ISO 13485 · Expert · 13 min lesson
Bioburden, sterility assurance level, endotoxin and LAL testing in routine control
ISO 13485:2016 clauses 7.5.5, 7.5.7 and 4.2.5; ISO 11737-1:2018, ISO 11737-2:2019; FDA 2012 endotoxin guidance, USP 85 and Ph. Eur. 2.6.14
What you will learn
- Explain how bioburden is determined and monitored under ISO 11737-1, including recovery efficiency and method suitability.
- Explain the sterility assurance level, the log reduction needed and why a sample sterility test cannot release a lot.
- Explain endotoxin and the lysate test, apply device limits in a worked calculation and design a routine monitoring programme.
Key takeaways
- Bioburden is measured with a method validated for recovery efficiency and method suitability (ISO 11737-1) and trended, because the sterilization basis depends on it.
- A SAL of ten to the minus six means the log reduction needed equals the log of the bioburden plus six.
- Sterility tests (ISO 11737-2) support validation; a small sample cannot release a lot, so rely on validated process control (clause 7.5.7).
- Normal sterilization does not remove endotoxin; prevent it upstream and test with the lysate assay in endotoxin units.
- Device limits from the FDA 2012 guidance are 20 EU per device (0.5 EU/mL) and 2.15 EU per device for cerebrospinal fluid contact; records must show controls and calculations.
Free practice question
Veldt's lab counts 40 colony forming units on a tubing set and reports it as the bioburden. The extraction method was never checked for recovery, and no study shows whether the plastic inhibits growth in the culture.
What does ISO 11737-1 expect before the number is used?
Recovery efficiency and method suitability studies, so that the count can be interpreted.
Correct. ISO 11737-1 expects the method to be shown to recover organisms and not to be inhibited by the product.
Nothing, because a colony count is exact.
Not quite. A count is exact only for recovered organisms, so ISO 11737-1 requires evidence on recovery and inhibition.
A sterility test on the same device.
Not quite. A sterility test answers a different question and cannot validate a bioburden counting method under ISO 11737-1.
An endotoxin test on the same extract.
Not quite. Endotoxin testing measures a different property and does not validate colony counting under ISO 11737-1.
Key terms
- bioburden: The population of viable microorganisms on or in a product or sterile barrier system before sterilization.
- sterility assurance level: The probability of a single viable microorganism on a product after sterilization; ten to the minus six by convention for sterile labels.
- endotoxin: Lipopolysaccharide from the outer membrane of gram negative bacteria; the main pyrogen tested in devices.
- pyrogen: Any substance that causes fever when introduced into the body.
- LAL: Limulus amoebocyte lysate, the reagent of the bacterial endotoxin test.
- recovery efficiency: The share of organisms present on a product that an extraction method retrieves.
- D value: The time or dose that reduces a microbial population by one log under stated conditions.
- maximum valid dilution: The endotoxin limit concentration divided by the lysate sensitivity; the largest dilution at which the limit can still be detected.
Official sources
- ISO 11737-1:2018 Sterilization of health care products - Microbiological methods - Part 1: Determination of a population of microorganisms on products
- FDA Guidance for Industry: Pyrogen and Endotoxins Testing: Questions and Answers (June 2012)
- ISO 17665:2024 Sterilization of health care products - Moist heat
- ISO 13485:2016 Medical devices - Quality management systems
Go further
The full lesson has narrated slides, more scenario questions with explained answers and a simulation.
Open the AcademyWatch on YouTubeIllustrative example. Educational only, not legal advice.