Nonsterile (P) 
Nonsterile (P) 
Units 
Percent chance to fail sterility test (F) 

1:10,000 
0.0001 
1 
0.01 

1:1000 
0.001 
1 
0.1 

1:100 
0.01 
1 
1.0 

1:10 
0.1 
1 
11.1 

1:10,000 
0.0001 
5 
0.05 

1:1000 
0.001 
5 
0.5 

1:100 
0.01 
5 
5.3 

1:10 
0.1 
5 
82.4 

1:10,000 
0.0001 
10 
0.10 

1:1000 
0.001 
10 
1.0 

1:100 
0.01 
10 
11.1 

1:10 
0.1 
10 
271.8 

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 I have highlighted the case where 1 in 1000 units are expected to be nonsterile. 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 1, 0.5 and 0.1% when 10, 5 or 1 units are tested. Thus, even when 10 units are tested the USP sterility test will miss the contamination event 99% of the time.
Nonsterile (P) 
Nonsterile (P) 
Units 
Percent chance to fail sterility test (F) 
1:1000 
0.001 
1 
0.1 
1:1000 
0.001 
2 
0.2 
1:1000 
0.001 
3 
0.3 
1:1000 
0.001 
4 
0.4 
1:1000 
0.001 
5 
0.5 
1:1000 
0.001 
6 
0.6 
1:1000 
0.001 
7 
0.7 
1:1000 
0.001 
8 
0.8 
1:1000 
0.001 
9 
0.9 
1:1000 
0.001 
10 
1.0 
1:1000 
0.001 
20 
2.0 
1:1000 
0.001 
40 
4.2 
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 the 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 nonsterile lot slightly increases to about 4.2%.
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.
Respectfully Submitted,
By: Daniel Prince