Positive Hole Correction Calculations

The maths for the correction of colony counts is based upon research by Macher J.A Am Ind Hyg Assoc J. 1989 Nov;50(11):561-8.

The statistics are based on the probabilities of multiple organisms landing through the same sampling hole onto the same point on the dish surface.

The number of sites an organism can land on the surface of dish can never exceed the number of holes in the sampling head being used. In the case of MicroBio samplers either 220 or 400 points. Each landing site can have multiple organisms. When counting post incubation, the landing sites will appear as a single colony. It is this colony count that needs to be statistically corrected to determine the likely number of organisms where multiple impacts on a single point ay have occurred.

Count Correction

The count can be corrected using the following equation: 

(1)

n_f is the number of counted colonies, n_c is the corrected count, N_h is the number of holes in the sampling head.

If the count exceeds the number of holes, Nj, then the mathematics will fail and the results cannot be trusted.

If this is the case, then the sample dish can be considered as overloaded with organisms and the user should consider lower sampling volumes.

Colony Concentration

The colony concentration is calculated as the corrected count per volume of air sampled. The results are normally expressed in colony forming units (CFU) per cubic metre. To convert the corrected count, use the equation:

(2)

Where Nc is the corrected number of colonies counted and Vs is the sampled volume in litres.

Combining the equations for (1) and (2) a concentration gives:

(3)

Online Tool

An online tool is available on the Cantium Scientific website to perform positive hole correction and CFU calculations.

Equations in this document