Comparison of In-Office Dental Unit Waterline Test Kits

The CDC recommends dental unit waterlines contain ≤ 500 Colony Forming Units (CFUs)/mL of heterotrophic, mesophilic bacteria for non-surgical procedures. The most common way to monitor dental unit waterlines is with in-office testing kits.  Objective:  To compare two in-office dental unit waterline test kits, Petrifilm AC Plate (3M, St. Paul, MN) and the Heterotrophic Plate Count (HPC) Sampler (Millipore, Burlington, MA) to R2A agar (gold standard).  Methods:  Water samples (40mL) were collected six times from air/water syringes of 25 dental units (n=150).  Samples were treated with 0.4 mL 1% sodium thiosulfate solution (final concentration 0.01%) to neutralize effects of daily waterline treatments (ICX, Adec).  Serial dilutions with sterile water were performed. Samples were cultured on HPC Samplers and Petrifilm AC Plates according to manufacturer's instructions and on R2A agar using standard spread plate method 9215C.  Sterile water was used for negative controls.  Samples were incubated at room temperature (23-27°C).  HPC Sampler and R2A plates were counted by hand (5X magnification) after incubation for 7 days.  Petrifilm AC plates were counted after 5 and 7 days using an electronic plate reader (Petrifilm Plate Reader, 3M).  Greater or less than 500 CFU/mL of bacteria was the endpoint criteria to assess the performance of each test kit to R2A plating methodology. Validity measurements for Petrifilm AC Plates (5 and 7 days) and HPC Samplers were calculated.  Results:  The table below contains the results of the study.  Conclusions:  Petrifilm AC Plate incubated for 7 days demonstrated increased validity compared to the HPC Sampler or Petrifilm AC Plate at 5 days.