance for Industry and FDA Staff Premarket Notification [510(k)] Submissions for Medical Devices that Include Antimicrobial Agents, July 19, 2007. http://www.fda.gov/MedicalDevices/DeviceRegulationandGuidance/Guidan ceDocuments/ucm071380.htm
  5. ASTM E 2315-03. Standard Guide for Assessment of Antimicrobial Activity Using a Time-Kill Procedure.
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