Sherlock Holmes' Approach to BIs & CIs

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Sherlock Holmes' Approach to BIs & CIs

By Martha Young, BS, MS, CSPDT

The mission of a department producing sterile medical devices is to improve the products and services provided continually and meet the changing requirements of the customer while ensuring that products are safe for patient use. The results of chemical and biological indicators (CIs and BIs), in addition to mechanical monitors and information included in record keeping, are used to detect failures in the sterilization process. When failures are detected, the next step is to try and determine the cause of the sterilization process failure.

Pay attention to the clues provided by the BIs and CIs and mechanical monitors to see if you can solve the problem as Sherlock Holmes and Dr. Watson work through these situations.

Situation One

A rapid readout BI and a chemical integrating indicator, commonly referred to as a chemical integrator, were run inside each sterilization container system. The container systems were routinely processed for 10 minutes in the 270°F/132°C gravity steam sterilizers. Sporadic positive BIs, detected after one hour of incubation, started occurring. The term sporadic positive BIs is used because not every BI from every container in every sterilizer is positive. Sometimes the BIs are negative and sometimes they are positive. This is an indication of a marginal sterilization process. In all cases, the CIs showed an accept. Why are the CIs showing an accept but the BIs are not always killed inside the container?

Sherlock Holmes discusses the situation with Dr. Watson:

  1. Was the correct BI and CI used?
  2. Is the container designed for use in a 270°F/132°C gravity steam cycle?
  3. Are the cycle parameters correct for the container system?
  4. Are positives occurring in all sterilizers or just a certain sterilizer?
  5. When did the positive BIs start occurring?
  6. Is the sterilizer actually reaching 270°F/132°C for 10 minutes?
  7. Can you get enough air out of and steam into the container system during the 10-minute cycle to achieve sterilization?
  8. Is the steam quality correct (i.e., not too wet or not too dry)?

The investigation revealed the answers to the above questions:

  1. The correct BIs and CIs were used.
  2. The container system is designed for use in a 270°F/132°C gravity steam cycle. It has a valve system that opens and closes to allow air removal and steam penetration.
  3. Pre-purchase testing of the container system had been performed at the time of purchase at the facility according to the Association for the Advancement of Medical Instrumentation (AAMI) recommended practice.1,2 At that time, all biological indicators were killed and the chemical integrators showed an accept. A 10-minute 270°F/132°C gravity steam sterilization cycle was determined to be effective for routine processing of the containers.
  4. Positive BIs were occurring in all 270°F/132°C gravity steam sterilizers.
  5. The positive BIs have been occurring for a week.
  6. The sterilizer repair person checked the equipment and determined that the sterilizer was reaching the appropriate time and temperature and that there were no air leaks.
  7. The AAMI pre-purchase testing protocol was repeated to determine if air was still being removed and steam was penetrating the container systems. Multiple BIs and chemical integrators were placed inside each container system used and processed for 10 minutes in a 270°F/132°C gravity steam sterilizer. After one hour of incubation, positive, rapid-readout BIs were observed from several of the containers. All the chemical integrators showed an accept. Upon inspection of the container systems by the hospital and container manufacturer, it was determined that the valves failed to open properly to allow complete air removal and effective steam penetration.
  8. The steam pressure corresponded to the temperature, which infers that the steam quality was correct. Further steam quality testing was not done.

Conclusion: The defective valve on the container system was detected by the positive rapid-readout BIs but not by the chemical integrators. This is because the spores of the BI are sensitive to small changes in the sterilization process. Spores are used in BIs to test the efficacy of the sterilization process because they die at a very slow rate and because they are affected by the same kinds of changes in cycle conditions that would effect the microorganisms commonly found on the items being sterilized.3

CIs designed around the "biological" view provide information about combinations of process parameters, such as temperature and time, but they may not detect small (but potentially significant) deficiencies in individual parameters.3 BIs integrate all the parameters of the sterilization process to ensure that adequate conditions are present during the process. CIs should still be used inside each package to determine that the sterilant has penetrated the package and should be used in conjunction with BIs to determine the effectiveness of the sterilization process.4,5,6

This situation required additional testing with BIs and CIs because the positives were occurring sporadically, and it was not immediately evident whether the problem was with the sterilizers, steam quality, or the container systems. Testing each container system with BIs and CIs in one sterilizer isolated the problem and identified the defective container systems.

The container systems were repaired and tested with BIs and CIs according to the AAMI recommended practices to determine that the container systems were working before they were put back into routine use.1,2

This hospital continues to use a rapid-readout BI in every load in the Operating Room and in Central Supply because they feel that only a biological indicator can tell the whole story. A chemical integrator is also run in each container. The Central Supply manager stated that "with a chemical integrator you know what sterilization conditions existed in the pack, but you don't have the assurance of knowing positively that the bacteria were killed."

Situation Two

A disposable rapid-readout biological indicator steam test pack was run in the first load of the day and with each load containing implantable medical devices in all steam sterilizers in Central Processing. The BI results indicated sporadic positives from one sterilizer. That sterilizer was an older-model, vacuum-assisted steam sterilizer that ran a 270°F/132°C 4-minute cycle. The Bowie-Dick test always showed a pass, and the mechanical monitors indicated that the correct cycle parameters were being met. All the chemical integrators inside the packages showed an accept.

Sherlock Holmes and Dr. Watson sat down for tea to discuss this situation:

Since the positive BI occurred only in one sterilizer, the discussion centered on that sterilizer.

  1. Was the correct BI test pack and CI used for the sterilizer cycle?
  2. How many of the loads run each day have positive BIs?
  3. Was the sterilizer reaching the appropriate time and temperature?
  4. Were there air leaks or steam quality problems?
  5. Was there anything different about the load contents or the amount of items in this sterilizer as compared to the other sterilizers that did not have positive biological indicators?

The investigation revealed the answers to the above questions.

  1. The correct BI test pack was used for the cycle, and the correct CI was used inside each pack.
  2. Since biological indicators were run only daily and with each load of implantable medical devices, the frequency of BI monitoring was increased to each load. After several days of testing, it was obvious that more sterilization process failures were occurring. Approximately 80% of the loads showed positive BIs.
  3. The sterilizer repair person checked the sterilizer and determined that the sterilizer was reaching time and temperature.
  4. No air leaks were detected by the sterilizer repair person. Further testing determined that the steam quality was fluctuating from cycle to cycle. In some cycles, the steam was too wet, and in other cycles, the steam was too dry to do an adequate job of killing the spores.
  5. Correct packaging and loading techniques were used in all sterilization cycles.

Conclusion: Since there was only one sterilizer showing positive BIs, the problem had to be related to the sterilizer, steam quality, or how that sterilizer was used. A steam quality problem was finally detected by the sterilizer repair person.

The sterilizer was repaired to correct the steam quality problem. Sterilizer efficacy testing was performed according to the AAMI recommended practices.4 A disposable rapid-readout BI steam test pack was run in three consecutive empty loads. After the rapid readout, BIs were negative, the CIs reached their appropriate endpoint response, and the sterilizer was put back into routine use.

As in situation one, the BI detected a problem while the chemical integrator did not. According to Dr. Jack Young, this is because the BI is more sensitive at detecting steam quality problems than a CI.7 This situation is an excellent example of the need to use both BIs and CIs to monitor the sterilization process. Running BIs in each load detects sporadic changes in the sterilization process.

This hospital now runs rapid-readout BIs in each load and chemical integrators in each package. From the hospital viewpoint, the side-by-side double check of tandem chemical and biological monitoring is the safest and wisest way to go.

Situation Three

The Central Service technician checked to see why the 270°F/132°C vacuum-assisted sterilization cycle was taking so long. The technician looked at the mechanical gauges and printouts and noticed that the sterilizer had been in the "come-up mode" for the past hour. The sterilizer never switched to the sterilization mode. The technician turned the sterilizer off and removed the load. The rapid-readout BIs from the disposable rapid-readout steam pack from the load were incubated, and the results were negative after three hours of incubation. The load was broken down, and all the chemical integrators inside the packs showed a reject. The load was repackaged and re-sterilized.

Sherlock Holmes and Dr. Watson discussed this situation while playing darts at the local pub.

  1. Why did the sterilizer not switch to the sterilization mode?
  2. Why was the BI killed when the chemical integrators showed a reject?

The investigation revealed the answers:

  1. The sterilizer never switched to the sterilization mode because not enough steam entered the chamber to reach the 270°F/132°C needed for the sterilization cycle to start. A valve was changed and the steam pressure was adjusted. Sterilizer efficacy testing was done according to the AAMI recommended practices.4 A disposable rapid-readout steam test pack was run in three consecutive empty loads. After all the rapid-readout BIs were negative and the CIs reached their endpoint response, the sterilizer was put back into routine use.
  2. The BIs were killed during the come-up time because the spores saw enough time and temperature (approximately 1 hour at 235°F/113°C) in the presence of steam to achieve kill. The CIs inside the packs failed because the temperature was not high enough for a long enough time in the presence of steam to melt the chemical pellet, allowing it to migrate and wick to the accepted position of the chemical integrator.

Conclusion: The sterilizer was repaired, re-tested, and put back into routine use. This is an example of why the results of all process monitoring controls must be read and interpreted to determine the efficacy of the sterilization process. Each of these monitors/indicators measures parameters of the sterilization process in different locations and by different methods. The results of the BI showed that the cycle had enough time and temperature in the presence of steam to kill spores even if the required sterilization parameters were not met as indicated by the mechanical monitor and the chemical integrators. This load would not be used because all the quality control checks did not indicate that the sterilization process was acceptable.

Situation Four

The Operating Room opened a package containing a scope and found an ethylene oxide (EO) internal CI with an incomplete endpoint color response. The package was returned to Central Service for reprocessing. Sherlock Holmes thought about this situation:

  1. What were the EO cycle parameters used, and did the mechanical gauges indicate that these were met?
  2. What was the relative humidity in the processing area?
  3. What other packages were in the cycle, and did the CIs inside those packages reach their endpoint color response?
  4. What were the load's BI results?
  5. What were the contents of the package, and how was it wrapped?
  6. How was the sterilizer loaded?

The investigation revealed:

  1. A warm EO cycle was used, and the mechanical printout showed that all parameters were met.
  2. The relative humidity (RH) in the processing area was 30%. When the RH is below 35%, packaging and medical devices become desiccated. When placed in the EO sterilizer, they may absorb so much humidity that there is not enough left for sterilization to occur. The room RH was increased to 50%.
  3. The CIs inside all other packages reached their endpoint color change.
  4. The test-pack BI was negative.
  5. The scope container had a piece of foam at the bottom with a 100% cotton towel on top. The scope was placed on a folded towel that was then folded over the top, creating four layers of the towel. The lid was placed on the container and the container was over-wrapped with two layers of a polyester/linen wrapper. This was a new scope and product testing according to AAMI recommended practices was never done before the scope container was put into routine use.4 So product testing according to the AAMI document was done by placing BIs and CIs into the scope container, which was then processed in a full load. The positive BIs and CIs with an incomplete color change obtained from this testing showed that the packaging material was too dense and absorbent to allow consistent EO and humidity penetration into the package. The towel was removed and the type of foam changed. Product testing was repeated. All BIs were killed and the CIs reached their endpoint color change. The scope container was put back into routine use.
  6. The sterilizer was not overloaded, but personnel were in-serviced on proper packaging and loading techniques. Product testing was written into the department policies and procedures.

Conclusion: For effective EO sterilization, you need adequate humidity in the processing area, inside the chamber, and the packages for EO to penetrate and effectively kill spores. In this situation, the packaging used for the scope was too absorbent and the low relative humidity in the processing area aggravated the situation. Whenever major changes are made in packaging, wraps, or load configurations that include the addition of a new type of package or tray such as this scope example, product testing, in accordance with AAMI recommended practices, must be done to ensure that the product change can be sterilized effectively.4

This problem was detected by the internal CI because it was inside a package that was too dense. If an internal CI is not used inside each package, this problem could go undetected, and a non-sterile scope would be used repeatedly on patients.

If you did not solve the problems, review the following tips and start thinking like Sherlock Holmes and Dr. Watson when you have a sterilization process failure.

Tips for Problem Solving:

  1. Brainstorm the possible causes of the process failure and start investigating those possible causes. There may be more than one reason. When the cause is found, correct it and monitor it.
  2. Don't call a service representative until you have done some investigation and eliminated all reasons for the failure except for a sterilizer or sterilant quality problem. The exception is if the Bowie-Dick test shows a problem. Then the only course of action is to call a service representative.
  3. Look for the obvious first. For example, if all the rapid-readout BIs are positive in all sterilizers on the same morning, what do they have in common? The steam source may be a good place to start. There could be air in the lines, so run an empty cycle in each sterilizer to blow the air out and then run another set of BIs.
  4. If you are only running a BI once a week or daily in each steam sterilizer, you could be missing a lot of problems. Run more BIs to see how extensive the problem is and to monitor changes that occur during the day in steam quality, loading, packaging, and operator functioning. Consider running a rapid-readout BI in each steam load and quarantining packages to prevent recalls or the use of non-sterile medical devices. According to Edwin Ross, this is the most practical and least expensive operational process that also meets your ethical responsibilities to the patient.8
  5. Use both BIs and CIs to detect problems. If they don't agree, think about why that is happening. Remember that BIs may detect small changes in the sterilization process, especially related to steam quality, that CIs may not detect. Also remember that the CI inside each pack is like the policeman looking for problems in those locations and that the one BI in the load cannot tell you what is happening in all locations.
  6. Never assume that a BI is a false positive (i.e., the positive is because of contamination of the BI) just because you cannot find an obvious reason for the sterilization process failure. Always proceed with the investigation.
  7. Never assume that just because you have one CI that did not reach its appropriate endpoint response that there is a CI quality problem.
  8. Always do pre-purchase testing of rigid containers or product testing when there is a change in the process before putting the rigid container or change into routine use. Also, test the sterilizer whenever it is installed, relocated, redesigned, after preventive maintenance, and after positive BIs to ensure the process is still effective. You will eliminate a lot of sterilization process failures by doing your homework.
  9. Run the correct BI, BI test pack, and CI for the cycle and load being processed. Don't over-challenge the process.
  10. Keep accurate records of the sterilization process, including maintenance. These records are invaluable when trying to determine the reason for a process failure.
  11. Check with the manufacturer of the products being used (i.e., sterilizer, wrapping material, container system, medical device, monitoring products) to determine if the products are being used correctly. If all else fails, read the package inserts and operating instructions.

Use common sense and your knowledge of the sterilization process to determine what the clues are telling you. Don't be frustrated if you cannot determine the problem. Sometimes transient situations occur that disappear before they can be identified. With practice and by asking the right questions, you will get better at solving the problems. If all else fails, have a cup of tea or play some darts and see if new thoughts cross your mind. That is what Sherlock Holmes and Dr. Watson did. Good luck.

Martha Young, BS, MS, CSPDT, is an international technical service specialist, Sterilization Products, for 3M Health Care (St. Paul, Minn). She has more than 20 years of experience playing Sherlock Holmes and solving sterilization process problems.

For references and Best Practices continuing education application form, see the ICT Web site.


  1. To list what questions to ask, how to investigate the situation, and how to find the answers to determine reasons for a sterilization process failure.
  2. To explain why the results of the biological and chemical indicators may not agree.
  3. To explain how a biological indicator integrates all the parameters of the sterilization process to ensure that adequate conditions are present during the process.
  4. To list how to conduct a pre-purchase product evaluation of rigid sterilization container systems and product testing if changes are made in the sterilization process.

Test Questions, True or False:

  1. If a BI survives a sterilization process, all the CIs can still reach their endpoint response.
  2. A rigid container system does not need to be tested with biological and CIs before placing it into routine use.
  3. BIs are sensitive to small changes in the sterilization process, especially steam quality.
  4. CIs should be used inside each pack to detect that the sterilant has penetrated the package.
  5. Do not increase the frequency of BI monitoring to determine the reasons for sterilization process failures.
  6. A load can be released if the BI was negative, the CIs failed, and the mechanical monitors indicate incorrect process parameters.
  7. After a sterilization process failure, the sterilizer should be re-tested with BIs and CIs in an appropriate test pack in three consecutive cycles and put into use only when all the BIs are negative and the CIs reach their endpoint response.
  8. Brainstorming is a great way to determine the possible causes of a sterilization process failure.
  9. Immediately call a sterilizer service representative to determine the reason for a sterilization process failure.
  10. Do product testing before any change in the sterilization process is put into routine use.

Answers for Test

1. T
2. F
3. T
4. T
5. F
6. F
7. T
8. T
9. F
10. T

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