Non-sterile (P) | Non-sterile (P) | Units | Percent chance to fail sterility test (F) | Percent False NegativeResult |
1:10,000 | 0.0001 | 1 | 0.0 | 99.99 |
1:1000 | 0.001 | 1 | 0.1 | 99.90 |
1:100 | 0.01 | 1 | 1.0 | 99.01 |
1:10 | 0.1 | 1 | 9.5 | 90.48 |
1:10,000 | 0.0001 | 5 | 0.0 | 99.95 |
1:1000 | 0.001 | 5 | 0.5 | 99.50 |
1:100 | 0.01 | 5 | 4.9 | 95.12 |
1:10 | 0.1 | 5 | 39.3 | 60.66 |
1:10,000 | 0.0001 | 20 | 0.2 | 99.80 |
1:1000 | 0.001 | 20 | 2.0 | 98.02 |
1:100 | 0.01 | 20 | 18.1 | 81.87 |
1:10 | 0.1 | 20 | 86.5 | 13.54 |
The above probability of a false negative sterility test F is calculated without respect to the sample volume tested. It is based on the number of units tested and the contamination rate P. Obviously, if a preparation has a contamination rate of P and N units are tested for sterility then the percent chance of correctly identifying a true sterility failure F is directly proportional to the volume of medium tested.
In this example refer to the case where 1 in 1000 units are expected to be non-sterile. Ideally, a sterility test will correctly identify this level of contamination. However, the USP <71> test does not accomplish this. The chance of correctly identifying this level of contamination is 2, 0.5 and 0.1% when 20, 5 or 1 units are tested. Thus, even when 20 units are tested the USP sterility test will miss the contamination event 98% of the time
Non-sterile (P) | Non-sterile (P) | Units | Percent chance to fail sterility test (F) | Percent False NegativeResult |
1:1000 | 0.001 | 1 | 0.1 | 99.9 |
1:1000 | 0.001 | 2 | 0.2 | 99.8 |
1:1000 | 0.001 | 3 | 0.3 | 99.7 |
1:1000 | 0.001 | 4 | 0.4 | 99.6 |
1:1000 | 0.001 | 5 | 0.5 | 99.5 |
1:1000 | 0.001 | 6 | 0.6 | 99.4 |
1:1000 | 0.001 | 7 | 0.7 | 99.3 |
1:1000 | 0.001 | 8 | 0.8 | 99.2 |
1:1000 | 0.001 | 9 | 0.9 | 99.1 |
1:1000 | 0.001 | 10 | 1.0 | 99.0 |
1:1000 | 0.001 | 20 | 2.0 | 98.0 |
1:1000 | 0.001 | 40 | 3.9 | 96.1 |
In this table it is shown that the chances of correctly identifying a contaminated lot when the contamination rate is 1 in 1000 as a function of sample size. When only a single unit is tested then there is a 0.1% chance of an accurate result or a 99.9% chance of a False negative result. By expanding the test to include 40 units as the sample size the chance of correctly identifying the non-sterile lot slightly increases to about 3.9%.
Accordingly, any rational to justify a reduced number of units tested should take into account the limits of the statistical meaning of the USP sterility test.
