ASTM D5465-93(2012)

1.1 These practices cover recommended procedures for counting colonies and reporting colony-forming units (CFU) on membrane filters (MF) and standard pour and spread plates.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Significance and Use:

These practices provide a uniform set of counting, calculating, and reporting procedures for ASTM test methods in microbiology.

The counting rules provide a best attainable estimate of microorganisms in the sample, since the samples cannot be held and reanalyzed at a later date.