Count Correction

Use this calculator to determine positive hole count correction on visible colony growths in a petri dish or contact plate following microbial air sampling. It is suitable for correcting counts obtained from microbial air samples taken using the MicroBio MB1, MB2 and MB2-RSH bioaerosol samplers with any sampling head option.

Determining Results

Once the sample has been taken, the contact plate or petri dish should be removed immediately from the MicroBio and the lid replaced on the plate and sealed. A note should be made on the lid regarding time, location and volume sampled. The plate should then be incubated for a period of time and temperature dependant on the requirements of the media. Once incubated, the colony growths are then counted, either manually or using an automatic colony counter. Due to the statistical nature of the sampling method and the chance more than one colony impinged at one point on the dish, a count correction needs to be performed.  This can be done using the calculator below or referring to the count correction tables in the MicroBio MB1 or MB2 manuals.

How are the results calculated?

Based upon work published (Macher, Am Ind Hyg Assoc J. 1989 Nov;50(11):561-8) the positive-hole correction count is calculated using:

Count Correction Equation
Count Correction Equation

Where Nh is the number of holes on the sampling head, Nf is the number of counted colonies and Nc is the corrected count.

If the counted colonies, (Nf) exceeds the number of sampling head holes (Nh), 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 per cubic metre. To convert the corrected count to CFU/cu.m use the equation

Colony Forming Unit per Volume Equation

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

Further Analysis

Spreadsheets to automate the correction process and to collate and present results are available for free download in Apple Numbers or Excel format.