Feller Correction Calculator

Feller Correction Equation

Download Excel Spreadsheet: Click Here

Feller Correction

What is the Feller Correction?

When using a microbial air sampler, there is a statistical probability that one or more viable microorganisms enter the same hole or slit in the sample head, and impact in the agar forming only 1 colony forming unit, rather than 2 colony forming units. The Feller Correction equation was developed in 1968 by William Feller, and first applied to multi-jet microbial air impactors (200 or more holes) in 1989 by Janet Macher. The equation provides a statistical adjustment to account for the probability of two or more microorganisms entering the same inlet.

The main premise is the larger the number of holes and smaller diameter of holes, the lower the probability of 1 or more microorganisms entering the inlet. Also, as can be seen above, the higher the CFU count, the larger the correction. And, in the pharmaceutical industry, after 250 CFU's the sample is classified as, "Too Numerous To Count" (TNTC). 

The Feller Correction for most microbial air samplers will generally not start making adjustments until 18 to 21 colony forming units are enumerated, depending on the manufacturer. Grade A and B clean areas generally have recommended CFU limits of LESS THAN 1 or 10 respectively. If CFU counts in these clean areas approach the Feller Correction limits, you obviously have a much larger problem than a Feller Correction.

Therefore, in the life science industry, the Feller Correction has somewhat limited impact on production.


Weaknesses of the Feller Correction

One of the main complaints, particularly among manufacturers of high efficiency samplers, is that the equation does not take into consideration design efficiency or inefficiency among the various manufacturers. Simply because a manufacturer claims a low Feller Correction, does by no means confirm they have superior collection and incubation efficiency. Indeed, this a balancing act, really a dance between physical collection efficiency, biological collection efficiency, and Feller Correction. Indeed, we know that among all manufacturers of microbial air samplers that there's vastly different physical and biological collection efficiency. This is further evident in differing theoretical and experimental d50 values (Yao, 2006).  With low efficiency samplers, the experimental d50 will be below their theoretical d50 value.  In certain cases, because of design errors users will need to order petri dishes with a custom agar fill of upwards of 50ml to fulfill the new requirements of ISO 17141. 

Also, we encourage end users to double check manufacturer's correction tables using the downloadable spreadsheet above. You may be surprised at what you learn, particularly with some manufacturers of slit-to-agar samplers, which Macher confirms have substantially unfavorable Feller Correction values.

It is Climet's opinion that the Feller Correction values are an important consideration when choosing a microbial air sampler, and equally, a high efficiency sampler should have a theoretical d50 of about 1 micron, with a lower experimental d50 (Yao, 2006). Next, the manufacturer must be able to provide an independent laboratory's biological collection efficiency study that compares their instrument's performance against a high efficiency microbial air sampler in order to compare apples-to-apples.