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SOP for Operation and Verification of a Colony Counter

Last reviewed 19 Oct 2026 · 4 min read

Contents
  1. Objective
  2. Scope
  3. Responsibility
  4. Procedure
  5. 1. Pre-use checks
  6. 2. Count a plate
  7. 3. Check counter registration
  8. 4. Handle a failed check
  9. 5. Complete records
  10. Training and document control
  11. Related procedures
  12. Abbreviations
  13. References

In short: This illustrative SOP covers routine use and a basic electronic count-registration check for a manual colony counter. A marker-press check confirms that the counter registers presses; it does not by itself establish visual counting accuracy or replace the manufacturer’s calibration method.

Objective

Describe how to operate, check and document the use of a manual colony counter in a microbiology laboratory.

Scope

Use this example for a manual colony counter with a digital display and marker pen, such as the instrument described in the source procedure. Adapt it to the specific model, manufacturer’s instructions, validated microbiological method and site equipment-control procedure.

Responsibility

Example in-house criterion (not a pharmacopoeial requirement): a trained microbiologist operates the counter and completes its usage record; a designated microbiology lead oversees the process; and QA reviews the controlled SOP under the site’s document system.

Procedure

1. Pre-use checks

Confirm the instrument identity and status. Check that the platform and marker are intact, the counter is clean, and the display and controls respond normally. Do not use a damaged instrument or one marked unavailable under the site’s equipment-control process.

Example in-house criterion (not a pharmacopoeial requirement): place the counter on a clean, dry surface; clean it before use with isopropyl alcohol (IPA); connect the marker; switch on the instrument; and set the display to zero using its reset control.

2. Count a plate

Follow the microbiological test method for plate identification, countable range and result reporting; this SOP does not set a universal countable range. Place the plate in the position specified by the instrument instructions. Mark each colony once with the connected marker and read the displayed count. If the plate cannot be assessed reliably, document the issue and follow the test method’s invalid-result or repeat-testing procedure.

Example in-house criterion (not a pharmacopoeial requirement): the source procedure places the Petri dish in an inverted position and records the final display in the instrument usage log.

3. Check counter registration

Schedule M Part I §17.3.10.16 requires calibration and maintenance records for major and critical equipment, as appropriate, including dates and the identities of people who performed the work (Drugs Rules, 1945, Schedule M, substituted by G.S.R. 922(E), 28 December 2023). The precise calibration method and frequency for a specific counter depend on its model and the site’s approved equipment programme.

Example in-house criterion (not a pharmacopoeial requirement): the source procedure sets a monthly check. Starting from a zero display, it compares displayed counts with marker-pen press totals of 5, 10, 15, 20 and 25 and accepts the counter when each display matches the corresponding press total. It calls for an “Under Calibration” tag during the check and entries in the calibration and usage records.

4. Handle a failed check

Example in-house criterion (not a pharmacopoeial requirement): if the display does not match the approved check criterion, stop using the instrument, identify its status, notify the designated supervisor and document the result. Return it to use only after the required assessment, repair or successful repeat check is authorised under the equipment-control procedure.

5. Complete records

Record instrument identity, date, operator, check or use performed, results and any issue or action in the approved equipment record. Schedule M Part I §17.3.10.16 (G.S.R. 922(E), 28 December 2023) describes equipment records for calibration, maintenance, cleaning and repair where applicable.

Batch-associated GMP records must be kept for at least one year after the expiry date of the respective product batch. EU batch documentation must be retained one year after batch expiry or at least five years after Qualified Person certification, whichever is longer (EudraLex Volume 4, Part I, Chapter 4 §4.11, revision 1, 2011). The corresponding U.S. finished-drug requirement is 21 CFR §211.180(a), current eCFR text (2026), with a separate period for certain OTC products without expiry dating. Apply any longer period required by applicable local law or marketing authorisation.

Training and document control

Example in-house criterion (not a pharmacopoeial requirement): train affected analysts on the current controlled procedure and distribute copies under the site’s document-control system.

Abbreviations

  • GMP: Good Manufacturing Practice
  • IPA: Isopropyl alcohol
  • QA: Quality Assurance

References

  1. Government of India, Ministry of Health and Family Welfare. Drugs Rules, 1945, Schedule M Part I §§17.2.7 and 17.3.10.16, substituted by G.S.R. 922(E), 28 December 2023. Official Gazette PDF.
  2. European Commission. EudraLex, Volume 4, Part I, Chapter 4: Documentation, revision 1 (January 2011; effective 30 June 2011), §4.11. Official PDF.
  3. United States. 21 CFR §211.180(a), current eCFR text (2026). Official eCFR.

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