Microbiology testing: test variability and reproducibility

For test methods that specify colony counts such as membrane filter or plated media, duplicate counts shall be performed monthly on one positive sample, for each month that the test is performed. If the lab has two or more analysts, each analyst shall count typical colonies on the same plate. Counts shall be within 10% difference to be acceptable. In a laboratory with only one microbiology analyst, the analyst shall count the same plate twice, with no more than 5.0% difference between the counts. Statutory Authority: § 2.2-1105 of the Code of Virginia. Historical Notes: Derived from Virginia Register Volume 25, Issue 7 , eff. January 1, 2009.