Validation of the microbiological methods hygicult dipslide, contact plate, and swabbing in surface hygiene control: A Nordic collaborative study

A collaborative study on total aerobic bacterial count was conducted to validate the Hygicult^® TPC dipslide against contact plates and swabbing, using stainless-steel surfaces artificially contaminated with different microbes at various levels. Twelve laboratories took part in the validation procedure. The total number of collaborative samples was 108. The microbial level in each sample was assessed in triplicate using the 3 above-mentioned methods under 3 different incubation conditions (at 25 ± 1°C for 48 and 72 h and at 30 ± 1°C for 48 h). Surface sampling methods detached 25–30% at the lowest (theoretical yield, 1.4 cfu/cm²), 18–20% at the middle (theoretical yield, 10.7 cfu/cm²), and 16–21% at the highest (theoretical yield, 43.6 cfu/cm²) levels of microbes from the test surfaces. The percentage of acceptable results after removing outliers was 89%. Repeatability standard deviations ranged from 27.2 to 74.6% and reproducibility standard deviations ranged from 42.1 to 97.5%. There were no significant differences between results obtained at different incubation temperatures (25 and 30°C) or incubation times (48 and 72 h) for all 3 methods. The Hygicult TPC dipslide, contact plate, and swabbing methods gave similar results at all 3 microbial levels tested: 0.35–0.43 cfu/cm² at the lowest level, 1.9–2.2 cfu/cm² at the middle level, and 7.1–9.1 cfu/cm² at the highest level.