TABLE 2.
Mean, CV, and percentage of Poisson variability for agar reference plates and mean, median, and CV of RE by surface material, target concentration, and sampling method
| Surface | Targeta | Reference agarb
|
RE (%)c
|
|||||||
|---|---|---|---|---|---|---|---|---|---|---|
| No. of samples | Mean (CFU/100 cm2) | CV (%) | % of Poisson variability | Sampling method | No. of samples | Mean | Median | CV (%) | ||
| Steel | Low (3 runs) | 48 | 8.3 | 230 | 510 | Swab | 36 | 3.4 | 0 | 550 |
| Wipe | 27 | 31 | 13 | 160 | ||||||
| Vacuum | 27 | 5.5 | 4.2 | 91 | ||||||
| Medium (3 runs) | 48 | 38 | 30 | 140 | Swab | 36 | 6.5 | 6.1 | 80 | |
| Wipe | 27 | 22 | 17 | 82 | ||||||
| Vacuum | 27 | 4.7 | 3.2 | 82 | ||||||
| High (3 runs) | 48 | 270 | 19 | 240 | Swab | 35 | 5.0 | 3.8 | 98 | |
| Wipe | 27 | 18 | 15 | 56 | ||||||
| Vacuum | 27 | 3.7 | 3.0 | 78 | ||||||
| Carpet | Low (2 runs) | 32 | 1.8 | 190 | 190 | Swab | 24 | 12 | 0 | 250 |
| Wipe | 18 | 120 | 4.5 | 220 | ||||||
| Vacuum | 18 | 6.3 | 2.1 | 130 | ||||||
| Medium (4 runs)d | 64 | 17 | 97 | 300 | Swab | 48 | 14 | 7.7 | 120 | |
| Wipe | 36 | 21 | 12 | 110 | ||||||
| Vacuum | 36 | 3.7 | 2.0 | 120 | ||||||
| High (3 runs) | 48 | 190 | 26 | 270 | Swab | 36 | 12 | 8.6 | 96 | |
| Wipe | 26 | 23 | 17 | 66 | ||||||
| Vacuum | 27 | 4.7 | 2.7 | 91 | ||||||
Target deposition densities were approximately 3, 30, and 200 CFU/100 cm2 for low, medium, and high, respectively.
The CV for agar results is 100% × the within-run standard deviation/agar mean; the percentage of Poisson variability is 100% × CV/Poisson CV, where Poisson CV = 100% × expected standard deviation/mean = 100% × (mean)1/2/mean.
RE (%) based on the reference agar is 100% × the mean (over all runs) of the mean (over all samples) of the ratio of the method result (based on a single sampling pass) to the mean agar result.
A single carpet run was intended to be low but was considered to be medium based on the agar results.