Heterotrophic plate count (HPC) is a procedure for estimating the number of live heterotrophic bacteria in water. The test is used to monitor water quality and disinfection effectiveness in water distribution systems, cooling towers, and process water. The time from sample collection to analysis should not exceed 24 hours. Samples must be kept cool during holding and in transit at approximately < 10° C (not frozen) to prevent variable bacteria counts.

For compliance testing of drinking water regulated by Clean Water Act, see Compliance Testing for Drinking Water.


Water Systems and Cooling Towers

For all sample collections:  

  1. Complete SPL’s Chain of Custody form.
  2. Label bottles with sample location, description, date and time with waterproof pen.

Collecting Samples

Water Distribution System: Quality of Water at Tap

  1. Turn on water (hot or cold) and immediately fill bottle to ≥120 ml.
  2. Close bottle and then invert to mix the sodium thiosulfate neutralizer.
  3. Keep samples cold (< 10° C ) within 15 minutes of collection.

Water Distribution System: Quality of Water in Pipe (Regulatory Compliance Testing of Drinking Water)

  1. Turn on water (hot or cold) and flush for 2–3 minutes, then fill bottle to ≥120 ml.
  2. Close bottle and invert to mix the sodium thiosulfate neutralizer.
  3. Keep samples cold (< 10° C) within 15 minutes of collection.

Cooling Tower and Non-potable Water

  1. Submerge open bottle just under the surface of water to obtain ≥120 ml of sample. Avoid sediment. 
  2. Close bottle and invert to mix the sodium thiosulfate neutralizer.
  3. Keep samples cold (< 10° C) within 15 minutes of collection.

Holding and Shipping

  1. After collection and in transit, keep samples in cooler with wet ice to prevent variable bacteria counts.
  2. Same day delivery: Place sample on wet ice in a cooler within 15 minutes of collection, and deliver to SPL with completed Chain of Custody within six hours.
  3. Ship overnight: Pack chilled samples (< 10° C)  with frozen cold packs in SPL box with insulated container or cooler.
  4. Include completed SPL Chain of Custody in plastic document holder in box.
  5. Ship Monday–Thursday only to Special Pathogens Laboratory 1401 Forbes Avenue, Suite 401, Pittsburgh, PA 15219

For questions or to request cold packs call 412-281-5335.


Compliance Testing for Drinking Water

Heterotrophic plate count (HPC) is a procedure for estimating the number of live heterotrophic bacteria in water. It is used for compliance testing regulated by the Clean Water Act as part of the required monitoring of public water systems to ensure water is suitable for drinking. To meet the requirements for regulated compliance testing, the time from sample collection to initiation of analysis should not exceed 8 hours. Samples must be kept cool during holding and in transit.

For all sample collections, first complete SPL’s Chain of Custody form. Label bottles with sample location, description, date and time with waterproof pen.

Collecting Samples

  1. Check the tap. If cleanliness is questionable, choose another tap or disinfect the faucet (inside and out) with a bleach solution prior to sampling. Run water for an additional 2–3 minutes after treatment and before sampling.
  2. Open cold water tap fully and flush line for 2–3 min or for a time sufficient to clear the service line. Reduce flow to fill bottle without splashing.
  3. Fill the bottle (≥ 120 ml) leaving ample air space in bottle.
  4. Close bottle and then invert to mix the sodium thiosulfate neutralizer.
  5. Keep samples cold (< 10° C) within 15 minutes of collection.

Holding and Shipping

  1. After collection and in transit, keep samples keep samples cold in cooler with wet ice to prevent variable bacteria counts.
  2. Same day delivery: Place sample on wet ice in a cooler within 15 minutes of collection, and deliver to SPL with completed Chain of Custody within six hours.
  3. Ship overnight: Pack chilled samples (< 10° C)  with frozen cold packs in SPL's insulated liner in box or cooler. 
  4. Ship Monday–Thursday only to: Special Pathogens Laboratory 1401 Forbes Avenue, Suite 401 Pittsburgh, PA 15219

Questions or to request cold packs contact SPL at 412-281-5335.