Dear Des Walsh,
Thks for yr input on this topic. I was not previously aware of the procedural ISO standard you mentioned. Various (primarily APC) numerical swab guidelines exist as frequently discussed on this forum. IMEX these are usually “keyed” to swabbed areas. Even so, the numbers given vary widely. However I hv also seen arbitrary reference “units” utilised in some industries for convenience, eg for rating the internal cleanliness of standard milk vessels.
I suspect that in this specific thread, as already suggested, different “people” are simply using different starting assumptions / enumeration methods.
I had a look at ISO 18593 which is not the easiest read I hv ever met . The extracts below summarise IMO the numerical aspects of their swab option using a swab stick / pour plate procedure.
3.1 Because these methods are not quantitatively reliable or reproducible, results should only be used in a “trend analysis”.
3.3 Using the swab method, a specified area of the surface to be examined is marked (e.g. using a template)
and then wiped. The swab sticks are broken into a tube or bottle containing a sterile dilution fluid or
neutralizing fluid and mixed by hand.
If high numbers of microorganisms are expected, prepare further decimal dilutions in peptone water diluent to obtain countable number of colonies (see ISO 6887-1).
Inoculate duplicate plates of media with initial suspension, using 1 ml of inoculum for pour plates. Treat any further dilutions in the same manner. Invert the dishes and incubate the plates for the appropriate time and temperature.
9.2.1 Calculate the number of colony-forming units (CFU) per millilitre of initial suspension, as described in the relevant International Standard.
9.2.2 Calculate the number of CFU per square centimetre of surface investigated, NS, using the formula
NS = N x F x D / A
where
N is the number of CFU in 1 ml of dilution liquid (or neutralizing liquid);
F is the amount, in millilitres, of dilution fluid (or neutralizing liquid) in the tube
A is the surface investigated, in square centimetres;
D is the reciprocal of the dilution used.
9.2.4 If the area swabbed was not defined, to calculate the number CFU per swab, NSW, use the formula
NSw = N x F x D
CommentsPara. 3.1 above is worth noting.
I assume the “initial suspension” in 9.2.1 refers to the holding fluid/swab as introduced in 3.3 above.
The preferred calculation is seemingly to measure the area swabbed and calculate the result as per the equation given, eg as CFU/cm
2.
If the swabbed area is unknown/not readily measurable, equation 9.2.4 is permitted. I guess this may have potential for defined but irregular shaped objects. Hands ?
For either N
S or N
Sw, the initial volume of suspension fluid (F) seems to be used in the calculation.
If Tesco were using the above basis, Poppy’s original criticism that “cfu/swab is <1 is illogical” seems well justified to me (the intriguing aspect is that Tesco provide different "nil" limits for APC and Enterobacteriaceae).
@Poppy - I deduce from yr post(s) that Tesco did not (procedurally) explain fully their requirements.?
Rgds / Charles.C
PS If anyone is interested in the detailed meaning of "dilution theory" a nice explanation IMO with examples is here -
http://www.jlindquis...ro/102dil1.htmlhttp://www.jlindquis...cro/102P1S.html