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How to Calculate Ethylene Oxide D-values

Ethylene oxide is a flammable gas that is found in solvents, antifreeze, pharmaceuticals, textiles and detergents. Ethylene oxide is also used for sterilization of hospital equipment. The D-value for ethylene oxide is the amount of time is takes to reduce the microbial population by 1 log after the sterilization process. The D-value calculation, therefore, determines the resistance levels of microorganisms to sterilization procedures. The D-value can be calculated using data from the time of exposure to the sterilant, the number of test samples exposed and the number of test samples showing no growth.

Things You'll Need

Scientific calculator

Instructions

- 1
  Create a table with three columns representing the exposure times of each sample (t), the number of samples exposed to the sterilzation during each time period (n) and the number of test samples exhibiting no growth at the specific time period (r). Label the different exposure times and samples as t1, r1, n1, t2, r2, n2 and t3, r3, n3, until rows for time intervals are complete.
- 2
  Calculate the x and y factors for each period of time (t) of exposure to the sterilant. Write the equations as x = (ti + ti + 1) / 2 and y = (ri + 1) / (ni + 1) - (r1 / ni). The "i" in the equation represents the time interval, such as t1, t2 or t3. Fill in as appropriate.
- 3
  Calculate the mean time to attain no growth for each time interval using the x and y factors that you calculated. The mean time to attain no growth is represented by a "u." Write the equation as ui = (xi) (yi). Complete the calculation for each time interval sample set.
- 4
  Determine the mean time to attain no growth from all of the test samples using the sum of all of the "u" values for each test sample. Calculate the mean D-value using a scientific calculator using the mean time to attain no growth value "u." Write the equation as D = u / 0.2507 + log10 x No. "No" represents the initial number of challenge organisms per test sample.

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