A. The Presumptive Test.
1. When definitely positive, that is showing more than 10 per cent.
(10%) of gas in 24 hours, is sufficient:
(a) As applied to all except the smallest gas-forming portion of
each sample in all examinations.
(b) As applied to the smallest gas-forming portion in the
examination of sewage or of water showing relatively high
pollution, such that its fitness for use as drinking water does
not come into consideration. This applies to the routine
examinations of raw water in connection with control of the
operation of purification plants.
2. When definitely negative, that is showing no gas in 48 hours, is
final and therefore sufficient in all cases.
3. When doubtful, that is showing gas less than 10 per cent. (10%) (or
none) in 24 hours, with gas either more or less than 10 per cent. in
48 hours, must always be confirmed.
B. The Partially Confirmed Test.
1. When definitely positive, that is, showing typical plate colonies
within 24 hours, is sufficient:
(a) When applied to confirm a doubtful presumptive test in cases
where the latter, if definitely positive, would have been
sufficient.
(b) In the routine examination of water supplies where a
sufficient number of prior examinations have established a
satisfactory index of the accuracy and significance of this test
in terms of the completed test.
2. When doubtful, that is, showing colonies of doubtful or negative
appearance in 24 hours, must always be completed.