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Positive Spore Test Troubleshooting Guide

Dear Customer:

Thank you for your concerns regarding Propper’s Biological Test Pack and Culture Services Program (i.e. spore test). There are many subtle problems that can lead to a positive spore test. You should immediately check if your sterilizer is capable of meeting the following performance benchmarks. 

  • Steady temperature and pressure throughout the sterilization exposure phase; (i.e. the phase during which temperature and pressure cease to rise, and remain constant.)
  • Calibrated temperature and pressure sensors.
  • Complete air removal from chamber.
  • Fully saturated steam. Over-saturated steam is indicated by wet packs or pouches after sterilization. See table below for saturated steam temperature and pressure.

Steam Temperature

Saturated Steam Pressure

121°C (250°F)

15.32 psi

132°C (270°F)

27.19 psi

134°C (273°F)

30.19 psi


Please go through the following checklist to better understand what may be causing sterilization failure.

Sterilization Failure Check List

Possible causes of spore test failure


Leaking Gasket

Ensure that gasket is fitted perfectly over its socket. If a gasket is too dry, wet it with mild detergent and water. Change gasket if leaking persists.

Temperature Fluctuation

Ensure that sterilizer temperature remains constant as it reaches its preset level (e.g. 270°F or 250°F). Fluctuations in excess of 4°F will reduce sterilization efficacy considerably.

Pressure Fluctuation

In steam sterilizers, pressure should hold constant at constant temperature (see above). Pressure fluctuations in excess of 3 PSI (21 kPa) will reduce sterilization efficacy considerably.

Water Tank

Ensure that water tank is full. Furthermore, fill water tank with only de-ionized or distilled water. Change water inside tank on a monthly basis.

Load Configuration

When placing the biological test pack (spore test) inside sterilizer, do not overload the chamber with too many other instruments. For consistent test results, always place the biological test pack near the door. Do not stack other items over the biological test pack. Finally, ensure that while sterilizing, the biological test pack is always oriented with its paper side up and plastic side down. This will optimize steam flow into the pack interior.

Inadequate Processing Time

It is possible for autoclave to be prematurely shut down because of power/mechanical/human errors. Before sending out the spore test, always ensure that the cream colored thermochromic ink (on the biological test pack) has changed color from cream to black/dark brown.

Sterilizer Cleaning / Warmup Issues

Whenever the autoclave has been cleaned via a manufacturer-approved chemical solution, the autoclave must be warmed up prior to running any spore tests. A sterilizer can be warmed up by running a normal cycle without instruments included.


If the sterilizer has not been calibrated for more than one year, wear and tear can cause temperature and pressure sensors to drift. Schedule a calibration service if your sterilizer is periodically yielding positive spore tests.


There are other mechanical faults associated with the sterilizer which only an authorized repair technician can accurately diagnose. We suggest that you contact a sterilizer repair professional as soon as possible if two or more spore test are positive from the same machine.

Our lab extension is 159; we will be delighted to help you if you have any questions.