Non-sterile (P)
Rate

Non-sterile (P)
Rate

Units
Tested (N)

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)
Rate

Non-sterile (P)
Rate

Units
Tested (N)

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.

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